FOUS11 KWBC 010617
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
217 AM EDT Thu Oct 01 2020

Valid 12Z Thu Oct 01 2020 - 12Z Sun Oct 04 2020

Days 1-3...

The probability of heavy snow or icing is less than 10 percent.


Petersen


$$
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FOUS11 KWBC 021918
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Fri Oct 02 2020

Valid 00Z Sat Oct 03 2020 - 00Z Tue Oct 06 2020




Days 1-3...

The probability of heavy snow or icing is less than 10 percent.


Jackson



$$
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FOUS11 KWBC 030803
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Sat Oct 03 2020

Valid 12Z Sat Oct 03 2020 - 12Z Tue Oct 06 2020



Days 1-3...

The probability of heavy snow or icing is less than 10 percent.

Petersen



$$
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FOUS11 KWBC 041917
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 PM EDT Sun Oct 04 2020

Valid 00Z Mon Oct 05 2020 - 00Z Thu Oct 08 2020



Days 1-3...

The probability of heavy snow or icing is less than 10 percent.

Jackson




$$
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FOUS11 KWBC 050610
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 AM EDT Mon Oct 05 2020

Valid 12Z Mon Oct 05 2020 - 12Z Thu Oct 08 2020


Days 1-3...

The probability of heavy snow or icing is less than 10 percent.

Petersen




$$
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FOUS11 KWBC 060805
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Tue Oct 06 2020

Valid 12Z Tue Oct 06 2020 - 12Z Fri Oct 09 2020


Days 1-3...

The probability of heavy snow or icing is less than 10 percent.

Petersen




$$
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FOUS11 KWBC 070716
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 AM EDT Wed Oct 07 2020

Valid 12Z Wed Oct 07 2020 - 12Z Sat Oct 10 2020


Days 1-3...

The probability of heavy snow or icing is less than 10 percent.

Petersen




$$
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FOUS11 KWBC 080825
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Thu Oct 08 2020

Valid 12Z Thu Oct 08 2020 - 12Z Sun Oct 11 2020



Days 1-2...

The probability of heavy snow or significant icing is less than 10
percent.

Day 3...

...Pacific Northwest to the Northern Rockies...

Snow levels are expected to drop noticeably as an upper level
shortwave trough and its associated surface frontal front move
across the Northwestern CONUS Saturday and Saturday night.
Measurable snow is likely across the higher elevations of the
Olympics and WA-OR Cascades, with significant accumulations
possible in areas above 7,000 ft. Light-to-moderate
higher-elevation snowfall (heaviest above 7,500 ft) will then
spread into the Blue, Salmon River, Sawtooth, Bitterroot, Teton,
and Wind River mountain ranges late Saturday into early Sunday
morning.

The probability of significant icing is less than 10 percent.

Hurley



$$
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FOUS11 KWBC 081949
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EDT Thu Oct 08 2020

Valid 00Z Fri Oct 09 2020 - 00Z Mon Oct 12 2020


Day 1...

The probability of significant icing and/or snow is less than 10
percent.

Days 2-3...

...Pacific Northwest to the Northern Rockies...
An upper level shortwave trough and its associated cold front will
move across the Northwest on Saturday. While snow levels are
expected to drop considerably -- from above 8000 to below 6000ft
-- heavy snow accumulations through late Saturday will likely
remain confined to the highest peaks of the Cascades.
Post-frontal, onshore flow will continue to support showers across
the Olympics and Cascades, where snow levels are expected to hover
around 5000ft into Sunday. Meanwhile, mountain snows are forecast
to spread west into the eastern Oregon mountains and northern
Rockies by Saturday night and then continue into Sunday.
Significant accumulations are possible across the higher peaks of
the Blue Mountains in eastern Oregon, as well as the central Idaho
and western Wyoming ranges.

The probability of significant icing is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 091933
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EDT Fri Oct 09 2020

Valid 00Z Sat Oct 10 2020 - 00Z Tue Oct 13 2020


Days 1-3...

...Pacific Northwest to the Northern Rockies...
A well-defined mid-to-upper level shortwave trough is forecast to
move across the northwestern U.S. on Saturday. Favorable upper
level jet dynamics, in addition to a long fetch of moisture, are
expected to support widespread precipitation across the region,
with orographic effects forecast to help produce locally heavy
amounts. While snow levels are expected to drop significantly
behind the associated cold front, heavy snow accumulations through
late Saturday will likely be confined to the highest peaks of the
Cascades. Onshore flow will continue to support showers across
the Northwest through Sunday before a mid-level shortwave brings a
return of organized heavier precipitation back into the region
Sunday night. This will likely produce several more inches of
snow across the highest peaks of the northern Cascades.

Meanwhile to the east, mountains snows are forecast to advance
east into the mountains of northeastern Oregon and the northern
Rockies beginning Saturday night and then continuing through
Sunday into Monday. Locally heavy accumulations are possible
along the higher elevations the Wallowa Mountains in northeastern
Oregon, as well as along the central Idaho, western Montana and
western Wyoming ranges.

The probability of significant icing is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 100838
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 AM EDT Sat Oct 10 2020

Valid 12Z Sat Oct 10 2020 - 12Z Tue Oct 13 2020


Days 1-3...

...Pacific Northwest to the Northern Rockies...
A well-defined mid-to-upper level shortwave trough is forecast to
move across the northwestern U.S. on Saturday. Favorable
left-exit region upper level jet dynamics into the trough base, in
addition to a long fetch of moisture, are expected to support
widespread precipitation across the region, with orographic
effects forecast to help produce locally heavy amounts. While
snow levels are expected to drop significantly behind the
associated cold front, heavy snow accumulations through late
Saturday will likely be confined to the highest peaks of the
Cascades. Onshore flow will continue to support showers across
the Northwest through Sunday before a mid-level shortwave brings a
return of organized heavier precipitation back into the region
Sunday night. This will likely produce several more inches of snow
across the highest peaks of the northern Cascades.

Meanwhile to the east, mountains snows are forecast to advance
east into the mountains of northeastern Oregon and the northern
Rockies beginning Saturday night and then continuing through
Sunday into Monday. Locally heavy accumulations are possible
along the higher elevations the Wallowa Mountains in northeastern
Oregon, as well as along the central Idaho, western Montana and
western Wyoming ranges.

The probability of significant icing is less than 10 percent.

Hurley/Pereira

$$
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FOUS11 KWBC 111920
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Sun Oct 11 2020

Valid 00Z Mon Oct 12 2020 - 00Z Thu Oct 15 2020

Days 1-3...

...Pacific Northwest to the Northern Rockies...
A reinforcing shortwave trough in a strengthening westerly jet
aimed right as Vancouver Island/northern WA will reach Vancouver
Island/the Olympic peninsula this evening and push across the
northern Rockies to central MT Monday. This strong and deep zonal
flow will persist across the northwest CONUS into midweek, until
the next embedded shortwave trough reaches Vancouver
Island/northern WA late Tuesday (and MT Tuesday night). This will
make for two distinct waves of precip with mountain snows for the
Cascades and northern Rockies tonight into Monday and again late
Monday night through Tuesday night.

Snow levels of 7000 to 9000ft are ahead of the shortwave that
reaches the coast this evening and quickly drop to around 5000ft
late tonight through Monday (as precip tapers off) then rise again
to 7000 to 9000ft Monday night/Tuesday ahead of the next wave.
Therefore, heavy snowfall tonight and Monday night/Tuesday will be
limited to the higher WA Cascades and the Bitterroot Range and
northern Rockies of eastern ID and western MT, along with the
Tetons, Sawtooth, and Wind River ranges in northwest WY.
Probabilities of at least 6 inches in 24 hrs are highest over
these higher ranges -- exceeding 50% over the highest peaks. By
day 2.5 (12Z Tue-12Z Wed), 24hr probs of 6+ inches will be
confined to the northern WA Cascades and far northern Rockies in
northwest MT.


The probability of significant icing is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 120806
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Oct 12 2020

Valid 12Z Mon Oct 12 2020 - 12Z Thu Oct 15 2020

Days 1-3...

...Pacific Northwest to the Northern Rockies...
A 130+ kt upper jet streak and associated flat shortwave will
quickly traverse the northern Rockies and High Plains on Monday.
Flat upper riding will follow in an otherwise continued fast,
quasi-zonal flow aloft, before another fast-moving shortwave
reaches Vancouver Island and WA on Tuesday. This second wave will
allow for more appreciable upper height falls across the northern
Rockies and into the Plains Tuesday night and Wednesday.

Higher elevation snowfall associated with the initial shortwave
and surface front will taper from west to east today across the
northwestern U.S. Highest totals during day 1 are expected above
5500 ft over the WA Cascades and northern Rockies in northwest MT,
though 24hr probabilities of 6+ inches will again be localized to
the highest peaks. Snowfall coverage picks up on day 2 (Tue-Tue
night) associated with the next powerful upper jet streak (150+
kts at 250 mb) and associated vort lobe. Snow levels initially
climb to 8000-9000 ft ahead of the shortwave, then drop to
4500-5500 ft across the WA Cascades and northern Rockies Tuesday
night. Day 2 24hr probabilities of 8+ inches peak between 60-80%
over the northern WA Cascades and northern Rockies in northwest
MT. By day 3 (Wed-Wed night), as the shortwave energy deepens the
longwave trough east of the Divide, snowfall over the northern
Cascades and Rockies will dissipate both in coverage and
intensity. Day 3 24hr probabilities of 8+ inches drop to 30% or
less across northwest MT.

The probability of significant icing is less than 10 percent.

Hurley

$$
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FOUS11 KWBC 131942
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Tue Oct 13 2020

Valid 00Z Wed Oct 14 2020 - 00Z Sat Oct 17 2020

Day 1-2...

...Pacific Northwest to the Northern Rockies...
Progressive, zonal flow will carry the last in a series of
shortwave troughs across the northern Cascades on Tuesday and then
across the northern Rockies late Tue into the overnight. Ahead of
a 150kt upper jet streak, this shortwave will help amplify the
longwave trough east of the Continental Divide Wed-Thu.

Ongoing, high elevation snowfall across the WA Cascades and the
northern Rockies will start to wane Tuesday night into Wednesday
as the amplifying wave digs over the Great Plains (and the ensuing
veering, deep-layer northwesterly flow cuts off the direct onshore
moisture connection). Snow levels will drop to 4000-5000ft across
the region Tuesday night. Day 1 24hr snow probabilities of 8+
inches above 50 percent will be limited to elevations above 6000ft
in the northern WA Cascades, along with the northern Rockies in
northwest MT.

By Day 2, the potential for any additional significant snow
accumulations will likely be confined to the higher elevations of
the western and central MT ranges, as well as the Bighorns in
northern WY.

Day 3...

The probability of receiving at least 6 inches of snow is less
than 10 percent.

The probability of receiving at least a tenth inch of ice is less
than 10 percent during days 1 through 3.

Pereira/Hurley/Jackson

$$
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FOUS11 KWBC 140809
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Wed Oct 14 2020

Valid 12Z Wed Oct 14 2020 - 12Z Sat Oct 17 2020

Day 1...

...Pacific Northwest to the Northern Rockies...
The 150-170+ upper level jet streak and associated shortwave
energy will gradually lift east-northeast of the Pacific NW and
northern Rockies Wednesday and Thursday. The ongoing, high
elevation snowfall across the northern Rockies in northwest MT
will gradually wane today as the amplifying wave digs over the
Great Plains (and the ensuing veering, deep-layer northwesterly
flow cuts off the direct onshore moisture connection). 24 hr
probabilities of 50% or greater of at least 6 inches of snow will
be confined to the higher elevations (7000+ ft) of the far
northern Rockies in northwest MT, along with the Bitterroots along
the ID/MT border.

Day 2...

The probability of receiving at least 4 inches of snow is less
than 10 percent.

Day 3...

Another upper level shortwave dropping southeast out of British Columbia/western Alberta on the western periphery of the longwave
trough will bring generally light snowfall across the far northern
Rockies in northwest MT. 24 hr snow probabilities of exceeding 4
inches range between 30-50%; however during the same time probs
of greater than 8 inches are less than 25%.

The probability of receiving at least a tenth inch of ice is less
than 10 percent during days 1 through 3.

Hurley

$$
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FOUS11 KWBC 150847
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Thu Oct 15 2020

Valid 12Z Thu Oct 15 2020 - 12Z Sun Oct 18 2020


...Northern Rockies and Northern Plains to Lake Superior...
Days 2-3...
The next shortwave packet to round a deep low centered over
northern Manitoba is currently a low over northern Alaska and a
trough approaching the Gulf of Alaska. The Gulf of Alaska wave is
expected to dart ahead of the northern Alaska low and track down
the Canadian Rockies, reaching western MT late Friday. Surface low
pressure then tracks east across the northern Plains to Lake
Superior through Saturday. Wrap around flow behind the low,
combined with anti-cyclonic flow ahead of a strong high diving
down the lee of the Canadian Rockies combine to form upslope flow,
along with low-to-mid level frontogenesis and upper jet support
will support the potential for locally significant accumulations
along the western Montana to the northern Wyoming ranges east of
the Divide Friday night through Saturday night. Day 2.5 and 3 snow
probs are moderate to high for six or more inches over only the
highest eastern Rockies in MT and the Absaroka/Big Horns in
northern WY.

Farther east, the surface low tracks across northern SD Friday
night and reaches Lake Superior Saturday evening. This is a very
similar track and strength of low to what crossed this area on
Wednesday though with much colder conditions in place and behind
the system. This should enable a swath of moderate to potentially
locally heavy snow north of the low track over ND/northern MN.
There are now 20 to 30 percent probabilities for 4 or more inches
over northeastern ND on Day 2.5 and over northern MN/the Arrowhead
on Day 3.

...Northeast...
Day 3...
A shortwave trough currently over the northern Rockies amplifies
the broad-scale trough attendant to the anchored and deep closed
low over Manitoba takes on a negative tilt as it moves across the
Mid-Atlantic Friday night. This system will continue to amplify as
it lifts across the Northeast late Saturday. Surface low pressure
is forecast to develop as it tracks north along a tight baroclinic
zone in the Gulf of Maine on Saturday. Guidance continues to
tighten the precip swath over New England and eastern NY Saturday.
Enough cold air shifts east to bring some mountain snows to the
highest Adirondacks, Greens, and Whites. Day 2.5 snow probs are 10
to 20 percent for four or more inches along the higher White
Mountains from northern NY into Maine. Snow levels drop to around
3000ft after the low passage Saturday with diminishing wrap around
snows for the New England and Adirondack terrain.

For Days 1-3, the probability of significant icing is less than 10
percent.

Jackson

$$
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FOUS11 KWBC 190853
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
452 AM EDT Mon Oct 19 2020

Valid 12Z Mon Oct 19 2020 - 12Z Thu Oct 22 2020


...Northern Rockies/High Plains...
Day 1...

Waves of energy embedded within progressive, northwesterly flow
aloft will continue to support periods of unsettled wintry weather
across much of MT through the next week.

The next shortwave trough will push southeast across BC today.
Southerly flow over cold air in place to the north of a stalled
frontal boundary is forecast to support snow across the higher
elevations of the northern Rockies into the High Plains with
freezing rain common for valleys and much of central MT through
this evening. Snow levels are then forecast to increase from
southwest to northeast with the heaviest snow accumulations are
expected to fall across the higher elevations of the western and
central Montana to the northern Wyoming ranges -- where WPC PWPF
continues to show moderate probabilities for local accumulations
of 6-inches or more. Ice probabilities for a tenth inch or more
are limited to around 20% in south-central MT, though the
probability for one hundredth is low to moderate for much of MT
east of the Rockies.

...Northern Plains and Upper Midwest...
Days 1/2...

Shortwave energy swinging east from the northern Rockies is
expected to amplify over the northern Plains on Tuesday. A weak
surface reflection along with favorable upper forcing will help
support a wide swath of light to increasingly moderate
precipitation as it shifts across the Dakotas late tonight and
across MN/WI/the UP Tuesday. Enough cold air will be in place to
support accumulating snow on a southerly flow ahead of the
shortwave. Days 1.5/2 snow probabilities are moderate for four or
more inches across central MN, the MSP metro into western WI and
for the North Shore of Lake Superior (enhanced from the southerly
flow over the lake). A low-to-mid level frontal band lifting
across the region looks to support some heavier totals across
central to northern Minnesota and northern Wisconsin. As of now
the Day 2 snow probs are up to 10% for 6 or more inches in central
MN.


...Northern Rockies to northern Great Plains...
Days 2/3...

The next wave rounding the deep low over Hudson Bay amplifies as
it digs over the Pacific NW Wednesday, reaching the northern Great
Basin Wednesday night. This promotes less side surface
cyclogenesis over WY on Thursday which shifts south to CO
Wednesday night. This provides yet another opportunity (and
perhaps the strongest yet) for snow in the northern Rockies as
Gulf of Mexico sourced moisture lifts up the Plains and wraps
around the developing lee-side cyclone. Furthermore, a ridge from
a 1040mb high shifts south into northwestern MT Wednesday night,
further promoting low level convergence and upslope flow. Day 3
snow probabilities for 6 or more inches are moderate to high for
much of central MT. Probabilities for a foot or more are moderate
to high for the northern Absarokas and Crazy Mtns of southern MT.

An inverted surface trough under a developing southwesterly jet
spreads this focus east over the Dakotas Wednesday night where
there are moderate Day 3 probabilities for 4 or more inches over
southern ND.

For Days 2/3, the probability of significant icing is less than 10
percent.

Jackson

$$
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FOUS11 KWBC 201942
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Tue Oct 20 2020

Valid 00Z Wed Oct 21 2020 - 00Z Sat Oct 24 2020


...Northern Rockies to northern Great Plains...
Days 1-3...
A significant early season snow event is likely stretching from
the Northern Rockies and into the northern Great Plains Wednesday
night through Friday morning.

Shortwave trough rotating around the large cyclonic gyre centered
near the Hudson Bay will dig into the Northern Rockies late
Wednesday and then pivot across the northern Plains, exiting the
western Great Lakes on Friday. This trough will be accompanied by
a strong jet max which will place its RRQ atop the region to
enhance deep layer ascent, with the interaction of the two driving
surface low development in the High Plains, and elongating into
the upper MS VLY. Northwest of this low, a steeply sloped
mid-level wave will move from WY northeast into the Arrowhead of
MN, and a swath of heavy wintry precip is likely.

Guidance has trended a bit south this aftn, and the preferred
blend included a heavy weight of the GFS/GEFS/ECENS/ECMWF due to
their consistency and agreement. This suggests there will be heavy
snowfall in the terrain of ID/MT southward into WY. With a more
broad band of snow from far northern SD into ND, eastward across
MN. Some of this snowfall could be very heavy as isentropic ascent
at 290-295K lays beneath an fgen band which is likely to intensify
through the RRQ of the upper jet streak, and some -EPV suggesting
potential CSI. The combination of these forcings should produce
snowfall rates which could well exceed 1"/hr at times. However,
the strongest ascent may overlap the p-type transition zone, and
forecast soundings indicate the potential for sleet, with some
freezing rain also possible. It is possible that some dynamic
cooling will overcome this subtle warm nose, especially in ND/SD,
with more robust WAA driving a better chance for mixed precip into
MN.

The heaviest snow from this event is likely in two areas. The
first is in the high terrain of the Northern Rockies, Absarokas,
and Big Horns, initially at 4000-5000 ft but falling to 1000-2000
as the event winds down. WPC probabilities are high for 8" across
these ranges, with amounts in excess of 12" possible. The second
maxima is more associated with the better mesoscale lift NW of the
surface wave into SD/ND where WPC probabilities for 6" exceed 40%
from south-central ND, along the ND/SD border, eastward towards
the Arrowhead of MN. Locally 12" is possible in a few locations.


...Washington Cascades...
Day 3...

A shortwave embedded within the broad cyclonic flow, and an
associated Pacific jet streak will dig down from British Columbia
Friday to spread moisture into Washington State. Ascent through
height falls and an associated diffluent LFQ will produce ascent
as mid-level RH briefly saturates invof a 700mb wave tracking into
Idaho. Snow levels are forecast to be generally 2000-3000 ft, so
precip should be rain in the lowlands. However, the synoptic
ascent combined with low-level upslope flow could produce heavy
snow, primarily in the Washington Cascades. WPC probabilities for
6" are greater than 50%.


For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss

$$
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FOUS11 KWBC 232012
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Fri Oct 23 2020

Valid 00Z Sat Oct 24 2020 - 00Z Tue Oct 27 2020


...Northern Rockies to the Central Plains...
Days 1-2...

As a well-defined system drops south from western Canada into the
western U.S., widespread snows, with moderate to heavy
accumulations, are expected from the Pacific Northwest to the
north-central U.S. over the coming days.

Heavy snow will become widespread across the Northern Rockies to
as far south as Wyoming as a potent shortwave digs through the
mean longwave trough across the CONUS. This shortwave will be
accompanied by a robust Pacific jet streak transporting anomalous
PW (edge of the AR) into the region, and the combination of the
ascent through height falls, PVA, diffluence, and low-level
upslope flow will produce heavy snow on day 1 /Saturday/. WPC
probabilities for 12 inches are high in the highest terrain of the
Northern Rockies, Absarokas, and Tetons.

Late D1 and into D2 /Sunday/ the swath of snow will likely
separate into two main areas. The first will be associated with
the surface low moving southeast through the Rockies beneath the
shortwave and the LFQ of the subtropical jet streak from the
Pacific. This will continue to provide robust ascent for heavy
precipitation, which will be snow in the higher terrain
throughout, and gradually become snow as the column cools at lower
elevations of the High Plains in MT/WY/CO. WPC probabilities are
high for 12 inches in the Tetons and CO Rockies on Sunday, with
lower probabilities for 12 inches along the Cheyenne Ridge and
Front Range. Lower accumulations, but still in excess of 4 inches,
are likely in the High Plains, with a relative minimum possible in
eastern WY where the split jet streaks may produce an overall
weakness in synoptic ascent.

Further to the east the leading jet streak will race off towards
the Great Lakes, leaving the favorable diffluent region for ascent
atop the northern and central Plains. At the same time, isentropic
ascent will increase at 300K to spread precipitation north and
east coincident with a band of fgen, possibly enhanced by the
aforementioned jet. While forcing for CSI does not appear present
for excessive snow rates, an exceptionally deep DGZ with a cold
and nearly isothermal profile suggests high SLRs with the
potential for large aggregate snowflakes despite temps well below
0C. This had led to an increase in snowfall amounts, especially
across SD and NE where the fgen band sinks slowly southward within
this favorable environment. WPC probabilities are high for 6"
across parts of SD and NE, with isolated amounts to 10" possible.
As this band sinks southward by D3, it should weaken, but will
also encounter a more marginal temperature profile so
probabilities for heavy snow accumulations wane rapidly across
Kansas.

...Southern Rockies into the Southern Plains...
Day 3...

The same upper trough will amplify while remaining positively
tilted late Sunday into Monday. Surface low pressure dropping
along the Rockies will continue to produce heavy snow from the San
Juans and Sangre De Cristos into the Wasatch of Utah, with lighter
amounts spreading eastward into the High Plains of NM. Moisture
will be slightly anomalous embedded within the Pacific jet streak,
noted by PWs that are 0.5 standard deviations above the climo
mean. WPC probabilities for 8 inches are high in the Sangre De
Cristos and San Juans Sunday night into Monday, with local amounts
to 20 inches possible. Lighter snows are likely spreading eastward
into the High Plains.

Additionally, as a surface cold front drops into Texas, persistent
SW flow aloft will produce an overrunning precipitation event from
north Texas through southeast Kansas. Guidance differs
considerably in the location and amounts of mixed precip during
this event, but there are increasing chances for some light
accretion of freezing rain. WPC probabilities are currently less
than 10% for 0.1", highest in central Oklahoma.

Weiss

$$
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FOUS11 KWBC 240837
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 AM EDT Sat Oct 24 2020

Valid 12Z Sat Oct 24 2020 - 12Z Tue Oct 27 2020

...Western and Central U.S. Winter Storm...

As an upper trough continues to amplify over the western U.S., a
significant winter storm will continue to move south along the
Rockies and extend east into the northern and central Plains over
the weekend.

As snows continue across the northern Rockies, a mid level low and
favorable upper jet forcing associated with an amplifying trough
over the northwestern U.S., will carry snows eastward into the
portions of the northern Plains on Saturday. Supported in part by right-entrance region upper jet dynamics, several inches are
possible across portions of southeastern North Dakota and western
to central South Dakota. Meanwhile to the west, favorable upper
forcing along with strong low-to-mid level frontogenesis will
contribute to heavy snows moving south through the northern
Rockies on Saturday, with accumulations of a foot or more likely
for portions of the western Wyoming ranges.

As the upper trough continues to amplify and settle south across
the western U.S., the threat for heavy snow is forecast to shift
into the central Rockies and High Plains on Sunday. Heavy
accumulations are likely across portions of the Colorado Rockies,
with WPC PWPF indicating high probabilities for accumulations of a
foot or more for the north-central Colorado ranges. Impactful
accumulations are also expected from the Front Range eastward into
the I-25 corridor, with WPC PWPF showing probabilities of 50
percent or higher for accumulations of 6-inches or more for this
area, including the Denver metro. While generally lighter amounts
are expected farther east, there is the potential for locally
heavy accumulations well out into the Plains, with WPC PWPF
showing high probabilities for accumulations of 4-inches or more
extending into central Nebraska.

By early Monday, expect the greater potential for heavy snow to
shift south into the southern Colorado and northern New Mexico
ranges. As an upper low begins to close off over the Southwest,
accumulating snows will become likely along the Mogollon Rim in
central Arizona.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 250826
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Sun Oct 25 2020

Valid 12Z Sun Oct 25 2020 - 12Z Wed Oct 28 2020

...Winter Storm impacting the western and central U.S....

An amplifying upper trough will continue to bring sharply colder
temperatures, along with moderate to heavy snows farther south
through the central Rockies and Plains on Sunday. A well-defined
shortwave currently centered over the northwestern U.S. will
continue to dig south into the Great Basin on Sunday. Increasing
lift and moisture ahead of the system, interacting with cold air
pushing south through the Plains and Rockies, will support
widespread snows across the central Rockies and Plains. While the
heaviest totals are expected across the higher elevations of the
Colorado Rockies, where WPC PWPF is indicating widespread amounts
of a foot or more, impactful accumulations are also expected
farther east, including along the I-25 corridor. Increasing low
level convergence/easterly flow, is expected to help support
heavier totals, with WPC PWPF continuing to show probabilities of
50 percent or greater for accumulations of 8-inches or more across
this region, including the Denver metro area.

On Monday, models show the upper wave continuing to drop farther
south, with a closed circulation beginning to develop near the
Four Corners region. By early Monday, drier conditions can be
expected across northern Colorado. However, snows are expected to
continue across southern Colorado while extending south into New
Mexico, with additional heavy accumulations likely across the San
Juan and Sangre de Cristo mountains. Meanwhile, snows will also
begin to develop farther southwest across northern and central
Arizona, with significant accumulations possible along the
Mogollon Rim and White Mountains.

By early Monday, southwesterly flow aloft, moving warm moist air
over shallow cold air sliding south, will set the stage for a
wintry mix, with accumulating ice becoming likely across portions
of Northwest Texas and western to central Oklahoma. Periods of
freezing rain are likely to continue through Tuesday into early
Wednesday across portions of western Texas and Oklahoma, while
spreading farther north into Kansas. While confidence in the
details is limited, latest WPC PWPF shows 50 percent of higher
probabilities for ice accumulations exceeding 0.10 inch from the
eastern Texas Panhandle through much of western Oklahoma.

Pereira

$$
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FOUS11 KWBC 260850
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
449 AM EDT Mon Oct 26 2020

Valid 12Z Mon Oct 26 2020 - 12Z Thu Oct 29 2020

...Southern Rockies/Plains...

As an upper low closes off over the Southwest, heavy snows are
expected to continue moving south into the southern Rockies, while
a wintry mix develops out over the Plains, with significant ice
accumulations possible across portions Texas and Oklahoma.

As a deep, positively-tilted upper trough continues to settle
south, models show an upper low closing off early in the period
near the Four Corners region. Strong lift and ample moisture,
interacting with very cold air spilling south, will support heavy
snows continuing Monday morning across the San Juan and Sangre de
Cristo mountains, before shifting farther south into the central
New Mexico ranges later in the day. WPC PWPF shows high
probabilities for additional snow accumulations of a foot or more
across portions of the San Juan, Sangre de Cristo, Jemez and
Sandia Manzano mountains. Meanwhile farther to the east, surface
to low level northeasterly flow and southwesterly flow aloft will
set the stage for a wintry mix from western and central Texas,
through western and central Oklahoma, and into eastern Kansas and
Missouri. Right-entrance region upper jet forcing, in addition to
low-to-mid level frontogenesis, are expected to support a period
of heavier precipitation across the region early Monday, raising
the potential for significant ice accumulations from the Texas
Hill Country northward to central Oklahoma. WPC guidance reflects
the overnight model trend, led by the hi-res guidance, toward a
colder solution across this region.

Following a period of scattered, light precipitation late Monday,
organized heavier precipitation is expected to return portions of
the southern Plains early Tuesday. As the upper low begins to
turn east, increasing lift and a surge of deeper moisture are
expected to once again raise the potential for higher
precipitation rates across portions of western and central Texas
into Oklahoma, with another round of significant ice accumulations
possible from the Texas Hill Country northward into central
Oklahoma. The heaviest storm total ice accumulations are expected
to center across southwest into central Oklahoma, where WPC PWPF
shows high probabilities for ice accumulations exceeding a
quarter-inch. Meanwhile within the deeper cold air, periods of
heavier snow are expected across the Texas Panhandle and eastern
New Mexico.

As the upper low moves east of the southern Rockies on Wednesday,
southerly winds ahead of a developing low level circulation will
end the potential for additional snow/ice accumulations across
central Texas and Oklahoma. However, cold air on the backside of
the system should be sufficient for additional light snow and ice
accumulations continuing through early Wednesday across the
Panhandle region and northeastern New Mexico, before lifting
farther north into eastern Colorado and western Kansas late in the
period.

Pereira


$$
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FOUS11 KWBC 271951
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Tue Oct 27 2020

Valid 00Z Wed Oct 28 2020 - 00Z Sat Oct 31 2020

...Southern Rockies and Plains...
Days 1-2...

A robust closed upper low over New Mexico will gradually begin to
fill and eject to the east tonight into Thursday. Ahead of this
feature, mid-level divergence combined with height falls and a
potent upper jet streak will drive deep layer synoptic ascent,
being enhanced by isentropic lift as mid-level WAA tops the cold
surface temperatures behind a cold front. This setup has already
produced significant snow and ice across the region, with more
likely through Wednesday aftn. The WAA aloft riding atop the cold
surface dome will continue a freezing rain threat through late
morning/early aftn Wednesday before increased mid-level divergence
and increasing low-level WAA erode the wet-bulbs to above
freezing. Persistent lift becoming more intense suggests rainfall
rates within the region of freezing rain will begin to intensify
towards morning, but with heavier rates and subtly warming wet
bulb temperatures, accretion efficiency is likely to wane. Still,
WPC probabilities indicate a moderate risk for an additional
0.25"+ of freezing rain, highest potential in western OK into the
eastern Panhandles of TX/OK.

Further to the NW, cold mid-level temperatures and intense
synoptic ascent will drive rounds of moderate to heavy snowfall
from New Mexico E/NE into the TX/OK Panhandle and far western
KS/eastern OK. The heaviest snow on D1 is likely across eastern NM
where the presence of the upper low will drive better instability
and colder temperatures aloft, while LFQ diffluence and more
robust height falls provide intense ascent. WPC probabilities are
moderate for an additional 6" of snowfall after 00Z this evening.
Further to the east, lighter snows are expected as the low begins
to fill and eject eastward, causing a reduction in both the
strength and temporal duration of forcing. WPC probabilities
indicate a low to moderate risk for an additional 4" of snow in
the Panhandles and spreading into southeast CO and far western KS.

The threat for winter weather is expected to finally come to an
end as the system begins to accelerate rapidly east by early
Thursday.

...Northeast...
Day 3...

The slowly filling upper low moving out of the Desert Southwest
will lift into the Mid-Atlantic and pass south of New England by
Saturday. This mid-level feature will combine with an intense jet
streak strengthening over southeast Canada (190kts, above the
climatological record) to drive lowering pressure, and a wave of
low pressure is likely to move south of New England Friday. Robust
WAA and isentropic lift ahead of this feature combined with strong
synoptic deep layer ascent will spread precipitation in the form
of rainfall late Thursday into Friday across the Mid-Atlantic and
into the Northeast. As the low moves offshore and begins to depart
early Friday, strong CAA from the north will combine with a band
of modest fgen and deformation to cool the column and begin to
chance the precip from rain to snow. While there should be a sharp
cutoff in precip across northern New England, rain may transition
to a period of moderate to heavy snow in the terrain of the
Adirondacks and ranges of northern/central VT/NH, before slowly
shifting southward until precip winds down and ends Friday aftn.
The heaviest snowfall is likely in the terrain of the Southern
Adirondacks, Catskills, Southern Greens, Berkshires, and into the
Worcester Hills and Monadnocks of SW NH where the best overlap of
precip with cooling temperatures exists. WPC probabilities in
these elevated areas are 30-50% for 4 inches. In the lower valleys
and into southeast Maine, some lighter accumulations are likely,
with some slushy accumulation possible approaching the coast of
Southern New England before the event winds down.

Weiss

$$
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FOUS11 KWBC 281926
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Wed Oct 28 2020

Valid 00Z Thu Oct 29 2020 - 00Z Sun Nov 01 2020

...Texas Panhandle...
Day 1...
Strong upper low which has been meandering across the Desert
Southwest will gradually shift eastward tonight into Thursday. As
this feature moves towards the Texas Panhandle overnight, intense
synoptic and mesoscale forcing will occur in the vicinity. Deep
layer ascent through the LFQ of a downstream jet streak combined
with mid-level divergence and height falls will be robust.
Additionally, a strong TROWAL is likely to wrap around the
mid-level low, focused across the Panhandle of TX, coincident with
a deformation axis and modest fgen. The atmospheric column will be
marginal in the low levels for snow, however, intense ascent aloft
should dynamically cool the column, and persistent moisture
wrapping through the TROWAL and within the WCB is likely to fall
as heavy snow across parts of the region. Snowfall rates could
reach 2"/hr as evidenced by the HREF mean hourly snowfall
probabilities, and a narrow corridor of heavy snow is likely
through Thursday morning. There is still some uncertainty as to
where the heaviest snow band will setup, as well as the temporal
duration of that band. However, confidence is increasing in a band
of more than 4" of snow, and WPC probabilities are moderate for
this amount in the TX Panhandle. Isolated amounts of 6-8" are
possible.

...Northeast...
Day 2...

As it begins to interact with the northern stream, the previously
noted upper low is forecast to transition to an open wave and
accelerate eastward from the southern Plains early Thursday to the
Mid-Atlantic coast by Friday morning. Moisture lifting to the
north, interacting with cold high pressure over Canada and an
associated low-to-mid level fgen zone, is expected to support a
stripe rain changing to snow across portions of northern
Pennsylvania, Upstate New York and southern to central New England
Thursday night into Friday morning. The general trend of the
models has been toward a more suppressed solution, with a more
rapid drying of the DGZ coincident with CAA during the p-type
transition to snow. This suggests a lesser potential for valley
snow accumulations, and the risk for impactful snowfall seems to
be more confined to the terrain/elevated regions. There is still
high confidence in some accumulating snow in the Catskills,
Berkshires, southern Greens and southern Whites, where WPC
probabilities for 4 inches are moderate. Elsewhere, some light
accumulations are possible at lower elevations and along the coast
of Maine.

The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.


Weiss

$$
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FOUS11 KWBC 290830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Thu Oct 29 2020

Valid 12Z Thu Oct 29 2020 - 12Z Sun Nov 01 2020

Day 1...
...New York/Vermont/New Hampshire...
An open mid level wave is forecast to accelerate northeastward
across the Appalachians today to the Mid-Atlantic coast and
coastal waters by Friday morning. Moisture lifting to the north
into an associated low-to-mid level frontal zone, is expected to
support a stripe of precipitation across upstate New York to
central New England Thursday night into Friday morning. Cold air
advects south across interior NY/Vermont/New Hampshire into the
area of precipitation tonight, causing the rain to change to snow.

There are model differences regarding precip amounts, timing and
length of precip after rain changes to snow.
The models continue to show a brief 3 or 4 hour period of snow
before drying aloft and sinking motion cause the snow to taper off
and end.

The risk for impactful snowfall seems to be more focused on the terrain/elevated regions. There is still high confidence in
accumulating snow in the northern Catskills of NY/southern
Adirondack Mountains, southern Greens in VT and southern
Whites/Monadnocks in NH, where WPC probabilities for 4 inches are
moderate.

Once the secondary wave of low pressure moves off the coast and
out to sea Friday, the snow should end.

Day 2...

...WA Cascades...

The models show a well defined cold front progressing onshore and
inland across western WA state.
Widespread precipitation amounts roughly averaging half an inch
liquid equivalent are forecast in the WA cascades.
It should be cold enough at higher elevations for the precip fall
as snow, with confidence that it would be cold enough at lower
elevations dropping at lower elevations. Rapid drying aloft the
latter half of Fri night as the 70 mb wave moves east out of WA
leads to the snow tapering and ending by Sat morning.

The probability of heavy snow is less than 10 percent Day 3.

The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.

Petersen


$$
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FOUS11 KWBC 310744
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Sat Oct 31 2020

Valid 12Z Sat Oct 31 2020 - 12Z Tue Nov 03 2020

...Great Lakes/Central Appalachians...
Days 1-3...

A shortwave and accompanying upper jet streak will drive a strong
cold front across the Lakes Saturday night through Sunday from
west to east. Behind this front, strong cold advection over still
warm lake waters will produce a period of favorable conditions for
Lake Effect Snow (LES). Favored belts south of Lake Superior into
the U.P. of MI will develop first on Saturday night, before
spreading into the L.P., and then downwind of Lakes Erie and
Ontario Sunday.

On Sunday, periods of moderate to heavy snow are likely across the
U.P. and northwest lower MI where N/NW winds will produce lake
effect snow bands. The heaviest snow is forecast to shift into the
eastern U.P. late as a modest deformation axis serves as a region
of secondary ascent. Later Sunday, snow will develop in northwest
lower MI as an effective fetch from Lake Superior moves across the
northern part of Lake Michigan to increase cross-lake fetch and
moisture fluxes, with ascent aided by upslope flow inland from
Lake MI in northwest lower MI.

On Sunday, LES will spread downwind of Lakes Erie and Ontario,
with a multiband setup likely. Very warm lake waters and 850mb
temps crashing below freezing to as low as -9C over Lake Erie will
produce significant instability as high as 1000 J/kg and lake
equilibrium levels above 700mb. Modest shear across these lakes on
NW winds will support multi-bands of snow showers.

As the mid level trough approaches the central Appalachians, the
low-mid level convergence and deformation combines with upslope
flow to produce showers, with rain quickly changing to snow. The
high amplitude trough of the NAM produces higher QPF, with better
clustering of the GFS FV3/03z SREF mean, and Canadian regional
GEM, which show potential for 4-8 inches of snow in the higher
elevations of central to northern WV. Snow should taper on Monday
as the upper trough departs and drier air aloft advects in from
the Ohio Valley.

On Sunday night into Monday, the upper trough continues east
across New York. The GFS shows trajectories crossing Lake Ontario
into the Tug Hill and western slopes of the Adirondacks, so
additional snow showers are expected with potential for several
more inches.

...Northern New York and Northern New England...
The trough moving across the Great Lakes Sunday will induce
cyclogenesis over eastern New England with the low moving north
across eastern Maine and into Canada Monday. Cold air being
advected south will cause precipitation to changeover from rain to
snow in the northern portions of VT, NH, and western Maine.
Accumulating snowfall is likely, enhanced by upslope flow on NW
winds into the terrain of the northern Adirondacks and northern
Green/White Mountains. Several inches are possible in windward
terrain. Downslope flow should result in lower accumulations in
the Champlain Valley of NY/VT.

The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.

Petersen

$$
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FOUS11 KWBC 010738
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Sun Nov 01 2020

Valid 12Z Sun Nov 01 2020 - 12Z Wed Nov 04 2020

...Great Lakes/Central Appalachians/New York...
Days 1-3...

A shortwave and accompanying upper jet streak will drive a strong
cold front across the lower Lakes and moving into New England
today. Behind this front, strong cold advection over still warm
lake waters will produce a period of favorable conditions for Lake
Effect Snow (LES). Favored belts south of Lake Superior in the
U.P. of MI will develop first, before spreading into northwest
lower MI, and then downwind of Lakes Erie and Ontario later this
afternoon and tonight.

The heaviest snow is forecast to shift into the eastern U.P. late
as a modest deformation axis serves as a region of secondary
ascent. Later today, snow will develop in northwest lower MI as an
effective fetch from Lake Superior moves across the northern part
of Lake Michigan to increase cross-lake fetch and moisture fluxes,
with ascent aided by upslope flow inland from Lake MI in northwest
lower MI. WPC probabilities are moderate to high for 4 inches.

LES will spread downwind of Lakes Erie and Ontario, with a
multiband setup likely. Very warm lake waters and 850mb temps
crashing below freezing to as low as -9C over Lake Erie will
produce significant instability. Modest shear across these lakes
on NW winds will support multi-bands of snow showers, although an
effective fetch from Lake Huron may cause an enhancement SE of
Lake Erie as additional moisture gets transported downwind. A
local max of more than 6" is probable here, and WPC probabilities
show a high risk for 4" inland from the Lake Erie shore in
northwest PA/adjacent southwest NY, and a moderate risk into the
Tug Hill east of Lake Ontario.

As the mid level trough approaches the central Appalachians, the
low-mid level convergence and deformation combines with upslope
flow to produce additional periods of snow in the favored terrain
of SW PA and along the Appalachians into WV. Although the snow may
be short-lived across this area, at times briefly heavy rates are
possible and WPC probabilities have increased to show a
low-moderate risk of 4".

On Monday night into Tuesday, a secondary but potent trailing
shortwave is progged to cross from the Great Lakes into the
northeast once again. An enhanced area of snow is likely east of
Lake Ontario where fetch/LES enhancement is likely, so several
inches is expected int he Tug Hill, with isolated maxima to a foot
o snow possible.
Moist northwest flow with 300 mb divergence maxima and 700 mb
vertical velocity maxima favors accumulating snow showers across
the western to northern Adirondacks into the northern Green
Mountains. WPC probabilities on day 2 are high for 4 inches in the
Adirondacks and into the northern Greens of VT.

The event winds down on day 3/Tue as low level ridging crosses the
lakes and drier air aloft advects into New York/New England.

...Northern New England...
Days 1-2...

The trough moving across the Great Lakes Sunday will induce
cyclogenesis over eastern New England with the low moving north
across eastern Maine and into Canada Monday. In addition to this
low, strong diffluence within a robust jet streak and rapid height
falls will likely lead to a broad precipitation shield, with some
enhancement and prolonging of precipitation likely in a mid level
deformation zone.

The precipitation may initially start as rain across much of NH/ME
on warm air advection, but will rapidly change over as the column
cools. WPC probabilities have increased and now show a moderate
chance for 6" in the terrain of VT/NH/ME, and upslope enhancement
could lead to higher totals.

Behind the departing synoptic system, moist cyclonic flow crossing
the mountains of northern VT/NH should bring periods of snow
showers Monday. The stronger 700 mb vertical velocity maxima in
northern VT leads to greater snow fall potential there.

The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.

Petersen



$$
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FOUS11 KWBC 012041
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Sun Nov 01 2020

Valid 00Z Mon Nov 02 2020 - 00Z Thu Nov 05 2020

...Great Lakes/Central Appalachians/New York...
Days 1/2...

A potent upper trough takes on a negative tilt over the eastern
Great Lakes this evening with rapid surface cyclogenesis in the
Gulf of Maine tonight that lifts north into the Gulf of St.
Lawrence Monday. Strong cold advection over rather warm Lake Erie
and Ontario waters (generally mid 50s to 60F lake temps) will
produce a period of rather favorable conditions for multi-band
Lake Effect Snow (LES) off these two lakes through tonight. Day 1
snow probabilities for 8 or more inches are in northwest PA and in
the Tug Hill area of NY.

As the upper trough approaches the central Appalachians this
evening, the low-mid level convergence and deformation combines
with upslope flow to produce additional periods of snow in the
favored terrain of SW PA and along the Appalachians into WV/MD.
Some low Day 1 probabilities for 4 or more inches are present in
the highest Allegheny Highlands.

A secondary but potent trailing shortwave is progged track
southeast from north of the Great Lakes Monday night, crossing New
England early Tuesday. An enhanced area of snow is likely east of
Lake Ontario where fetch/LES enhancement is likely, so several
inches is expected in the Tug Hill area, with moderate Day 1.5
snow probabilities for a foot in the highest part of the Tug Hill,
but given mesoscale processes a foot is likely where the largest
band sets up Monday night.

Moist northwest flow with 300 mb divergence maxima and 700 mb
vertical velocity maxima favors accumulating snow showers across
the western to northern Adirondacks into the northern Green
Mountains where there are low to moderate Day 2 snow probabilities
for 6 or more inches in the highest Adirondacks and Green Mtns.

The event winds down on Tuesday as low level ridging crosses the
lakes and drier air aloft advects into New York/New England.

...Northern Maine...
Day 1...

The upper trough moving east from the the Great Lakes Sunday will
induce cyclogenesis over eastern New England with the low moving
north across eastern Maine and into Canada Monday. In addition to
this low, strong diffluence within a robust jet streak and rapid
height falls will likely lead to a broad precipitation shield,
with some enhancement and prolonging of precipitation likely in a
mid level deformation zone. However, guidance has shifted farther
north with the precipitation shield, likely ending precip Monday
afternoon over northern Maine where before Monday afternoon was
looking like a prime time to have additional wrap around snow.

Therefore Day 1 snow probabilities are now low for 4 or more
inches over the northern White Mtns and across northern Maine.


The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.


Jackson



$$
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FOUS11 KWBC 020804
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Mon Nov 02 2020

Valid 12Z Mon Nov 02 2020 - 12Z Thu Nov 05 2020

...Great Lakes/New York/Vermont...
Day /2...

The initial strong cold advection over rather warm Lake Ontario
waters (generally mid 50s to 60F lake temps) is forecast to
produce additional snow showers in the Tug Hill area of NY. A
potent shortwave is progged track southeast from north of the
Great Lakes today, crossing New England tonight, and offshore
Tuesday. An enhanced area of snow is likely east of Lake Ontario
where fetch/LES enhancement is likely, so locally heavy snow is
expected in the Tug Hill area. Boundary layer west winds optimize
cross lake fetches late this afternoon and evening, which then
turn upslope into the Tug Hill and produce expected heavy snow.
Day 1 snow probabilities are moderate to high for a foot in the
Tug Hill.

Moist northwest flow with 300 mb divergence maxima and 700 mb
vertical velocity maxima favors accumulating snow showers across
the western to northern Adirondacks into the northern Green
Mountains where there are low to moderate Day 1 snow probabilities
for 6 or more inches in the highest Adirondacks and Green Mtns.
The progressing shortwave crossing interior New England tonight
brings burst of ascent to the White Mountains for a period
tonight before departing tot he coastal water Tue morning.

The event winds down on Tuesday as low level ridging crosses the
lakes and drier air aloft advects into New York/New England.

...Northern Maine...
Day 1...

The rapidly deepening low in Maine is forecast to move quickly
north into Canada today. Strong diffluence within a robust jet
streak and rapid height falls will likely lead to a broad
precipitation shield, with some enhancement of precipitation
likely in a mid level deformation zone in western Maine. However,
with the steady north movement of the low, the precip shield
should also move steadily north. The snow should end with the
precipitation shield this afternoon over northern Maine. Because
of the short duration of snow, Day 1 snow probabilities are low
for 4 or more inches over the northern Maine.

The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.

Petersen



$$
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FOUS11 KWBC 030808
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Tue Nov 03 2020

Valid 12Z Tue Nov 03 2020 - 12Z Fri Nov 06 2020

...Pacific Northwest/Northern Rockies...
The initial surge of precip with the atmospheric river on Days 1-2
will be associated with warm conditions. As a result, snow levels
are expected to remain high, confining significant snow
accumulations to the highest peaks of the northern Cascades
through late Wednesday.
A mid level shortwave and associated cold front progresses across
the northwest on Thu and into the northern Rockies Thu night.
Post-frontal lift with mid level deformation/upper divergence
maxima support snow becoming locally heavy in the WA/OR Cascades
and then into the ranges of northern ID and northwest MT. Snow
levels are expected to lower, expanding the threat for heavy
amounts across portions of the Cascades on Thursday and northern
Rockies Thu night into early Friday.

Much of the uncertainty revolves around how quickly cooling occurs
to support the change over from rain to snow. This favors higher
elevations with probabilities decreasing in valley areas due to
the limited duration of snow. A second factor is in the QPF
amounts, as the ECMWF/ECMWF Mean/UKMET forecast more QPF than the
NAM due to a stronger 700 mb low on Thu. More weighting was given
to the heavier solutions.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Petersen


$$
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FOUS11 KWBC 031945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
244 PM EST Tue Nov 03 2020

Valid 00Z Wed Nov 04 2020 - 00Z Sat Nov 07 2020

...Pacific Northwest to the Northern Rockies...
Progressive onshore flow with embedded mid-level energy and deep
moisture will continue to support periods of heavy precipitation
along the orographically favored regions of the Northwest through
midweek. However, snow levels are expected to remain high through
Wednesday -- confining the threat for heavy snowfall accumulations
to the highest peaks of the northern Cascades. Then on Thursday
as an amplifying shortwave begins to move onshore, snow levels
will begin to lower across the Northwest -- expanding the
potential for heavy snow accumulations across the northern
Cascades.

Overall consensus of the models shows an upper trough continuing
to amplify over the northwestern U.S., with an upper low closing
off over Oregon/Northern California on Friday. This will bring
the threat for high elevation heavy accumulations farther south
into the southern Cascades. Meanwhile, strong low-to-mid level
frontogenesis extending from western Montana to eastern Oregon,
along with falling snow levels, will help set the stage for
locally heavy snows beginning to develop from the northeastern
Oregon mountains to the ranges of the northwestern Montana.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 040825
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Wed Nov 04 2020

Valid 12Z Wed Nov 04 2020 - 12Z Sat Nov 07 2020

...Pacific Northwest/California/Northern Rockies...

Sustained upper level jet maxima and moisture fluxes and coupled
upper divergence/lower convergence maxima support periods of heavy precipitation along the orographically favored regions of the
Northwest through midweek. However, snow levels are expected to
remain high through Wednesday -- confining the threat for heavy
snowfall accumulations to the highest peaks of the northern
Cascades. Then on Thursday night and Friday as an amplifying
shortwave begins to move onshore, snow levels will begin to lower
across the Northwest -- expanding the potential for heavy snow
accumulations across the WA Cascades and in the front range of MT.

Overall consensus of the models shows an upper trough continuing
to amplify over the northwestern U.S., with an upper low closing
off over Oregon/Northern California on Friday and drifting inland
into the Great Basin Friday night. As the low passes, cold
advection aloft will lower snow levels and lead to rain changing
to snow. This will bring the threat for high elevation heavy snow accumulations into the OR/northern CA Cascades. Meanwhile, strong
low-to-mid level frontogenesis extending from western Montana to
eastern Oregon, along with falling snow levels, will help set the
stage for locally heavy snows beginning to develop from the
northeastern Oregon mountains to the ranges of northwestern
Montana, particularly the front range/Glacier National Park. The
primary uncertainty is duration of snow, and secondarily amounts
of QPF once precip becomes snow.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 052124
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 PM EST Thu Nov 05 2020

Valid 00Z Fri Nov 06 2020 - 00Z Mon Nov 09 2020


...Western U.S....
A strong winter storm is expected to impact portions of the
northern Rockies and High Plains this weekend, with heavy snows
and strong winds likely to develop across north-central to
northeastern Montana Saturday night, continuing through Sunday.

A closed low now developing off of the Northwest coast will
continue to deepen as it digs farther south along the coast into
Northern California late Friday and Saturday. While onshore flow
ahead of the system will support widespread precipitation from
western Washington into Northern California early in the period,
initially high snow levels will keep any heavy snow accumulations
confined largely to the high peaks of the central and southern
Cascades. Meanwhile, southerly flow into a lingering low-to-mid
level front will support precipitation east of the Cascades
through northeastern Oregon into northern Idaho and western
Montana. Here also, high snow levels will keep the heavy snow
threat confined largely to the higher peaks of the northwestern
Montana ranges.

Models show the low continuing to drop south through California on
Saturday, transitioning to an open wave as it pivots across
Southern California into the Southwest late in the day. As the
flow becomes more alongshore, the threat for heavier precipitation
is expected to wane, with relatively light amounts expected across
California into Nevada, including generally light snow
accumulations across the Sierra and the Nevada mountains Friday
night into Saturday. The Transverse and coastal ranges of
Southern California may be an exception, where onshore flow is
expected to bolster amounts, with heavy snow accumulations
possible over the higher peaks. As the system begins to pivot
east, directing deeper moisture across the Southwest into the
Great Basin and Rockies, snows will likely begin to develop
Saturday across the northern Arizona high terrain and the Utah
ranges. High snow levels at the onset are expected to keep
accumulations across these areas in check through late Saturday.
However, heavy snows are likely to develop early Saturday across
San Juan mountains in southwestern Colorado. With snows likely to
continue through the remainder of the weekend, accumulations of
two feet or more are likely across portions of San Juan mountains
before the end of the weekend.

As a second amplifying shortwave digs south into the Northwest
Saturday, the system over the Southwest is forecast to lift
northeast across the central Rockies into the northern High Plains
Saturday night. By early Sunday, models show a powerful surface
cyclone forming over the northern High Plains, with wind-driven
heavy snows developing on the backside of the system. Powerful,
easterly to northeasterly low level inflow, along with an upper
jet couplet are expected to support heavy snows spreading east
from mountains of northwestern Montana Saturday night and across
north-central into northeastern Montana on Sunday. WPC Day 3 PWPF
(00Z Sunday - 00Z Monday) shows 50 percent or greater
probabilities for accumulations of 8-inches or more covering a
large portion of north-central Montana.

For Days 1-3, the probability of significant icing is less than 10
percent.

Pereira


$$
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FOUS11 KWBC 062126
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 PM EST Fri Nov 06 2020

Valid 00Z Sat Nov 07 2020 - 00Z Tue Nov 10 2020

...A major winter storm is expected for much of Montana this
weekend with heavy snow and strong winds...

...Overview...
A cold core mid-level low currently stretched meridionally over
the CA/OR border will continue to shift south to central CA into
Saturday before opening and ejecting northeast across the Great
Basin through Saturday night. This feature closes off again over
eastern MT Sunday and slowly tracks northeast into the Canadian
Prairies into Monday. Meanwhile, a reinforcing shortwave currently
over southwest Yukon and riding a powerful jet streak draped
across southern AK will amplify as it digs over BC through tonight
and closing off as it shifts down the West Coast Saturday
afternoon through Sunday. Interaction with the low shifting
northeast from eastern MT Sunday night makes for a broad, deep and
positively tilted trough stretching from the northern Rockies and
across the Great Basin Sunday night. This feature then shifts east
onto the Great Plains Monday.

...Western U.S....
Days 1-3...

Elevated, tropically sourced moisture spreading into The West
ahead of the low moving into CA will allow brief heavy snow to
shift south down the Sierra Nevada (snow level around 4000ft)
tonight and southern CA ranges (including the San Gabriels and San
Bernadinos with snow level around 5000ft) Saturday. Snow levels
dip to around 5000ft as the system shift northeast across the
Southwest through the UT ranges including the Wasatch, the San
Juans and western ranges of CO, into WY through Saturday night.
WPC probabilities for snowfall greater than 4" on Day 1 and 1.5
are moderate to high across these areas as well as western NV.

The reinforcing low dropping from WA down the West Coast Saturday
night/Sunday producing the larger mid-level low will bring more
widespread and longer duration heavy snow to the Sierra Nevada,
Southern CA ranges, Mogollon Rim, and Wasatch and San Juans. Some
lighter snows may also accumulate down into the valleys as snow
levels fall up to 2000ft lower than the system tonight into
Saturday. Day 2 WPC snow probabilities for 6 inches or more are
across are high for the Sierra Nevada and higher ranges across NV
to southern UT before being moderate on Day 3 for southern UT, the
Mogollon Rim and White Mtns of AZ as well as high for the San
Juans in CO.

...Northern Rockies and Montana...
Days 1-3...

The combination of sharp height falls and intense upper divergence
will drive pressure falls, and surface cyclogenesis Saturday in WY
with only a slow shift northeast across the northern High Plains
through Sunday night. Strong advection ahead of this system will
transport anomalous moisture (PWs of +2 standard deviations) up
the Great Plains as Pacific and Gulf moisture lift northward. The
establish stationary front near the MT/WY border will maintain
cold air over MT with advective precip forming on the High Plains
late Saturday. Initially, the column outside of the terrain will
be too warm for snow, but intense forcing through synoptic and
mesoscale ascent, including a developing and intense TROWAL and
isallobaric acceleration into the deepening low will dynamically
cool the column to allow for a changeover from rain to snow.
Periods of freezing rain are also possible in eastern MT into
northwest ND Sunday with a snow/mix/rain line forming over eastern
MT. Day 2.5 ice probabilities for a tenth inch of ice are 10 to 20
percent over far northwest ND to far northeast MT.

However, the majority of the precipitation from the Northern
Rockies and points northeast will be snow, and this should be
extremely heavy at times. Strong ascent coupled with anomalous
moisture should produce snowfall rates that will exceed 2"/hr as
the TROWAL pivots overhead, and WPC probabilities have increased
further today. There are now high probabilities for 12 or more
inches across the eastern slopes of the MT Rockies and across
north-central MT on Day 2 and 2.5. Locally 20" is possible in a
few locations, and this event could challenge November record
snowfall across parts of Montana. Additionally, strong winds will
accompany this snowfall to produce significant blowing and
drifting.



Jackson


$$
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FOUS11 KWBC 070800
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Sat Nov 07 2020

Valid 12Z Sat Nov 07 2020 - 12Z Tue Nov 10 2020

...Northern Rockies and Montana...
Days 1-3...

...Major winter storm to impact Montana this weekend...

A mid-level impulse ejecting out of the Southwest will amplify as
it approaches the Northern Rockies late tonight into Sunday,
closing off over Montana before lifting into Manitoba by Monday.
Rapid height falls in advance of this feature combined with
intense mid-level divergence and upper diffluence within the LFQ
of a potent jet streak will lead to pressure falls and surface low
developing in WY Saturday. This low will then deepen as it advects
northeast into Montana/ND and eventually Canada by Monday.

Moisture being drawn northward ahead of this system will become
anomalous as noted by PWs of +2 standard deviations above the
climo mean and mean mixing ratios along the 300K isentropic
surface as high as 6g/kg. This moisture will be streamed from both
the Pacific and Gulf, and will be drawn northwestward within the
WCB and into a robust TROWAL developing on Sunday. Deep layer
ascent through the aforementioned synoptics will combine with
intense mesoscale forcing through upslope flow, isentropic
upglide, and strong deformation to cause heavy precipitation to
develop across the Northern Rockies and then gradually spread
eastward. Initially, precipitation outside of the terrain will
likely be rain as the column is too warm to support snow. However,
rapid dynamic cooling of the column, especially as the low
strengthens and isallobaric acceleration occurs from the north,
will cause a p-type transition to snow, with a brief period of
freezing rain possible in far NE MT and NW ND. Freezing rain
should only feature minimal accretion, and WPC probabilities for
0.1" are less than 10%. Snowfall rates are likely to be intense as
strong omega intersects a saturated DGZ. The latest HREF
probabilities suggest at least 1"/hr, but it is likely that 2"/hr
rates could develop on Sunday, beyond the current HREF forecast
time range. It is likely that most of the precipitation will fall
as snow, and with heavy rates continuing within the slowly
advecting system, accumulations are forecast to be significant.
WPC probabilities are greater than 70% for 12 inches across much
of the Northern Rockies eastward into the High Plains of MT,
including Great Falls and Glasgow. Accumulations in excess of 20"
are possible in the favored upslope terrain as well, or where any
banding lingers for a long duration, and this event could
challenge November record snowfall across parts of Montana. In
addition to this heavy snow, strong winds will likely produce
blizzard or near blizzard conditions at times.

...Western U.S....
Days 1-3...

An anomalous and large upper low will develop across the West
through Monday as two separate but potent upper lows moves across
the area. As the latter ejects into the Plains late Monday, yet
another shortwave will begin to approach the Pacific Northwest by
Tuesday morning.

For Days 1-2, the two potent upper lows will cause height
anomalies that reach as low as -3 standard deviations from the
climo mean. Each of these upper lows will be accompanied by
Pacific moisture within associated jet streaks, through which the
combination of moisture and ascent will produce widespread
moderate to heavy snow across much of the western terrain. The
anomalously low heights will produce falling snow levels all the
way to the Mexican border, so heavy snow accumulations are
possible as far south as the San Gabriels and San Bernadinos, as
well as the Mogollon Rim, and most other ranges of the Great
Basin. The heaviest snow through Sunday is likely in the San Juans
where orthogonal mid-level flow will produce upslope enhancement,
and WPC probabilities are high for 18", with more than 3 ft
possible through the first 2 days. Other ranges that will likely
receive significant snowfall include the Sierra and San
Gabriels/San Bernadinos of CA, the Uintas and southern Wasatch in
Utah, and parts of the Mogollon Rim. Here, WPC probabilities are
high for 8 inches, with local amounts greater than 12 inches
likely above 4000-5000 ft.

By D3, the complex upper low begins to fill and shift east cutting
off most of the forcing across the West, leaving lingering light
to moderate snow across the Four Corners. However, quickly behind
this exiting system yet a third shortwave will begin to dig into
the PacNW with an accompanying Pacific jet streak to provide
moisture. Snowfall Monday will increase across the Olympics and
Washington Cascades, where WPC probabilities feature a high risk
for 4 inches.

Weiss


$$
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FOUS11 KWBC 072059
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EST Sat Nov 07 2020

Valid 00Z Sun Nov 08 2020 - 00Z Wed Nov 11 2020

...Northern Rockies and Montana...
Days 1-3...

...Blizzard winter storm to impact Montana through Sunday night...

An opening low will eject northeast out of the Southwest and cross
the Northern Rockies late tonight into Sunday, closing off again
over east-central Montana which will cause it to slow before
lifting into Manitoba Sunday night. Height falls in advance of
this feature combined with intense mid/upper-level divergence and
upper diffluence within the left exit region of a jet streak will
enhance lee-side surface cyclogenesis over northern WY before
tracking across western ND Sunday and Manitoba Sunday night.

Gulf of Mexico and Pacific moisture being drawn northward in the
warm conveyor belt ahead of this system is anomalous with PWs of
+2 standard deviations above the climo mean and mean mixing ratios
along the 300K isentropic surface as high as 6g/kg. This moisture
will drawn into a rapidly developing TROWAL tonight through Sunday
over central MT which is further focused by a 1030mb surface high
shifting south along the Canadian Rockies. Deep layer ascent
through the aforementioned synoptics will combine with intense
mesoscale forcing through upslope flow, isentropic upglide, and
strong deformation to cause heavy precipitation to develop across
the Northern Rockies and then gradually spread northeastward.
Initial precipitation is snow in the northern Rockies and will be
rain on the High Plains of central MT this evening in a warm
column. However, rapid dynamic cooling of the column, especially
as the low strengthens and isallobaric acceleration occurs from
the north, will cause an abrupt transition to snow, with a swath
of mixed rain/sleet/freezing rain in a stripe northeast of the
surface low over northeast MT and northwest ND through Sunday.
Freezing rain should only feature minimal accretion with Day 1 WPC probabilities for 0.1" less than 10%.
Snowfall rates are likely to be intense as strong omega intersects
a saturated DGZ. HREF probabilities continue to depict high
probabilities for 1"/hr over a couple swaths of west-central MT
through tonight and north-central MT Sunday afternoon with low
probabilities for 3"/hr rates, so it is likely that 2"/hr rates
develop tonight and Sunday. It is likely that most of the
precipitation will fall as snow, and with heavy rates continuing
within the slowly advecting system, major snow accumulations are
forecast. Day 1/1.5 WPC probabilities are greater than 60% for 12
inches for much of west-central and north-central MT, including
Great Falls and Glasgow with isolated probabilities for 18" or
more over 40%. Accumulations in excess of 20" are possible in the
favored upslope terrain as well, or where any banding lingers for
a long duration, and this event could challenge November record
snowfall across parts of Montana. In addition to this heavy snow,
strong winds are expected to produce blizzard conditions at times.

...Western U.S....
Days 1-3...

A reinforcing trough over the Pacific NW will track south just
inland of the West Coast through Sunday when by then will have
formed an anomalous and large upper gyre that spans the Great
Basin and northern Rockies. This feature opens and shifts east
onto the Great Plains Monday with a northern Plains/Upper Midwest
low expected to develop Tuesday. The next shortwave reaches the
Pacific Northwest Monday night. This looks to be just the next
disturbance in an active pattern for the Northwest and central
CONUS that lasts through at least the next week.

Both of these upper lows will be accompanied by Pacific moisture
within associated jet streaks, through which the combination of
moisture and ascent will produce widespread moderate to heavy snow
across much of the western terrain through Sunday night. The
anomalously low heights will produce falling snow levels all the
way to the Mexican border, so heavy snow accumulations are
expected for the San Gabriels and San Bernadinos, as well as the
Mogollon Rim, and most higher ranges of the Great Basin.
Particularly heavy snow is expected in the San Juans tonight where
orthogonal mid-level flow, and instability will enhance
precipitation in many ways. There is a likelihood of 0.5"/hr
precipitation over the San Juans tonight which should lead to
particularly heavy convective snow. Day 1 WPC probabilities are
high for 18" there and the next round Sunday night into Monday
brings the risk of storm total snow of 48". Other ranges that will
likely receive significant snowfall include the Sierra Nevada and
San Gabriels/San Bernadinos of CA, the Uintas and southern Wasatch
in Utah, and parts of the Mogollon Rim. Here, Day 1/2 WPC
probabilities are moderate high for 8 or more inches, with local
amounts greater than 12 inches likely above 4000-5000 ft.

On Monday the complex upper low begins to fill and shift east with
ridging shifting into the West, cutting off most of the forcing
and leaving lingering light to moderate snow across the Four
Corners. However, quickly behind this exiting system the next
shortwave digs into the Pac NW begin to dig into the PacNW with an
accompanying Pacific jet streak to provide moisture. Snowfall
Monday night/Tuesday will increase across the Olympics and
Washington Cascades as well as the Blue Mtns of OR and northern ID
Rockies, where Day 3 WPC probabilities are moderate for 8 or more
inches.

...Central Plains to Upper Midwest...
Day 3...

The reinforcing low currently over the Pacific Northwest swings
northeast from the Four Corners Monday night, amplifying over the
southern High Plains and allows rapid surface cyclogenesis over
the Upper Midwest Tuesday. A swath of snow develops in the comma
head of this system late Monday night over western KS/NE and
shifts northeast to MN Tuesday. Differences in track are leading
to lower probabilities at this time (generally 20 to 30% for 2" on
Day 3), but given ample Gulf of Mexico moisture ahead of this
system and rapid cyclogenesis, mesoscale banding is likely to
produce locally heavy snow and a swath of mixed precipitation and
should be monitored closely.

Jackson

$$
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FOUS11 KWBC 080829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sun Nov 08 2020

Valid 12Z Sun Nov 08 2020 - 12Z Wed Nov 11 2020

...Northern Rockies and Montana...
Days 1-2...

...Blizzard expands across Montana today...

A mid-level shortwave which has ejected out of the Southwest will
amplify and close off across Montana today and then lift into
Manitoba Monday. Rapid height falls and PVA associated with this
feature combined with mid and upper level divergence will enhance
cyclogenesis across WY, with this low moving slowly to the
northeast through Monday.

Gulf of Mexico and Pacific moisture being drawn northward in the
warm conveyor belt ahead of this system is anomalous with PWs of
+2 standard deviations above the climo mean and mean mixing ratios
along the 300K isentropic surface as high as 6g/kg. This moisture
will drawn into a rapidly developing TROWAL over central MT which
is further focused by a 1030mb surface high shifting south along
the Canadian Rockies. Deep layer ascent through the aforementioned
synoptics will combine with intense mesoscale forcing through
upslope flow, isentropic upglide, and strong deformation to cause
heavy precipitation to spread across much of eastern and central
MT. Initial precipitation is snow in the northern Rockies and will
be rain on the High Plains of central MT. However, rapid dynamic
cooling of the column, especially as the low strengthens and
isallobaric acceleration occurs from the north between this low
and the aforementioned high, will cause an abrupt transition to
snow, with a swath of mixed rain/sleet/freezing rain in a stripe
northeast of the surface low over northeast MT and northwest ND.
Freezing rain should only feature minimal accretion with Day 1 WPC probabilities for 0.1" less than 10%.

Snowfall rates are likely to be intense as strong omega intersects
a saturated DGZ. The WPC snowband probabilities indicate a high
likelihood for 1-2"/hr snowfall rates through this evening,
leading to rapid accumulation across parts of the state. It is
likely that most of the precip will occur as snow, and WPC
probabilities continue to indicate a high likelihood of 12 inches
for much of north central and northeast MT, including Great Falls
and Glasgow. Within this region there are probabilities for 18
inches as high as 40%. Accumulations in excess of 20" are possible
in the favored upslope terrain, or where any banding lingers for a
long duration, and this event could challenge November record
snowfall across parts of Montana. In addition to this heavy snow,
strong winds are expected to produce blizzard conditions at times.

...Western U.S....
Days 1-3...

A reinforcing closed low over the Pacific NW will drop south from
Washington early Sunday to Arizona early Monday, when by then will
have formed an anomalous and large upper gyre that spans the Great
Basin and northern Rockies. This feature opens and shifts east
onto the Great Plains Monday with a northern Plains/Upper Midwest
low expected to develop Tuesday. A subsequent shortwave in advance
of a weak AR will approach the Pacific NW coast on Tuesday.

Both of these upper lows will be accompanied by Pacific moisture
within associated jet streaks, through which the combination of
moisture and ascent will produce widespread moderate to heavy snow
across much of the western terrain through Sunday night. The
anomalously low heights will produce falling snow levels all the
way to the Mexican border, so heavy snow accumulations are
expected for the San Gabriels and San Bernadinos, as well as the
Mogollon Rim, and most higher ranges of the Great Basin. The
heaviest snow is expected in the San Juans today and tonight where
orthogonal mid-level flow, and instability will enhance
precipitation, and the HREF probabilities show a high chance for
snowfall rates reaching 4"/hr there. Day 1 WPC probabilities are
high for 12" in the San Juans, with additional snow Day 2 bringing
event totals in excess of 3 feet. Other ranges that will likely
receive significant snowfall include the Sierra Nevada and San
Gabriels/San Bernadinos of CA, the Uintas and southern Wasatch in
Utah, and parts of the Mogollon Rim. Here, Day 1/2 WPC
probabilities are moderate high for 8 or more inches, with local
amounts greater than 12 inches likely above 4000-5000 ft.

On Monday the complex upper low begins to fill and shift east with
shortwave ridging shifting into the West. This will shunt the
forcing, at least briefly, leaving lingering light to moderate
snow across the Four Corners. However, quickly behind this exiting
system the next shortwave digs into the Pac NW with an
accompanying Pacific jet streak and weak AR to provide moisture.
Snowfall will spread across the Olympics Monday night and then
spread into the WA and OR Cascades, and eastward into the Northern
Rockies on Tuesday. WPC probabilities are moderate for 8 inches in
the higher terrain.

...Central Plains to Upper Midwest...
Day 3...

A cold front moving across the Plains will leave a sharp
baroclinic gradient into the Midwest Tuesday. Forcing aloft
including a potent shortwave and diffluence within the LFQ of a
potent jet streak will impinge upon this baroclinic zone to drive
surface cyclogenesis Tuesday morning, and this low will likely
deepen quickly as it shifts northeast into the Great Lakes by the
end of D3. There remains considerable spread in the track guidance
of this system, affecting the thermal structure of the column.
However, strong synoptic ascent suggests a quickly deepening low
will be slightly less progressive, and a blend of the ECMWF/GFS
was preferred for this forecast. This suggests some light freezing
rain is possible along the transition zone between rain and snow,
with a band of heavy snow possible to the NW. WPC probabilities
are low for 0.1" of freezing rain Tuesday, highest in northern IA
and into southern MN. NW of there, the potential exists for some
mesoscale banding as robust deformation/fgen overlap within a
developing comma head. WPC probabilities have increased for 4
inches of snowfall, with the great potential along Lake Superior
from NW Wisconsin into the Arrowhead of MN.

Weiss

$$
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FOUS11 KWBC 082034
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Sun Nov 08 2020

Valid 00Z Mon Nov 09 2020 - 00Z Thu Nov 12 2020

...Montana...
Day 1...

...Blizzard continues into tonight...

A 990mb surface low over northwest ND will lift into Manitoba this
evening as a TROWAL under the closed H5 low over northeastern MT
continues to bring moderate to locally heavy snow across
northeastern MT this afternoon. This setup, combined with the
1030mb surface high over northern BC/Alberta will keep snow
falling and the wind blow strong over north-central/northeastern
MT into tonight. By Monday morning, however, the elongated
mid-level trough will have shifted the focus for precipitation to
northern Ontario and the surface high building in will allow the
winds to ease, thus ending the blizzard. Day 1 WPC snow
probabilities for an additional 6 inches after 00Z are moderate
over portion of north-central MT.

...Four Corners....
Day 1...

A reinforcing closed low now over the northern Sierra Nevada will
swing east over the southern Great Basin tonight and eject east
across the south-central Rockies and onto the southern High Plains
Monday night. Pacific moisture wrapping in south/east of this low
and dropping snow levels will produce widespread moderate to heavy
snow across the higher terrain (snow levels generally 6000ft now
drop to 4000ft tonight) of the Four Corners states into Monday.
The heaviest snow is again expected in the San Juans of CO tonight
where orthogonal mid-level flow, and instability will enhance
precipitation where Day 1 WPC probabilities are high for 12"
bringing 36hr snow totals in excess of 3 feet. Elsewhere on Day 1,
6 or more inch probabilities are moderate in AZ (for the Kaibab
Plateau, Mogollon Rim, White Mtns), UT (southern and eastern UT
ranges and Wasatch), western CO ranges, and and NM part of the San
Juans.

...Pacific Northwest and Northern Rockies...
Days 2/3...

The next shortwave trough digs across WA Monday night and it and a
reinforcing trough following close behind and slowly shift east
across the northern Rockies into Wednesday. Decent Pacific
moisture ahead of the initial trough will make for the highest
snow probabilities in the WA Cascades on Day 2 where 6 or more
inches are likely with snow levels down around 2000ft. The
expanding trough over the Northwest CONUS shifts the snow focus
south and east with Day 3 snow probabilities for 6 or more inches
moderate for the OR Cascades, OR Blue Mtns, northern ID Mtns
(Clearwater and Bitterroots), Lewis Mtns in MT, and high for the
Tetons in WY.

...Central Plains to Upper Midwest...
Days 2/3...

The cold front from the low shifting north from ND this afternoon
will slow in its progression east on Monday as the reinforcing low
currently over northern CA/NV will eject east across the
south-central Rockies Monday night and allow surface low pressure
to ride northeast along the baroclinic zone from the central
Plains late Monday night to the Upper Midwest Tuesday. A fairly
narrow swath of locally moderate snow is expected in the comma
head of the developing low from western the western KS/NE border
Monday night to southern/central MN through Tuesday. Rapid
cyclogenesis Tuesday over WI will allow for the swath to become
more intense with additional enhancement from Lake Superior
Tuesday evening allowing heavy snow to develop over far northern
WI/the Arrowhead of MN before quickly exiting into western Ontario
Tuesday night. It should be noted that the GFS remains farther
east with its track over the Upper Midwest while the
ECMWF/CMC/UKMET are farther west/inclusive of Duluth and are
preferred at this time. Day 2.5 snow probabilities for 4 or more
inches increase up to 10 percent in south-central MN, but increase
to likely from the Duluth area to Grand Marais, MN.

Ahead of the developing low, a wintry mix transition zone is
expected to develop with low Day 2 probabilities for a tenth inch
of ice extending from southeast Neb across northwest IA and over
southern MN.


Jackson


$$
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FOUS11 KWBC 090746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Mon Nov 09 2020

Valid 12Z Mon Nov 09 2020 - 12Z Thu Nov 12 2020

...Four Corners....
Day 1...

Anomalously deep upper low will pivot across the Great Basin and
the Four Corners on Monday. Embedded within this larger gyre, a
shortwave will open briefly as it ejects from AZ northeast, before
sharpening once again as it moves into the Southern Plains. This
feature will be accompanied by a poleward arcing jet streak
placing the favorable LFQ for ascent atop the Four Corners,
providing a brief but intense period of ascent coincident with
some enhanced pacific moisture. This combination will produce
periods of precipitation across the region, with snow occurring at
elevations above 3000-4000 ft. Periods of snow could be heavy at
times, especially within the San Juans due to orthogonal mid-level
flow to enhance ascent, but may persist longer across the CO
Rockies and even into the Uintas of UT. So despite weaker snow
rates, total accumulations Monday could be about the same as in
the San Juans, and WPC probabilities are moderate for 6", with
isolated amounts to 10" possible. As the low pulls away late
Monday, the forcing for ascent ejects eastward bringing an end to
the snow.

...Pacific Northwest and Northern Rockies...
Days 1-3...

Yet another shortwave will dig into the Pacific Northwest tonight,
moving onshore WA state before elongating and rotating within the
larger cyclonic flow into the Northern Rockies by Wednesday night.
This shortwave will be accompanied by a weak Pacific jet streak to
transport moisture onshore, with confluent mid-level flow focusing
moisture from the WA/OR coast inland into MT and WY through
late-week. Periods of snow and snow showers are likely each day,
first in the Olympics and WA Cascades, and then spreading eastward
as far as the Tetons and Uintas by D3, with snow levels generally
1000-3000 ft, lowest near the Canadian border. The guidance
indicates periods periods of stronger omega which would enhance
snowfall at times, and 3-day totals could approach 2 ft in the
Cascades and Tetons. In general, however, WPC probabilities each
day are high for 6 inches, first in the Cascades, and then
spreading southeast through the forecast period.

...Central Plains to Upper Midwest...
Day 2...

A cold front will move quickly across the Central Plains and into
the Midwest today into Tuesday, leaving a sharp baroclinic zone
across the region. Atop this gradient, a potent shortwave will
lift northeast into the Midwest, while a secondary shortwave
closes off behind it to cause a slowing and interaction of the
two. At the same time, a jet streak downstream of the primary
trough axis to the west will intensify, placing a divergence max
in the LFQ atop the gradient as well, working in tandem to drive
surface cyclogenesis on Tuesday. While the guidance still features
considerable spread in timing and position, a blend that weighs
heavily towards the slower/stronger guidance was preferred today.
As this low develops, the column will initially be too warm for
frozen precipitation. However, as CAA continues behind the front
and dynamic cooling occurs through forcing and into the
strengthening low pressure, precipitation will expand and
changeover to snow, especially well NW of the surface low. The
fast movement of the system, antecedent warm temperatures, and
modest signal for banding suggests the heavy snow will be somewhat
limited. However, WPC probabilities do indicate a moderate chance
for 4 inches of snowfall, highest in the Arrowhead of MN.
Southeast of this snow, a transition area will occur and some low
level CAA could lead to light freezing rain accretion across parts
of IA and MN. WPC probabilities for 0.1" of accretion are
generally 10% or less.

Weiss


$$
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FOUS11 KWBC 101840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
140 PM EST Tue Nov 10 2020

Valid 00Z Wed Nov 11 2020 - 00Z Sat Nov 14 2020

...Upper Midwest...
As the associated upper level shortwave continues to lift
northeast -- assuming a negative-tilt as it moves from the central
Plains into the upper Mississippi valley -- ongoing precipitation
will move north across the Upper Midwest, with winter weather
impacts likely Tuesday evening into the overnight across much of
eastern Minnesota, northwestern Wisconsin and the western U.P. of
Michigan. In addition to ample upper support, guidance shows good
low-to-mid level frontogenesis helping to support banded
precipitation lifting north across the region Tuesday evening into
the overnight. WPC PWPF shows 50 percent or greater probabilities
for additional accumulations of 4-inches of snow or more during
the Day 1 period (00Z Wed - 00Z Thu), with 30 percent or greater
probabilities for 6-inches or more, across portions of
northeastern Minnesota and northwestern Wisconsin. In addition to
snow, a wintry transition zone is likely to produce at least
measurable freezing rain accumulations over portions of northern
Wisconsin into the U.P.

...Western U.S....
A well-defined shortwave currently dropping southeast into the
Pacific Northwest is expected to help produce locally heavy
mountain snows as it moves quickly from the Cascades to the
northern Rockies Tuesday night into Wednesday. Areas impacted
will likely include the Oregon Cascades, the mountains of
northeastern Oregon, as well as parts of the central and eastern
Idaho, western Montana and western Wyoming ranges. WPC PWPF
suggests widespread coverage of snow amounts of 4-inches or more,
with locally heavy amounts of 8-inches or more across these areas.
As this system moves east into the Plains, shortwave ridging will
support dry conditions across much of the Northwest into the
northern Rockies late Wednesday through Thursday. By late
Thursday however, wet weather is expected to return to the
Northwest with the approach of another shortwave trough. Models
show this system moving quickly east, however any break in
precipitation will likely be brief as the next system, positioned
on the leading edge of strong Pacific jet, approaches the
Northwest late Friday. This combination of systems is likely to
bring heavy amounts back into the Olympics and Cascades, with
accumulations of a foot or more possible during the Day 3 period
(00Z Fri - 00Z Sat) across the higher terrain.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 111921
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
220 PM EST Wed Nov 11 2020

Valid 00Z Thu Nov 12 2020 - 00Z Sun Nov 15 2020

...Western U.S....
A warm front associated with an approaching upper level shortwave
will bring organized heavier precipitation, including mountain
snows, back into the Olympics and northern Cascades by late
Thursday. While this system is expected to weaken and move
progressively downstream, it will be quickly followed a second
system. Models show an amplifying shortwave along the leading
edge of fast, Pacific flow moving into the Northwest by late
Friday, with another plume of deeper moisture spreading from the
Northwest into the northern Rockies. Areas impacted by
significant snow through late Friday are likely to include the
Olympics, Washington and Oregon Cascades, the Wallowa Mountains in
northeastern Oregon and the central Idaho ranges.

This second system is expected to move rapidly across the West as
well, with a well-defined cold front dropping south across the
Rockies and Great Basin on Saturday. However, upstream onshore
flow with embedded energy aloft, will support periods of high
elevation snow across the Northwest into the northern Rockies
through Saturday. Additional heavy snows are likely for portions
of the Cascades on Saturday, as well as the central Idaho ranges,
including the Sawtooth. Areas of heavy snow are also expected to
develop farther north across the northern Idaho and northwestern
Montana ranges, with favorable upper forcing helping to enhance
precpitation rates across the region Friday night into early
Saturday. Locally heavy accumulations are also possible across
the western Wyoming and northern Utah ranges as the aforementioned
front pushes through the region on Saturday.

WPC PWPF suggests many of the previously noted areas are likely to
see accumulations of a foot or more by late Saturday. The
Washington Cascades are expected to see the heaviest three day
totals, with WPC PWPF indicating widespread probabilities of 50
percent or higher for amounts of two feet or more.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 121919
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 PM EST Thu Nov 12 2020

Valid 00Z Fri Nov 13 2020 - 00Z Mon Nov 16 2020

...Western U.S....
An amplifying shortwave positioned along the leading edge of a
strong Pacific jet is expected to approach the Northwest coast
Thursday night. The combination of moist onshore flow and strong
upper forcing is expected to produce widespread precipitation,
including mountain snow, across western Washington and
northwestern Oregon beginning overnight, with heavy amounts across
the orographically favored terrain. As the upper trough moves
onshore, precipitation is likely to spread farther south into
northern California and east across the northern Intermountain
West into the northern Rockies on Friday into early Saturday.
Areas impacted by heavy snow through early Saturday are likely to
include the Olympics, Cascades, and the northeastern Oregon,
central and northern Idaho and northwestern Montana ranges. WPC
PWPF indicates the potential for at least localized amounts of a
foot or more across these areas, with several feet likely across
the higher terrain of the northern Cascades.

Snows will continue to push through the northern Rockies on
Saturday and south along the associated frontal band dropping
south through the central Rockies. This is expected to result in
locally heavy accumulations across the western Wyoming and
northern Utah ranges.

Even as the system moves east into the Plains by late Saturday,
progressive westerly flow, with embedded energy aloft will
continue to support periods of mountain snow from the Northwest to
the northern Rockies.

With periods of precipitation continuing, warm advection ahead of
a system moving inland is expected to result in rising snow levels
across the Northwest into the Rockies Saturday night into Sunday.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 131945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Fri Nov 13 2020

Valid 00Z Sat Nov 14 2020 - 00Z Tue Nov 17 2020

...Western U.S....
Satellite imagery shows a well-defined, amplifying, upper trough
currently moving into the Pacific Northwest. This system is
expected to move quickly across the western U.S., with models
showing a deep trough moving east of the Rockies by midday
Saturday. The upper forcing associated with this system, along
with with a strong cold front pushing south and east, are expected
to help support a period of heavy snow across portions of the
northern to central Rockies Friday evening and overnight. Areas
impacted are expected to include the western Wyoming and northern
Utah ranges, where WPC PWPF show high probabilities for localized
amounts of a foot or more Friday evening into Saturday.
Meanwhile, progressive onshore flow, with embedded energy aloft
will continue to support periods of mountain snow across the
Northwest, with additional heavy amounts likely across the
Cascades Friday night into early Saturday.

Showers are expected to continue across the Northwest on Saturday,
before a warm front associated with a rapidly deepening low
approaching British Columbia brings a return of more organized
precipitation back to the region. This will bring areas of heavy
snow back into portions of the Northwest and northern Rockies, but
with more limited impacts as warm air spreading across the region
pushes snow levels higher. This system is expected to quickly
weaken as it moves onshore on Sunday, leaving behind a weak front
over the Northwest. Fast zonal flow across the Northwest and
northern Rockies will continue to support shower activity along
and north of the boundary through Sunday. By early Monday, the
front is expected to surge north as a strong upper ridge begins to
build ahead of an amplifying upper trough and deepening low
offshore. With snow levels rising, any threat for heavy snow on
Monday will be largely confined to the higher peaks of the
northern Cascades.

...Northern New England...
The upper trough associated with the system currently moving into
the West will continue to move quickly across the U.S. through the
weekend, with a negatively-tilted upper trough moving across the
Midwest into the Northeast on Sunday. Cold air north of a
developing secondary low lifting across the Northeast, will
support a period of snow across northern New York and New England
late Sunday into early Monday, with some potential for impactful
accumulations across northern Maine.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 140840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Sat Nov 14 2020

Valid 12Z Sat Nov 14 2020 - 12Z Tue Nov 17 2020

...Western U.S....
Days 1-3...

GOES-16 mid-level water vapor imagery depicts a well-defined,
neutrally tilted, upper trough axis currently moving east from NV
into UT with an upper ridge just off the West Coast. The trough
will shift east of the Rockies today. Associated mountain snows
over CO/WY will continue this morning with snow levels currently
around 6000ft dropping to 4000ft as snow tapers off. Day 1 snow
probabilities for 6 or more inches after 12Z light to moderate
across the highest northern CO Rockies.

Meanwhile, the next shortwave trough will shift into central BC
tonight on strong zonal flow shifting elevated Pacific moisture
across the Northwest CONUS on the back side of the ridge this
afternoon through Monday when the next low/trough approaches the
Northwest. A frontal zone will provide a focus for precip with
high snow levels over western WA/OR tonight, southern OR/far
northern CA then through Sunday night before shifting north
through western WA Monday ahead of the approaching low/trough. Day
1/1.5 snow probabilities for 6 or more inches are high for the
WA/OR Cascades, Blue Mtns of northeast OR, central and northern
ID, and northwest WY. Day 3 snow probabilities for 6 or more
inches are moderate and limited to the WA Cascades.

...Northern Maine...
Day 2...

The upper trough associated with the system currently moving
across the West will continue to move quickly across the U.S.
through the weekend, with a negatively-tilted upper trough moving
across the Midwest/Great Lakes into Quebec Sunday night. Cold air
north of a developing secondary low lifting across the Northeast,
will support a period of snow across northern New York and New
England late Sunday into early Monday, with 30 percent Day 2
probabilities for 6 or more inches for far northern Maine.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 142018
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 PM EST Sat Nov 14 2020

Valid 00Z Sun Nov 15 2020 - 00Z Wed Nov 18 2020

...Western U.S....
Progressive onshore flow will direct another plume of deep
moisture across the Pacific Northwest, with a strong warm front
helping to organize heavy prepitation as it moves across the
region Saturday evening and overnight. Although snow levels will
begin to rise through the evening and overnight, cold air in place
at the onset is expected to support heavy snow across portions of
the Cascades, northeastern Oregon and central Idaho ranges. As
the system pushes farther east, it is expected to bring additional
heavy snows to portions of the eastern Idaho and western Wyoming
ranges Saturday night into Sunday. WPC PWPF indicates widespread
potential for 8-12 inches, with locally heavier snowfall
accumulations, across these areas during the Day 1 period (00Z Sun
- 00Z Mon).

Boundary lingering across the Northwest and northern Rockies, will
continue to support periods of snow into Monday before surging to
the north as an upper ridge begins to build ahead of an amplifying
trough moving across the eastern Pacific.

...Northeast/Great Lakes...
A deep upper trough currently centered over the Plains is expected
to continue to move quickly across the U.S., with models showing a negatively-tilted upper trough moving from the Great Lakes on
Sunday, into the Northeast Sunday night. Cold air on the backside
of the primary suraface low lifting across the upper Great Lakes
Saturday into Sunday morning is expected to support a changeover
to snow, with a few inches likely across the western U.P. of
Michigan.

As the associated surface front moves across the Northeast, a
secondary low surface low developing on the triple point is
expected to help hold enough cold air in place to maintain wintry
precpitation across northern New Hampshire and Maine, with
impactful snow accumulations possible Sunday night.

Deep cyclonic, westerly flow and cold air advection in the wake of
the system is expected to support developing lake effect snow
showers in the lee of lakes Erie and Ontario, with the potential
for locally heavy accumulations increasing Monday and Tuesday.

For Days 1-3, the probability of signficant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 150848
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Sun Nov 15 2020

Valid 12Z Sun Nov 15 2020 - 12Z Wed Nov 18 2020

...Great Lakes and Northeast...
Days 1-3...

A strong and becoming negatively tilted upper trough with a
rapidly developing surface low currently pushing into the Midwest
will push northeast across northern Michigan this afternoon. An
intensifying wrap around band of precip will continue to
experience dynamic cooling and should turn into a band of moderate
to locally heavy snow over eastern WI this morning and across
western and northern MI this afternoon. The orientation of the
band with motion of the system could cause multiple hours of
accumulating snow, particularly over the northern LP of MI. This
is a powerful storm with very strong westerly flow also adding
lake enhancement to western MI this afternoon. Day 1 snow
probabilities are low for 4 or more inches and contained in higher
elevations of the northern LP of MI from the WPC Super Ensemble,
but once the band switches to snow, elevation won't matter much
and accumulating snow is possible down to lake level.

As the associated surface front moves across the Northeast, a
secondary low surface low developing on the triple point is
expected to help hold enough cold air in place to maintain wintry
precipitation across northern New Hampshire and western/northern
Maine tonight with Day 1 snow probabilities for four or more
inches low for the Presidential Range in NH and far northern
Maine.

Deep cyclonic, westerly flow and cold air advection in the wake of
the system will support lake effect snow bands in the lee of lakes
Superior and Michigan particularly tonight and then lakes Erie and
Ontario late tonight through Tuesday with the low level wind
veering from westerly to northwesterly through that time slowly
shifting the snow zones southeast. Day 2 snow probabilities for
four or more inches are moderate in the Tug Hill east of Lake
Ontario and low over the Chautauqua and far northwest PA areas on
Day 3.

...Western U.S....
Days 1-3...

An strengthening zonal jet into WA/OR today will focus a stream of
enhanced precip along a baroclinic zone over OR into southern ID
into tonight with snow levels generally 5000 to 6000ft. This
moisture axis shifts north through WA late tonight through Monday
as a trough digs offshore causing the onshore flow to back to
southerly. This trough pushes into the WA/OR/northern CA coasts
Tuesday with broad onshore flow and snow levels decreasing from
around 7000ft to around 5000ft into Tuesday night. Day 1 snow
probabilities for 6 or more inches are high for the Sawtooth Mtns
of central ID, and the Tetons/Wind River Range of western WY as
well as the WA Cascades. There are moderate probabilities for 6 or
more inches in the northern WA Cascades on Day 2 then moderate for
the higher Cascades and northern Sierra Nevada on Day 3.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

$$
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FOUS11 KWBC 161929
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Mon Nov 16 2020

Valid 00Z Tue Nov 17 2020 - 00Z Fri Nov 20 2020

...Western U.S....
Days 1-3...

A modest Pacific jet streak of 110-130kts will move onshore the
Pacific Northwest beginning Tuesday night, with LFQ diffluence and
increasing moist advection driving precipitation across the West.
Much of the precipitation through D3 across the west should remain
north of this jet, from Northern California to Washington, and
stretching as far east as Wyoming. Embedded within this fast flow,
at least two shortwaves will move onshore driving modest cold
fronts into the West to enhance ascent and precipitation. The
first of these is likely Tuesday evening, with a secondary
shortwave/front moving onshore Wednesday night. Each of these
fronts will cause an enhancement in precipitation, with snow
levels falling in succession. Although heavy snow is limited D1,
by D2 /Tuesday night through Wednesday/ heavy snow is likely in
the mountains from the Olympics eastward to the Northern Rockies,
and southward as far as the Sierra Nevada, including all the
in-between terrain. WPC probabilities are high for 4 inches across
the terrain, with isolated amounts to 12 inches in the Sierra,
Sawtooth, and Cascades on D2. By D3, the first shortwave/front
decays and shifts east to be followed closed by the second. This
will bring another round of heavy snow to these same areas, with
high WPC probabilities for 4 inches persisting, and isolated
totals again eclipsing 12 inches possible in the WA and OR
Cascades. 3-day snowfall may reach 30 inches in the Cascades.

...Great Lakes...
Days 1-2...

A shortwave embedded within broad cyclonic flow across the eastern
CONUS will drive a cold front and associated wave of low pressure
across the Great Lakes overnight into Tuesday morning, bringing
rounds of lake effect snow (LES) to the region. There is likely to
be two waves of LES over the next 1.5 days. The first will be
associated with the more robust synoptic forcing associated with
the passing shortwave and cold front. The combination of height
falls and low-level convergence in a moistening column will drive
both modest synoptic snow showers and LES from the U.P. of MI into
New England. During this period of ascent, the EL atop the lakes
will be deepest, but instability through CAA will be modest and
total snowfall through Tuesday morning is likely to be light.

However, as CAA intensifies behind the front, LES should become
more robust and widespread despite lowering ELs as synoptic ascent
wanes. More intense low-level forcing off the lakes due to
increasing CAPE should produce favorable ascent into the DGZ for
periods of moderate LES. While the NW flow is not ideal for heavy
snow rates due to the limited fetch, guidance is producing at
least one more steady band thanks to an effective fetch from Lake
Huron across Lake Erie, and the heaviest snow through day 1.5 is
likely in the favored NW snow belts downwind of Lake Erie. WPC
probabilities for 4" are moderate in NW PA and SW NY, but less
than 10% elsewhere. While not accumulating much, there is
potential for isolated snow squalls behind the front across PA/NY
Tuesday as well, although low-level theta-e lapse rates around 0
suggest limited intensity of any squalls, and it may be more
snow-showers moving across the Mid-Atlantic and into New England
Tuesday night into Wednesday.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 171935
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 PM EST Tue Nov 17 2020

Valid 00Z Wed Nov 18 2020 - 00Z Sat Nov 21 2020

...Western U.S....
Days 1-3...

Persistent weakly cyclonic upper level flow with an embedded
130+kt jet will drive moisture into the west through the week.
Embedded within this flow, several shortwaves will rotate onshore
accompanied by modest surface waves and associated warm/cold
front. Each of these features will drive ascent to produce
mountain snow across much of the western ranges through Thursday,
before a sharper trough digs southward into the Great Basin D3
shunting forcing and moisture away from the region.

The first impulse will lift onshore the OR/WA coast early
Wednesday accompanied by robust diffluence within the LFQ of the
upper jet streak. This will spread snowfall across many of the
ranges from the Olympics of WA, southward through the Sierra
Nevada, and as far east as the Tetons of WY. Snow levels will
gradually fall D1 from 4000-7000 ft early, to 2000-5000 ft late,
lowest in WA. WPC probabilities for 4 inches are high across most
of these ranges, with the highest snowfall likely in the Sierra
and Sawtooth where locally in excess of 12 inches is possible.

A second impulse quickly follows the first on Thursday with
additional height falls and PVA aided by modest jet diffluence in
a moist column. Snow levels of 2000-5000 ft will continue to
spread eastward, allowing periods of moderate to heavy snow to
impact many of the same ranges as on D1. Additional snow
accumulations could produce 2-day totals in excess of 2 ft in the
Cascades and Sawtooth.

Additional moderate to heavy snow will continue in the Cascades on
D3, but in general the forcing will begin to shunt southeastward
towards the Great Basin as a deeper trough digs across CA Friday.
The overlap of moisture and synoptic ascent will weaken by D3, and
WPC probabilities for 4 inches are moderate in the WA Cascades and
eastward into the Northern Rockies.

...Great Lakes...
Day 1...

Continued CAA behind Tuesday's cold front will leave lingering
multi-bands of lake effect snow, primarily SE of Lake Erie and
Lake Ontario tonight. The environment will gradually become less
conducive for LES as ridging builds northeast from the Ohio
Valley, causing low-level winds to back to the W/SW while the DGZ
dries and EL's lower. Lingering LES could produce a few inches of
snowfall in NW PA and parts of W NY, with a potential heavier band
focused in NW PA thanks to an effective fetch tapping moisture
from Lake Superior and Huron. WPC probabilities for 4 inches are
moderate in a narrow corridor of Erie and Crawford counties in PA,
but otherwise are less than 10%. Any accumulating snow should end
by early Wednesday morning.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 180851
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Wed Nov 18 2020

Valid 12Z Wed Nov 18 2020 - 12Z Sat Nov 21 2020

Days 1-3...

...Western U.S....

No major changes to the previous thinking. A broad upper trough
moving over the western U.S. over the next few days will continue
to foster a weakly cyclonic upper-level flow pattern. Multiple
embedded shortwaves in this deeper layer flow and associated waves
of low pressure will help to maintain a fetch of mid-level Pacific
moisture across the Intermountain Region going through at least
Thursday. The combination of larger-scale forcing associated with
the shortwave energy and the moist, upslope flow into the west and
southwest facing slopes of the higher terrain should maintain a
threat for heavy accumulating snowfall as a result. By Friday, the
Great Basin will begin to dry out as high pressure builds over the
region, but there will be some shortwave energy/troughing
approaching the central Rockies, and also still a fetch of Pacific
moisture still impacting portions of western WA that will maintain
a threat of some wintry weather precipitation for the higher
terrain.

More specifically, the next shortwave impulse will advance onshore
the OR/WA coast early this morning accompanied by rather strong
upper-level divergence associated with the LFQ of the upper-level
jet-streak nosing into the region. This will spread snowfall
across many of the ranges from the Olympics of WA, southward
through the northern Sierra Nevada, and as far east as the Tetons
of WY. Snow levels will gradually fall going through today and
tonight, with snow levels falling to as low as 2000 to 3000 feet
farther north in the WA Cascades, and down to perhaps as low as
5000 to 6000 feet in the Sierra-Nevada. Snow levels will be a bit
higher farther east over the northern Rockies and generally in the
6000 to 7000 feet range. A second shortwave impulse will arrive on
Thursday with additional height falls and forcing associated with
this which will yield a second round of locally heavy accumulating
snowfall. The combination of these two shortwave impulse is
expected to foster additional snowfall totals going through early
Friday of as much as 1 to 2 feet, with the heaviest accumulations
over the WA Cascades and also locally over the southwest facing
slopes of the Sawtooth where some isolated additional amounts over
2 feet will be possible. Consequently, there are some locally
elevated WPC snowfall probabilities for exceeding 12 inches of
snow over these areas.

On Friday and through early Saturday, a moist Pacific fetch will
still be impacting portions of the WA Cascades which may yield an
additional 6 to 12 inches for this period here, but with higher
snow levels. Elsewhere, expect locally several inches of snow to
be possible over the central Rockies, and especially northern CO
as shortwave energy attempts to amplify toward this region.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Orrison

$$
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FOUS11 KWBC 191956
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Thu Nov 19 2020

Valid 00Z Fri Nov 20 2020 - 00Z Mon Nov 23 2020

...Pacific Northwest/Northern Rockies...
Day 1...

A weak shortwave embedded in confluent mid-level flow will advect
from the Pacific Northwest into the Northern Rockies on Friday
bringing a modest increase in mid-level RH as moisture spreads
inland from the Pacific. Periods of moderate to heavy snow are
likely in the higher terrain of the N-S oriented ranges which will
aid in some upslope enhancement. Modest snow accumulations are
likely through Friday, with WPC probabilities indicating a high
risk for 4 inches in the WA Cascades where locally 10"+ is
possible. Otherwise, a few inches are likely in the Olympics and
Northern Rockies.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss


$$
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FOUS11 KWBC 201943
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 PM EST Fri Nov 20 2020

Valid 00Z Sat Nov 21 2020 - 00Z Tue Nov 24 2020

...Pacific Northwest/Northern Rockies...
Day 3...

A sharp shortwave and accompanying potent 130+kt Pacific jet
streak will drop onshore Washington Sunday producing ascent and
heavy snow in the highest terrain of the WA and OR cascades as
well as the Olympics. The strongest LFQ ascent occurs late D3 so
additional snow is likely beyond this period, and heavy snow may
be ongoing Sunday night, especially in the WA Cascades. WPC
probabilities are high for 6 inches in the highest terrain of the
WA Cascades D3, with locally more than 12 inches possible.

...San Juan Mountains...
Day 3...

Mid-level shortwave will eject eastward into the Great Basin while
sharpening to produce height falls into the Four Corners region.
Atop this feature, a zonally oriented jet streak will push
eastward as well, extending into the Plains and strengthening to
drive in-situ RRQ diffluence to aid in ascent. These features
together will spawn weak cyclogenesis in the CO high plains, aided
lift which will be at times intense through 700mb southerly WAA
upsloping into the San Juans. Periods of intense omega with
increasing mid-level RH will produce rounds of snow, which will
likely be heavy at times. WPC probabilities indicate a high chance
for 6 inches, with locally more than 12 inches possible in the
highest terrain.

...Northeast...
Day 3...

An area of low pressure will develop along a cold front across the
Ohio Valley Sunday in response to a shortwave and RRQ jet dynamics
interacting with the low-level baroclinic zone. This low is likely
to lift rapidly northeast into New England by the end of D3
/Sunday night/ with robust ascent aided by WAA spreading
precipitation from Pennsylvania to Maine. Cold high pressure in
advance of this system will slowly retreat to the northeast, with
overrunning precipitation expected to help lock in the cold air,
at least initially, across the region. As the WAA intensifies, a
warm nose is likely, and a brief period of snow lifting northeast
will quickly transition to sleet and freezing rain from SW to NE.
The WAA should eventually overwhelm the column causing p-type to
change to rain except in the terrain where WPC probabilities for
0.1" of freezing rain are low to moderate in the Adirondacks.
Elsewhere, a light glaze of freezing rain is possible. Further
north, some heavier snow is likely where the WAA provides ascent
but the overall column is colder to prevent a warm nose exceeding
0C. WPC probabilities indicate a low chance for 4" of snow in the
White Mountains, with some heavier accumulations possible over far
northern Maine.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 210837
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Sat Nov 21 2020

Valid 12Z Sat Nov 21 2020 - 12Z Tue Nov 24 2020

...Pacific Northwest/Northern Rockies...
Shortwave ridging is expected to support dry conditions across the
region through Saturday before giving way to an upper trough and a
pair of frontal systems that will bring wet weather back into
western Washington and Oregon on Sunday. A weakening system
moving inland is expected to bring mainly light snow to the
Olympics and Cascades on Sunday. This will be quickly followed by
a second system moving inland Sunday night into Monday morning.
While both system are expected to produce relatively light
amounts, combined totals may reach several inches across the
higher elevations of the Olympics and northern Cascades.

...Central Rockies...
Southwesterly flow ahead of an amplifying trough moving across the
Great Basin/Southwest will support the increasing potential for
light to moderate precipitation, including high elevation snow
across western to central Colorado and far northern New Mexico on
Monday. Across the higher elevations of the San Juan Mountains,
amounts of a foot or more are possible by early Tuesday.

...Great Lakes/Northeast...
A warm front ahead of a deepening surface low lifting from the
Ohio Valley toward the lower Great Lakes, along with favorable
mid-to-upper level forcing will support precipitation moving
northeast across the region on Sunday. Cold air in place ahead of
the system will support a wintry mix, with minor ice accumulations
and/or a few inches of snow expected from southern Lower Michigan
and northern Ohio to northern New England, before strong warm
advection produces a changeover to rain across most areas.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 212025
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Sat Nov 21 2020

Valid 00Z Sun Nov 22 2020 - 00Z Wed Nov 25 2020

...Pacific Northwest/Northern Rockies...
Days 2-3...

A short wavelength flow will persist through early next week with
periods of shortwave ridging and troughing moving across the
Pacific Northwest. After ridging D1, a fast moving trough will dig
across the region Monday accompanied by brief but robust upper
diffluence in the LFQ of a 130kt Pacific jet streak. The
combination of height falls and this upper ventilation will allow
a wave of surface low pressure and associated cold front to move
from the Pacific across the Northwest and into the Northern
Rockies before dissipating. Ascent through these features will
cause moderate to heavy snow along the WA and OR Cascades,
spreading inland to ID and MT. WPC probabilities for 4 inches are
moderate, highest in the WA Cascades. After brief ridging again
late D2 into D3, yet another shortwave and accompanying jet streak
will lift onshore bringing additional moderate snow to the WA
Cascades on D3.

...Central Rockies...
Days 2-3...

Zonally oriented jet streak Monday will begin to intensify and
sharpen northward in response to a longwave trough amplifying and
digging through the Great Basin into the Four Corners. Embedded
weak mid-level impulses moving through the flow on Monday will
create period of moderate to heavy snow, especially in the San
Juans which will be more orthogonal to the flow to drive
significant upslope ascent. WPC probabilities on D2 are high for 8
inches in the San Juans, with locally more than 12" likely. By D3
/Tuesday/ the poleward arcing jet streak will produce a more
robust LFQ diffluent region which will interact with the
sharpening trough to drive low pressure development in the lee of
the Rockies. Ascent in the vicinity of this low will enhance
already robust deep synoptic lift to produce more widespread snow
across much of the CO Rockies. Amounts on Tuesday will be somewhat
muted compared to D2, but WPC probabilities are high for 4 inches
in the San Juans, with lower probabilities stretching into the CO
Rockies, Front Range, and Sangre De Cristos.

...Northeast...
Days 2-3...

A warm front ahead of a wave of low pressure lifting northeast
from Pennsylvania to Maine through Monday night spreading
precipitation across New York and New England. Initially, this
warm front and associated WAA will spawn an fgen band which will
cause a period of snow as it lifts northward. However,
accumulations are likely to be light. Strong WAA following this
front will quickly drive a warm nose northward to cause a p-type
changeover from snow to sleet to freezing rain, and then rain.
Cold high pressure to the north at onset may lock in at the
surface for a long enough duration that up to 0.1" of freezing
rain is possible, and WPC probabilities for this amount are
highest in the Adirondacks. Otherwise, expect the WAA to overwhelm
the column and cause primarily a rain event. Further north as the
low pulls into the Canadian Maritimes, stronger ascent and a
colder column should produce periods of light to moderate snow in
northern New England, especially Maine. WPC probabilities for 4
inches are as high as 20% across the terrain of these areas.

...Upper Midwest/Great Lakes...
Days 1-3...

On Sunday, a wave of low pressure will deepen as it lifts
northeast from Indiana into Pennsylvania. This low will deepen in
response to mid-level shortwave becoming negatively tilted beneath
a coupled jet structure. This deep layer ascent will allow the low
to intensify. As this occurs, a secondary shortwave moving across
MN will interact, causing a band of deformation to lay atop some
enhanced fgen on the NW side of the surface low. High-res guidance
this aftn has converged on a signal for a changeover from rain to
snow as dynamic cooling occurs through this intense mesoscale
ascent. While uncertainty continues into placement and amounts due
to the rain changing to snow scenario, WPC probabilities have
increased for accumulating snow, with a greater than 50% chance
for 2 inches, and a narrow window of 10% for 4 inches, highest in
SE Michigan.

After a break on D2, a larger scale system will develop across the
High Plains on Tuesday and lift northeast in response to the
poleward extending and intensifying jet streak downwind of a
longwave trough advecting eastward. A divergence maxima within the
LFQ will colocate with maximum height falls to intensify the
surface low as it lifts northeast into the Upper Midwest. Robust
WAA downstream of this system will spread moisture northward,
which will initially fall as snow. However, a LLJ of 40-50 kts
will drive strong fgen to cause a p-type changeover to rain across
most locations. However, strong WAA into the cold antecedent
column combined with the potential for some ageostrophic cooling
beneath the jet streak should allow for a burst of moderate to
heavy snow to accumulate several inches in WI and MN. WPC
probabilities for D3 are moderate for 4 inches in northern WI and
the Arrowhead of MN.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss

$$
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FOUS11 KWBC 222057
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 PM EST Sun Nov 22 2020

Valid 00Z Mon Nov 23 2020 - 00Z Thu Nov 26 2020

...Great Lakes/Northeast Day 1...

Low pressure tracking up the St Lawrence River tonight and into
Quebec should maintain a low-mid level warm advection pattern
across northern New York and northern New England. The primary
snow band is expected in Canada, with mixed precip occurring in
the NY Adirondacks to the VT Green mountains, NH White Mountains,
and ranges of western Maine.
Warm advection results in snow changing to sleet and freezing rain
before ending.
Multiple precip types limit amounts of any one type, so the threat
of heavy amounts is minimal.

The trailing cold front crosses the lower lakes, with a brief
period of cross lake flow within the cold advection pattern
allowing lake effect snow showers Monday morning in the lee of
Lakes Erie and Ontario. The setup is temporary, with the models
showing pockets of dry air aloft advecting across the region Mon
afternoon. This limits potential amounts.
Likewise a trough/secondary front in the upper lakes allows for
snow showers in the lee of Lake MI in northwest lower MI, bit the
approach of a low level ridge from the Upper MS Valley leads to
drying aloft and sinking motion, so the duration of snow should be
limited. Some light snow accumulations are likely.

...Upper MS Valley to the Great Lakes Days 2/3...
A low-to-mid level warm front associated with an approaching upper
trough will help generate widespread precipitation, with cold air
supporting snow across the upper Mississippi Valley into the upper
Great Lakes Monday night through Tuesday.
A surge of lift occurs with 850-700 mb theta-e advection across
the region, so snow may be moderate at the theta-e advection
peaks. The warm advection leads to a precip type change over
further south in SD/IA/southern MN/southern WI, limiting amounts
there. The NAM and Canadian regional GEM hold on the snow longer
than the GFS and ECMWF, so the duration of snow is causing
uncertainty.

Accumulating snows appear likely from eastern Minnesota to the
U.P. of Michigan, with a few inches most likely across portions of
northern Wisconsin into the U.P. of MI. The event winds down as
the continuing warm advection with the front crosses MI to the
Canadian border on Wed.

...Central Rockies/high Plains Days 1-2...
Deep southwesterly flow ahead of an approaching upper trough
combines with upper divergence maxima in the left exit region of a
jet over NM to support snow across the mountains of western and
central Colorado tonight into Monday before diminishing as the
upper trough moves east of the region on Tuesday. Areas impacted
will likely include the Elk and San Juan Mountains and Sawatch
Range. The long duration leads to high amounts of snow, with one
to two feet of snow in the San Juan Mountains.

On Day 2/Mon night-Tue, as the low level front produces cooling on
the central high Plains, a band of snow may occur within the band
of low-mid level frontogenesis/deformation in eastern CO. There
is potential for several inches, with the parent NAM higher than
the Nam Conus Nest, but the Canadian global/regional GEM/ECMWF/GFS
have increased amounts from last night. The event ends late Tue as
the 700 mb trough moves east of the area and ascent declines and
ends.

...Pacific Northwest/Northern Rockies Days 1-3...
An upper trough and a pair of weak frontal systems are expected to
produce periods of mainly light snow from the Olympics and
Cascades to the northern Rockies Sunday and Monday. A better
defined frontal system and upper jet maximum crosses the Olympics
and Cascades Tue, bringing potential for several inches to the
ranges. As the next 700 mb trough approaches on Wed, another round
of precipitation and a threat for heavier snow accumulations into
the Olympics and northern Cascades on Wednesday. As
heights/temperatures aloft fall, snow level decrease on
Wednesday. This allows for an increase in snow coverage/duration,
with the 09z SREF Mean indicating maxima near an inch liquid
equivalent in both the WA/OR Cascades, indicating potential for up
to a foot of snow for the 24 hour period ending 00z Thu.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 232109
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Mon Nov 23 2020

Valid 00Z Tue Nov 24 2020 - 00Z Fri Nov 27 2020

...Pacific Northwest/Northern Rockies...
Another wave and associated front will move into the Pacific
Northwest Tuesday afternoon and night. Snow levels will initially
be quite high, but will drop a bit post frontal. Onshore upslope
flow and additional embedded shortwaves moving ashore, will
continue the mountain snow threat through Wednesday night.
Generally looking at 1 to 2 feet of snow across the WA and OR
Cascades, generally at or above 4000 feet.

...Central Rockies/Plains...
As an amplifying upper trough moves across the Great Basin and
Southwest, areas of heavy snow are expected to develop and
continue through Tuesday across portions of the western and
central Colorado ranges. Currently not looking like a significant
event for the foothills or plains of eastern CO, with thermals
borderline and the area of low pressure remaining rather weak.
However some light accumulations are likely across this region
during the day Tuesday as colder air works in from the north. We
will likely have some areas exceed 2", and a lower probability we
could see a narrow swath of 4" of snow just south of Denver.

As the system ejects eastward across the Plains, dynamical cooling
within a developing deformation band may be sufficient enough to
produce a swath of wet accumulating snow across portions of
Nebraska and Kansas Tuesday and Tuesday night. Currently not
expecting anything too significant, but a narrow swath of 1-2" is
possible. This swath of accumulating snow will likely be quite
narrow, as given the marginal thermal profiles, it will take all
the dynamical cooling we can get to result in any accumulating
snowfall. Thus would only expect it to occur within the narrow
axis of any heavier precip that is able to develop. Again this is
not a sure bet given the marginal thermal profiles...however the
most likely solution at this time has a swath of around 1"...but
certainly a lower potential of 2"+ amounts within a narrow swath
from western KS into southern NE should precip intensity get high
enough on the northwest edge of the precip shield.

...Upper Mississippi Valley/Great Lakes...
Low to mid level WAA, combined with some upper level divergence,
will allow for an expanding area of light to moderate snow tonight
over portions of eastern IA, spreading east northeast through the
overnight and into the day Tuesday. The general trend among the
guidance is for a weaker system, and less of a northern and
southern stream connection. This is resulting in a more suppressed
system, with lighter snow amounts to the north, and an increase in
forecast snowfall further south. Current forecasts show a moderate
to high probability of 2"+ from northeast IA, into far southeast
MN, much of southern and central WI, and into north central MI.
Embedded within this area we may see a narrower swath of 4-5",
with the greatest threat from far southeast MN into south central
WI. Thermal profiles are marginal across this corridor as well,
with some model differences noted. WPC did blend in some of the
colder NAM and other 12z high res models...with the 12z GFS
appearing a bit too warm within the boundary layer resulting in
less snowfall than we think will happen.

...Northeast...
Light overrunning snowfall will spread across NY and central and
northern New England Tuesday night into Wednesday. Models have
trended a bit wetter with this system...indicating we should be
able to get a pretty widespread swath of 1" with this event...with
some of the favored terrain likely exceeding 2-3".

After a brief break, another round of precipitation should
overspread the region Wednesday night into Thursday. By this time
warmer temperatures will likely have moved in aloft, however the
colder air will try to hang on in the lower levels aided by some
CAD. Thus seems like a decent setup for some light freezing rain
across portions of central and northern New England. Given we are
three days out, uncertainty sill exists with this threat, and not
looking like anything too significant. However enough of a signal
in some of the higher res guidance to suggest a light icing
threat, so we blended in some of these colder models to generate a
few hundredths of icing in our day 3 forecast across this region.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Chenard

$$
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FOUS11 KWBC 242031
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Tue Nov 24 2020

Valid 00Z Wed Nov 25 2020 - 00Z Sat Nov 28 2020

Days 1 to 3...

...Pacific Northwest/Northern Rockies/Intermountain West...
An upper-level trough will be advancing inland across the Pacific
Northwest and the northern Rockies going through tonight and
Wednesday. The surge of Pacific moisture aided by the arrival of a
cold front, and the larger scale forcing with the upper trough
should yield moderate to heavy precipitation over the Washington
and Oregon Cascades. Snow levels will be lowering as the upper
trough and associated height falls cross the region, and this
should result in heavy accumulating snowfall for the higher
terrain. As much as 6 to 12 inches of snow is expected going
through Wednesday with locally heavier amounts over the favored
west-facing slopes. By Wednesday night and Thursday, the bulk of
the energy aloft and the influx of mid-level moisture should
fostering accumulating snow over the northern Rockies and adjacent
areas of the Intermountain West. This will include as much as 6
inches of snow for the Sawtooth, Tetons, and Wind River range with
even a few inches of snow possible farther south down into the
Wasatch. By Friday, the latest model guidance supports the idea of
an upper low digging down into the Four Corners region and this
should bring a threat for locally a few inches of snow to areas of
southern UT, southwest CO and northern NM.

...Central Plains/Midwest/Great Lakes/Northeast...
Initially there will be a rather weak, transient piece of energy
that will cross the Great Lakes region and the Northeast tonight
through Wednesday as a broad warm air advection pattern sets up
over the region ahead of a stronger upper-level trough and
embedded upper low ejecting east tonight across the central
Plains. The first surge of moisture and stronger isentropic
lift/ascent will favor an axis of light to moderate precipitation
with thermal rather marginal, but still sufficiently cold enough
for a few inches of wet snow to occur over northern Lower MI
tonight before warmer air arrives and allows for a changeover to
rain. On Wednesday, somewhat colder air will initially be in place
over northern New England, and locally a few inches of snow will
be possible here too with a focus on northern ME before this
impulse shears out.

With respect to the second stronger system back over the central
Plains, there will be the arrival and passage overnight of a
fairly robust mid-level deformation zone with impacts on areas of western/central KS and up across southeast NE going through early
Wednesday morning. Thermal profiles get to be very marginal and
somewhat conducive for a rain/snow mix or a changeover to wet snow
across these areas for a few hours as stronger dynamic cooling in
the deformation zone/comma-head region of the mid/upper-level low
arrives overhead. Meanwhile, cold air advection will be increasing
in the low-levels around the back side of departing surface low
pressure. It appears that there could at least locally be a few
hours of moderate snow that may foster a small snowfall
accumulation (1 to 2 inches) before the system exits Wednesday
morning across the Midwest. By Wednesday night and Thursday, the
energy will quickly move across the lower Great Lakes region and
into the Northeast. Strong warm-air advection ahead of the
advancing surface low will bring mainly rain along its path, but
as the system encounters shallow cold air across northern New
England Wednesday night and early Thursday, some light freezing
rain will be possible before changing over to all rain.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Orrison

$$
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FOUS11 KWBC 252039
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EST Wed Nov 25 2020

Valid 00Z Thu Nov 26 2020 - 00Z Sun Nov 29 2020

Days 1 to 3...

...Western U.S...

An upper-level trough moving through the Intermountain West
tonight and on Thanksgiving Day will bring lower snow levels and
sufficient energy/forcing in conjunction with a modest mid-level
Pacific moisture tap for some additional accumulating snow for the
higher terrain, and especially the northern Rockies with several
inches of new snow expected for the Sawtooth, Bitterroots, Tetons,
and the Wind River range. Farther south as energy begins to dig
southeastward across the interior, a few inches will be locally
possible across the Wasatch as well. On Friday and Saturday, the
latest model guidance agrees in developing an upper low which will
swing east over the Four Corners region and this will yield
locally a few inches of snow over the higher terrain of the
central and southern Rockies, and especially the San Juans of
southwest CO.

...Northern New England...

Energy associated with a relative compact, and robust mid-level
trough and associated closed low will quickly move across the
lower Great Lakes region and into the Northeast overnight and
through Thanksgiving Day. Strong warm-air advection ahead of the
advancing surface low will bring mainly rain along its path, but
as the system encounters persistent, yet shallow subfreezing
temperatures across northern New England, some light freezing rain
(with locally up to near a 0.10" of ice accretion) will be
expected before a changeover to rain occurs from south to north on
Thanksgiving Day. The heaviest ice accumulations will generally be
across areas of west-central to southwest ME where the combination
of cold air and somewhat heavier precipitation will be noted. All
of the precipitation should begin to taper off and come to an end
by tomorrow night.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Orrison

$$
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FOUS11 KWBC 260828
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Thu Nov 26 2020

Valid 12Z Thu Nov 26 2020 - 12Z Sun Nov 29 2020

Days 1 to 3...

...Southwestern U.S...

An upper-level trough moving through the Southwest today through
Friday has an embedded wave that may close off into a mid level
low over AZ that will then move east over New Mexico Sat and then
into the southern Plains.
Moisture circulating around the low into the ranges of northern AZ
to the ranges of New Mexico and adjacent southwest CO should
produce a few inches of snow each day. On Day 3, the precip in
most solutions starts as rain and changes over to snow in lower
elevations of eastern NM and the TX panhandle, so amounts will be
limited. The primary uncertainty is the timing of the low, with
good clustering among the GFS/Canadian global/ECMWF/UKMET
solutions, with spread provided by the faster Nam and slower SREF
Mean.

...Northern New England...

Precipitation has occurred i conjunction with low-mid level warm
advection over sub-freezing surface cold air
over interior NH and southern Maine. The warm front and low level
warm advection pattern should continue to move north today,
producing widespread light precipitation.

Some light freezing rain (with locally up to near a 0.10" of ice
accretion) will be expected before a changeover to rain occurs
from south to north today (Thanksgiving Day) across interior
sections of Maine. The primary uncertainty is the rate of warming
as cold air retreats, with the NAM nest typically showing the
longer duration of subfreezing cold air.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 262025
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Thu Nov 26 2020

Valid 00Z Fri Nov 27 2020 - 00Z Mon Nov 30 2020

Days 1 to 3...

...Southwest U.S...

For Friday and Saturday, the latest models are in very good
agreement in developing an upper low which will swing east over
the Four Corners region and then out into the southern High Plains
by Saturday night. The system overall will be rather
moisture-starved, but is expected to yield locally a few inches of
snow over the higher terrain of the central and southern Rockies,
with areas as far west as northern AZ seeing some light
accumulations.

...Pacific Northwest...

A shortwave trough will brush the Pacific Northwest Friday night
and Saturday and bring a weak cold front in across the WA
Cascades. Sufficient onshore flow is expected for locally several
inches of snow in especially the northern WA Cascades going
through early Saturday.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Orrison

$$
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FOUS11 KWBC 270837
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Fri Nov 27 2020

Valid 12Z Fri Nov 27 2020 - 12Z Mon Nov 30 2020

Days 1 to 3...

...Southwest U.S...

For Friday and early Saturday, the latest models are in very good
agreement in developing an upper low over Arizona that moves east
across New Mexico and then out into the southern High Plains by
Saturday night. The system overall will be rather
moisture-starved, but lift from the combined upper divergence/low
level convergence maxima near the low center and areas of upslope
flow is expected to yield a few pockets of 3-5 inches of snow over
the higher terrain of the mountains of central New Mexico and
adjacent east central Arizona. Amounts taper on the high Plains
with low QPF and mixed precip types and duration.

...Pacific Northwest...

A shortwave trough will brush the Pacific Northwest Friday night
and early Saturday and drive a weak cold front across the WA
Cascades. Sufficient onshore flow is expected to combine with
ascent focused as a jet maxima crosses Fri night to Sat morning,
supporting upper divergence maxima across the region. The
combination of jet-induced lift and orographics should result in
several inches of snow in the northern WA Cascades Friday night to
early Saturday.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 272109
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Fri Nov 27 2020

Valid 00Z Sat Nov 28 2020 - 00Z Tue Dec 01 2020

Days 1 to 3...

...NM/TX...
An upper low will bring light snowfall accumulations to portions
of NM and the TX Panhandle tonight into early Saturday. Generally
looking at an inch or two over portions of central and northern
NM, with localized 4" totals in some of the higher elevations. A
bit more uncertainty from far eastern NM into the TX Panhandle.
Most guidance suggests we should have enough cold air aloft
underneath the upper low for some light snow accumulations across
this corridor late tonight into Saturday morning. A swath of 1-2"
seems plausible near the NM/TX border, with a lower chance of a
narrow axis of 2-3" if rates get high enough underneath the upper
low.

...Pacific Northwest...
Snow will continue tonight across the higher elevations of the WA
Cascades, with another round of snow possible on Monday. This
system on Monday is a progressive but pretty dynamic system...with
a strong shortwave bringing the potential for a quick shot of
moderate to heavy snow to the higher elevations (mainly above 4000
feet). Some model spread with this system is noted...with the
latest ECMWF shifting the system south and missing the WA
Cascades. This is more of an outlier at this time, with the latest
snowfall probabilities depicting a 40-60% chance of 6"+ of snow
Monday.

...Ohio Valley...
An area of low pressure will track from the Gulf Coast northeast
into WV by 12z Monday. We should begin to see some phasing of the
southern and northern streams on Monday resulting in a deepening
of the low pressure system. This should support a deformation band
on the northwest side of the low, and an expanding area of lighter
snows/snow showers further west underneath the developing
mid/upper level low. Thus we should end up with some accumulating
snowfall across portions of the OH Valley by late Sunday night
into the day Monday.

Models are beginning to cluster on the low track and overall QPF
placement, however the snowfall accumulation remains uncertain.
Boundary layer temperatures will be marginal, and ground
temperatures warm. So will likely take decent snowfall intensity
to get accumulations. This will be possible within any deformation
band...and even underneath the upper low with temperatures cooling
enough to eventually get some light accumulations. Looking at the
last several runs of the ECMWF and GFS, along with the ECMWF
ensemble, suggests the highest probabilities for accumulating
snowfall through 00z Tuesday will be from far northern KY into
eastern IN and western OH...with moderate probabilities of 2"+
across this corridor. A broader area of coating to 1" is likely
within the more extensive light snow/snow showers underneath the
developing upper low...but any higher accumulations should end up
within any deformation axis on the west/northwest side of the low.
These dynamic systems can sometimes over perform with snowfall as
dynamic cooling takes over, so some risk of higher accumulations
certainly exists...but a general 1-4" seems most probable at this
time, through 00z Tuesday, over the aforementioned most favored
areas. Additional snow after this time will be possible over OH,
especially near Lake Erie.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Chenard

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FOUS11 KWBC 010833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Tue Dec 01 2020

Valid 12Z Tue Dec 01 2020 - 12Z Fri Dec 04 2020

...Great Lakes/Central to Southern Appalachians...
Days 1-2...

The models continue to signal a significant lake-effect/orographic
snow event in the lee of the eastern Great Lakes this period. A
low that is forecast to lift north across Lake Ontario today and
into adjacent Ontario and Quebec
allows a mid level deformation band to cross Lake Erie, with a
multi-lake trajectory across Georgian Bay and then Lake Erie
enhances available moisture. Lee shore convergence aids in
producing ascent with good model agreement in producing heavy snow
in northeast Ohio to northwest PA and southwest NY. Up to an
additional foot is possible in northwest PA near Erie PA.

As the low pulls further north late Tuesday into Wednesday, snows
are expected to develop east of Lake Ontario in the Tug Hill, with
moderate accumulations forecast. Several inches of snow are
expected before the low departs Wed.

Meanwhile, moist, cold, northwest flow will carry lake moisture
south and lead to periods of snow in the favored upslope regions
from western Pennsylvania to western Maryland and West Virginia,
with several inches possible along the favored upslope terrain
through early Wednesday. As the low lifts north, the strength of
upslope flow weakens in the southern Appalachians, and drier air
advects into the region. Consequently, snow should wind down in
terrain along the North Carolina/Tennessee border, with only a few
more inches today.

...Southern to South Central Plains...
Day 2...
The models show an elongated north-south 850-700 mb trough
extending down the plains on Wed. The models show a closed low
likely forming in the base of the trough in the southern Plains.
Lift occurring in the cold sector of the low in an 850-700 mb
deformation zone should bring snow to portions of south central to
southwest Kansas and western Oklahoma Wednesday into early
Thursday. Differences in the precip type in southeast KS and
eastern OK continue to limit forecast confidence with this system.

The 00z NAM was an outlier in producing the heaviest QPF and
resultant snow, which appear to be a result in an anomalous surge
in the 850-700 mb jet and resultant theta-e advection and
convergence. The NAM Conus Nest QPF was half the parent NAM,
which made it more in line with the magnitude of the
GFS/ECMWF/UKMET QPF. The NAM was given the least weighting as a
result. The ECMWF wraps heavier QPF west in OK than the GFS/UKMET
in spite of a similar 700 mb low track.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen



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FOUS11 KWBC 022047
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Wed Dec 02 2020

Valid 00Z Thu Dec 03 2020 - 00Z Sun Dec 06 2020

...Texas/Oklahoma Panhandle Region/Northwestern
Oklahoma/Southeastern Kansas...
Day 1...
Expect an ongoing deformation band to continue to reorient
east-west along the Kansas-Oklahoma border, with low-to-mid level
frontogenesis and favorable forcing aloft supporting a small area
of moderate to heavy snow through the evening hours. Several HREF
members indicate snowfall rates of 0.5-1 inch/hr persisting
through 06Z, raising the potential for additional heavy
accumulations across a small area over far southeastern Kansas,
the Oklahoma Pandhandle and the far northwestern Texas Panhandle
into northwestern Oklahoma. WPC PWPF is showing high
probabilities for additional accumulations of 4-inches or more
after 00Z. As the the associated low begins to move east,
snowfall is expected to diminish across the region during the
overnight hours.

...Northeast...
Day3...
The upper low impacting the Plains on Day 1 is expected to move
east into the lower Ohio and Tennessee valleys by late Friday.
Models continue to differ with the timing and degree of phasing
this system will have with a northern stream trough amplifying
over the Great Lakes at that time. Significant model spread and
poor run-to-run continuity limits confidence in any particular
solution. For the moment, the the GFS is a relative outlier with
its lack of phasing and suppressed surface low track across the
Mid-Atlantic states and then out to sea on Saturday. General
consensus of the remaining 12Z global deterministic runs and the
NAM is for a more phased solution, with a deepening surface low
tracking from the Mid-Atlantic to the New England coasts on
Saturday. While the NAM, ECMWF, UKMET and Canadian all agree on
bringing a low farther north than the GFS, timing and longitudinal
differences evolve, with significant impacts on QPF and snow
accumulations across the Northeast late in the period. Therefore,
while there appears to be at least some potential for impactful
snow, especially over the higher elevations of central to northern
New England, any details at this point are far from certain.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira

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FOUS11 KWBC 030818
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 AM EST Thu Dec 03 2020

Valid 12Z Thu Dec 03 2020 - 12Z Sun Dec 06 2020


...Northeast...
Day 3...

A significant winter storm is possible Saturday into Sunday, but
confidence is lower than normal. A southern stream impulse
ejecting out of the Plains this morning will open and lift
northeast while a northern stream shortwave digs out of central
Canada. The interaction of these two will likely lead to phasing
and a deep longwave trough, but the details of exactly where and
when this will occur are uncertain. The ECMWF continues to be
faster and further NW than the remaining guidance, while the
operational GFS and NAM are slow/east. For this iteration, the
UKMET and GEFS were used heavily as they provided a middle ground
with a reasonable solution.

As the energy lobes phase, a coupled upper jet structure will also
develop, and the collocation of these features will drive a
rapidly strengthening surface low lifting northeast from the
Southern Appalachians to near Cape Cod by the end of D3. This
track suggests the coast will remain liquid due to WAA and a
column above freezing, but heavy snow is possible well inland into
Northern New England and the terrain of Upstate New York and
Central New England. A track further SE would bring the snow
further south, while a track further NW would push rain
potentially into Canada. Using a blend of the UKMET/GEFS, there
has been an increase in snow amounts for northern VT/NH/ME, and
WPC probabilities now indicate a high risk for 4 inches in the
Northeast Kingdom of VT as well as the White Mountains of NH and
ME. A lower chance for 4" exists in the Adirondacks, Catskills,
and Berkshires, as well as much of the rest of Central New England
and northern Maine. There is potential for these amounts to be
significantly higher, but confidence in placement is too low to
introduce higher probabilities at this time.

Additionally, some light snow is likely in the terrain of WV,
where WPC probabilities suggest a less than 10% chance for 4
inches of snow accumulation as northerly flow behind the system
cools the column and provides some upslope enhancement.


For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Weiss

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FOUS11 KWBC 040830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Fri Dec 04 2020

Valid 12Z Fri Dec 04 2020 - 12Z Mon Dec 07 2020

...Northeast...

...Significant nor'easter to affect parts of New England Saturday
and Sunday...

All the ingredients are coming together for the first widespread
significant snow event of the season for New England. While
guidance still features above normal spread, consensus among the
preferred non NAM (and non hi-res outside the 00Z/4 HRRR) is for a
rapidly deepening cyclone to move off the Mid-Atlantic coast and
then lift just inside the benchmark (40N/70W) before slowing
through the Gulf of Maine on Sunday and eventually departing into
the Canadian Maritimes. NW of this system a swath of heavy snow is
becoming more likely, and many parts of New England are expected
to receive plowable snowfall this weekend.

The mechanisms driving this evolution are split flow with energy
in both the northern stream and southern stream phasing over the
Mid-Alantic producing a negatively tilting closed low over New
England by Sunday morning. In response to this amplifying trough,
downstream jet streaks will begin to couple, producing an intense
divergence maxima which drives rapid surface pressure drops. This
energy moving along a baroclinic zone supports a rapidly deepening
low pressure system, and precipitation spreading into the
northeast.

The antecedent column is marginal for snow, and initially
precipitation is likely to fall as rain, especially in southern
New England. As moisture spreads further northward it will advect
into a cooler column producing snow across central and northern
New England. The exception is likely to be eastern Maine where
onshore flow and a surface low track near the coast will produce
WAA and rainfall. However, as the low continues to intensify,
mesoscale forcing will become increasingly intense as a robust
TROWAL develops through the WCB wrapping NW around the surface
low, producing intense omega into the DGZ, and dynamic cooling
will occur. This should begin to transition most of the precip
from rain to snow, and there is likely to be an intense fgen band
collocated with the TROWAL just NW of the center. As the low
deepens and lifts northeast, this band should collapse
southeastward slowly, with the isallobaric acceleration into the
low potentially causing heavy snow all the way to the coast,
including the I-95 corridor from Boston to Portland. Recent WSE
plumes have indicated an increase in the means and 90th percentile
for eastern MA/NH/ME, with a decrease in NY and western New
England, and some enhancement in the terrain where orographics
will produce higher snowfall. WPC probabilities for 6" are highest
in northern NH and ME, where locally more than 15" is possible in
the higher terrain. Elsewhere across the terrain of upstate NY and
much of central New England, WPC probabilities are low to moderate
for 4 inches.

Although the heaviest snowfall may be across northern New England
due to a longer period of snow and less mixing, the heaviest snow
rates may occur within the strong fgen band, with rates
approaching 2"/hr possible as it pivots E/SE Saturday evening. A
secondary max of snowfall is possible from Worcester county, MA
eastward into Essex County, MA and towards Portland, ME where this
band may linger. Confidence at this time is low in the exact
placement of this band, but is high that it will develop.
Additional accumulations in excess of 4" are likely across this
area, potentially much higher, and this could include the Boston
to Portland metro areas. By Sunday afternoon the low will be
racing away into Canada, and only light snow will linger across
parts of eastern and northern Maine.

For Days 1-3, the probability of significant icing (0.25-inches or
greater) is less than 10 percent.

Weiss

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FOUS11 KWBC 050818
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 05 2020

Valid 12Z Sat Dec 05 2020 - 12Z Tue Dec 08 2020

...New England...
Days 1-2...

...Significant Nor'easter begins this afternoon and persists
through Sunday...

Guidance has clustered nicely this morning on a surface low track
moving inside the benchmark (40N/70W) this aftn, just east of Cape
Cod tonight, and through the Gulf of Maine towards Nova Scotia
Sunday. As this occurs, the low will rapidly deepen, potentially
"bombing out" by deepening 24mb in 24 hrs. While a track this
close to the coast will bring rain to eastern sections, it is
extremely likely that many parts of southern, central, and
northern New England will receive significant to major snowfall
this weekend.

The synoptics driving this development include phasing of southern
and northern stream energy in time to capture the low and swing it
northward up the Mid-Atlantic and New England coastlines. At the
same time, height falls and robust coupling of downstream jet
streaks will provide ample mid and upper diffluence to promote
rapid surface pressure falls. The antecedent airmass is marginally
warm, so without an arctic high pressure in place ahead of the
system, precipitation spreading northward this morning will
initially be all rain across SNE, with snow developing where its
colder across NNE later today. However, intense forcing will cause
rapid dynamic cooling of the column, and in addition to better
track consensus, the models have also begun to show a more rapid
transition from rain to snow later this morning.

There are likely to be two maxima of snowfall with this event. The
more certain region is across northern NH and much of central
northern ME where the entirety of the event will be snow.
Precipitation will begin as snow this aftn as mid-level WAA and
robust synoptic ascent cause an expansion of precipitation. As the
low pulls northeastward and deepens, this region will also
experience a pronounced TROWAL through the WCB wrapping
cyclonically around the low, reaching as far west as the Northeast
Kingdom of Vermont. Snowfall rates may reach 1"/hr at times late
Saturday into Sunday as 700mb omega maximizes during the
WAA/TROWAL overlap, but otherwise an extended period of moderate
snow is likely across this region. Long duration snow will
accumulate efficiently however, and WPC probabilities from the NE
Kingdom of Vermont across the White Mountains of NH and into much
of northern ME are high for 8 inches, with many spots likely
reaching 12". In the mountainous terrain where upslope enhancement
will also occur, isolated totals to 2 feet are possible.

The other area of heavy snow is becoming more focused as a strong frontogenetical band develops NW of the low across central/eastern
MA and into SE New Hampshire and SW Maine. There is very good
agreement among the high-res that this band will drop slowly
eastward as the low lifts northeast, potentially pivoting in place
across SW Maine into far NE Massachusetts as suggested by guidance
and the conceptual model for this event, and collocated with 700mb
omega maxima where WAA and the edge of the TROWAL overlap. Despite
marginal thermal profiles, intense dynamics are expected to
rapidly cool the column sufficiently for rain to change quickly to
snow even as surface temps remain near or even slightly above
freezing. The recent HREF mean and WPC snowband probabilities
indicate a high likelihood for 2"/hr snowfall rates within this
band. These rates are supported by intense omega of 30 ub/s into
the saturated DGZ coincident with a deepening near isothermal
layer in local soundings, and pockets of -EPV suggesting
thunder-snow and large aggregates which will accumulate rapidly.
Guidance has slowed the motion of this band as well, and a pocket
of very heavy snowfall is becoming more likely from Worcester
County, MA, to Essex County, MA northeast into Maine, potentially
impacting both the Boston and Portland metros. WPC probabilities
within this snowband are moderate to high for 8", with local
maxima potentially close to double this amount. There will be a
very sharp gradient in snowfall near the coast however, and any
deviation in the intensity of speed of this band could drastically
alter the snowfall in this area.

Elsewhere across New England, generally west of I-91 and south of
the CT/MA border, precipitation will be lighter and profiles
warmer. Light to moderate accumulations are likely in the southern
Greens and Berkshires where WPC probabilities are moderate for 4
inches. Otherwise, much of the precipitation will fall as rain,
with some light snow as the event winds down.

For Days 1-3, the probability of significant icing (0.25-inches or
greater) is less than 10 percent.

Weiss

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FOUS11 KWBC 060833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Sun Dec 06 2020

Valid 12Z Sun Dec 06 2020 - 12Z Wed Dec 09 2020

...New England...
Day 1...

Potent nor'easter will gradually pull into the Canadian Maritimes
through this evening. Continued warm/moist advection north of the
low will spread precipitation across much of Maine, with ascent
provided through a TROWAL, height falls, and a divergence maxima
associated with coupled jet streaks. Heavy snow will persist
across primarily northern Maine after 12Z today, and additional
significant accumulations are likely before snow winds down as
forcing ejects to the east and the column begins to dry. WPC
probabilities are high for an additional 4 inches across northern
Maine with local amounts of up to 8" possible in the highest
terrain, bringing local storm totals in excess of 12" across parts
of Maine.


...Pacific Northwest...
Day 3...

A Pacific jet streak will dive into the PacNW Tuesday accompanied
by a weak shortwave and modest mid-level confluence to drive
increased column moisture to the region. Ascent through weak
height falls and divergence along with a weak wave of low pressure
diving SE will spread moderate snowfall into the terrain of the WA
Cascades and Northern Rockies during D3. However, accumulations
greater than 4 inches are forecast only in the Cascades which has
a longer overlap of ample moisture and forcing. WPC probabilities
for 4 inches are around 30%.


For Days 1-3, the probability of significant icing (0.25-inches or
greater) is less than 10 percent.


Weiss

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FOUS11 KWBC 062035
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Sun Dec 06 2020

Valid 00Z Mon Dec 07 2020 - 00Z Thu Dec 10 2020

...New England...

Day 1...
The Nor'easter currently over New Brunswick will continue to lift
north this evening with ongoing light to possibly moderate wrap
around snow over northern Maine will taper off before midnight
tonight. Day 1 snow probabilities are low for an additional 2
inches this evening.

Day 2...
The next phasing system will develop as it tracks north-northeast
from off Cape Hatteras Monday afternoon to points east of Nova
Scotia Tuesday. However, strong northeasterly flow under the
northern stream upper trough should provide enough lift and
instability to allow some ocean effect snow bands to spread across
southeastern Mass late Monday night into Tuesday. Convective
allowing models are not too strong with these bands as of yet, but
it will be cold enough for accumulating snow so this is worth
monitoring.


...South-central Appalachians and Mid-Atlantic...

Day 1...
Phasing between northern and southern stream shortwave troughs
will occur tonight over the TN Valley with mid-level low pressure
closing off Monday morning over the upper OH Valley that tracks
east as a second, reinforcing, northern stream shortwave digs
across the Carolinas. This begins to develop into a potent system
off the NC coast later Monday, but before then some light to
moderate precip will develop as it spreads from the Appalachians
to the southern Mid-Atlantic Piedmont. A band or two of
accumulating snow is possible with the south-central VA Blue Ridge
to Richmond area the most likely target east of the Appalachians
for an inch or more of accumulation per the 12Z consensus. Upslope
flow allows for low Day 1 snow probabilities for two inches back
along the southern Appalachians.


...Pacific Northwest...
Days 2/3...

A Pacific jet streak looks to mainly remain north of WA Tuesday
with reduced risk of heavy snow in the Cascades and northern
Rockies with only low probabilities for 4 or more inches in the
Day 2.5 time frame. However, some light to locally moderate precip
spreads east of the northern Cascades Tuesday/Tuesday night which
given cold antecedent conditions in the northern Columbia Valley
may need to some thicker ice accretion. As of now the Day 2 ice
probabilities are around 10 percent for a quarter inch or more of
ice in north-central WA in the lee of the Cascades.


Jackson

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FOUS11 KWBC 072009
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Mon Dec 07 2020

Valid 00Z Tue Dec 08 2020 - 00Z Fri Dec 11 2020

...Pacific Northwest...
Days 1-3...

A zonal jet streak currently over BC/Alberta will continue to
allow Pacific moisture to stream into WA through tonight. While
snow levels are rather high (7 to 8 kft), lingering cold air in
the lee of the Cascades will only slowly retreat. Therefore, there
is a 20 percent chance for a tenth inch of freezing rain on Day 1
in north-central WA.

A shortwave approaches Tuesday night with snow levels decreasing
through Wednesday. Then the next shortwave approaches on Thursday.
Day 2/3 snow probabilities for 4 or more inches are 20 to 30
percent for the WA Cascades and ranges near Glacier NP.


...New York...
Days 2/3...

A shortwave trough on the leading edge of a jet streak rounding
the broad ridge over the western US pushes southeast across the
Northeast CONUS late Tuesday night into Wednesday. Some lake
enhanced snow may be moderate at times Tuesday night with some
lake effect snow Wednesday. Conditions are marginal thermally for
snow with most or all of the saturated layer below the dendritic
snow growth zone. Areas with greater lift, such as the Tug Hill
Plateau of NY do have a 30 percent chance of 4 or more inches on
Day 2.


...Southwest...
Day 3...

A cutoff low that shifted south off the southern CA coast today
will eject east Wednesday night, crossing AZ/NM on Thursday. ample
Pacific moisture is forecast and snow levels lowering to about
7500ft under the trough will allow some high elevation snows for
the Highest Mogollon Rim and White Mtns of AZ into NM. As of now
there are low probabilities for 4 or more inches for the highest
portions of the White Mtns on Day 3.


Jackson

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FOUS11 KWBC 082103
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 PM EST Tue Dec 08 2020

Valid 00Z Wed Dec 09 2020 - 00Z Sat Dec 12 2020

...Pacific Northwest to the Northern Rockies...

Days 1-3...

An upper level shortwave crosses from the eastern Pacific onshore
and inland into the northern Rockies tonight into early Wed.,
continuing on to the northern Plains Wed night-Thu. A jet max
cross the WA Cascades tonight and then inland through the northern
Rockies Wed, with divergence in the favorable right rear quadrant
for ascent as the jet crosses the terrain.
The shortwave will advect enhanced moisture across the northwest
and northern Rockies, which supports snow at higher elevations in
upslope areas. Snow levels will initially be high but gradually
lower with the height falls and cold front. The system is forecast
to be progressive so snowfall will be modest, with several inches
in portions of the northern WA Cascades and western MT Rockies.

As the moisture from the above forcing spreads eastward, it will
encounter a cold surface high stretched east of the WA Cascades
and into the Columbia Basin. This cold air will be slow to scour,
so an area of light freezing rain is likely across parts of
central WA on Tuesday.

The next shortwave approaches the WA/OR Coast Thursday night
bringing an enhanced moisture plume, but modest ascent. A weak mid
level wave progresses across the region, with modest lift
resulting in renewed chances for light snowfall to the Cascades of
WA and OR. WPC probabilities for Thu night-early Fri are low for 4
inches of new snowfall.


...Western to Northern New York...
Day 1...

A fast moving shortwave diving out of Ontario into Quebec will
drive a secondary front across Lake Ontario and western to
northern new York. Boundary layer flow will become westerly Wed
with the westerly flow turning upslope in the Tug Hill and western
Adirondacks, resulting in a swath of moderate snowfall. Several
inches of snow are expected, with a complimentary burst of
boundary layer vertical velocities forecast by the GFS centered on
18z Wed.
However, the flow starts to veer, weaken, and result in less lee
shore convergence Wed evening, and then drier air advection aloft
cuts down on snow showers Wed night, bringing the event to a
close.

...Southwest/Central High Plains...
Day 3...

A cutoff low will gradually cross the southwest Wed night, the
southern Rockies Thu, and the high Plains Thu night and central
Plains Friday. Low-mid level moisture advection and ascent ahead
of the approaching wave, supported by 300 mb divergence maxima,
lead to mountains snows starting in the higher elevations of AZ
and then increase in coverage/intensity crossing the ranges of
NM/southern CO. The highest amounts, locally around a foot of
snow, are expected in the highest terrain of the Sangre De
Cristos of northern NM/southern CO. Several inches are possible in
the White Mountains, and Mogollon Rim. On Fri, light snows extend
onto the central Plains as low pressure beings to develop,
resulting in a front increasing in strength. Amounts and
probabilities are low through Friday for the Plains.

Petersen

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FOUS11 KWBC 132126
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EST Sun Dec 13 2020

Valid 00Z Mon Dec 14 2020 - 00Z Thu Dec 17 2020


Day 1...
...Pacific Northwest/California to the Northern and Central
Rockies...
A deepening, fast-moving, upper trough is expected to bring
widespread snows, with the potential for locally heavy amounts
across the higher terrain as it moves from the West Coast to the
Rockies this period. Greatest potential for heavy amounts is
expected to center along the Sierra, where left-exit region upper
jet forcing is expected to accentuate lift Sunday evening into the
overnight. WPC PWPF shows high probabilities for additional
accumulations of 6-inches or more after 00Z, mainly for areas
above 5000 ft. Generally lighter amounts are expected elsewhere;
however, there is the potential for locally heavy amounts across
the higher terrain. Those areas include the northern Nevada and
southern Idaho mountains, as well as the Wasatch and Uintas.

...Central Appalachians into the Northern Mid Atlantic...
As a well-defined upper trough moves east across the lower
Mississippi into the Tennessee valley surface low pressure is
expected to deepen and track east of the southern Appalachians on
Monday. Strong upper jet forcing, along with low-to-mid level
frontogenesis is expected to support widespread moderate to heavy
precipitation west of the low, with the strong dynamics and low
level northerly flow supporting rain changing to snow across the
portions of the central Appalachians, with several inches possible
across the higher terrain of eastern West Virginia. While
widespread heavy amounts are not expected elsewhere, there is good
potential for light amounts, generally on the order of an inch or
two of accumulating snow, from the eastern West Virginia Panhandle
to southern New England.

Day 2...
...Pacific Northwest...
An weakening upper low is expected approach northern British
Columbia this period, will mid-level energy/an upper jet breaking
down a shortwave ridge to the south over the Pacific Northwest.
Increasing onshore flow along with favorable forcing aloft will
support the return of organized precipitation across western
Washington, with heavy snow accumulations possible across the
higher elevations of the Olympics and northern Cascades.

...Central to Southern Plains...
Upper trough traversing the West on Day 1 will continue to move
progressively east - moving east of the Rockies early Tuesday.
Models continue to show a swath of generally light snows
developing along an north-south, elongated trough/frontogenetical
axis extending from western Kansas into central Nebraska Tuesday
morning. By late Tuesday consensus of the models shows steadier
precipitation reorienting farther to the south - closer to the
center of a mid-to-upper level low moving along the
Kansas-Oklahoma border. Overall, expect accumulations to be
light, with WPC PWPF showing little potential for widespread
amounts of 4-inches or more.

Day 3...
...Southern and Central Appalachians/Mid-Atlantic...
A major winter storm is expected to develop on Wednesday,
impacting portions of region, with significant ice accumulations
and heavy snow. Precipitation will begin to develop across the
region by early Wednesday and become widespread by late in the day
as the previously noted trough over the Plains continues its
progressive track to the east. Models are showing a classic cold
air damming signature, with a cold surface ridge extending from
eastern Canada through the Mid Atlantic east of the mountains. A
developing coastal low will help to maintain the low level cold
air as it tracks from the Carolinas to southeastern Virginia.
While guidance differs on the details, there is overall agreement
on the potential for freezing rain, with significant ice
accumulations, developing across portions of the Mid Atlantic on
Wednesday. Latest WPC PWPF shows high probabilities for ice
accumulations of 0.10 inch or more occurring from the southern
Blue Ridge eastward into the North Carolina and southern Virginia
Piedmont. Heavy snow is more likely to develop farther to the
north, with several inches of accumulation likely by late
Wednesday from northwestern Virginia and the eastern West Virginia
Panhandle into south-central Pennsylvania. While the heaviest
amounts through late Wednesday will likely be well north and west
of the cities, accumulating snows are likely along the I-95
corridor from Washington to New York.

Pereira

$$
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FOUS11 KWBC 142129
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 PM EST Mon Dec 14 2020

Valid 00Z Tue Dec 15 2020 - 00Z Fri Dec 18 2020

...MAJOR NOR'EASTER/WINTER STORM TO IMPACT THE MID-ATLANTIC AND
SOUTHERN NEW ENGLAND STATES WEDNESDAY AND THURSDAY...

Day 1...

...Pacific Northwest/Northern Rockies...

An offshore Pacific trough and associated cold front will be
arriving across the Pacific Northwest and the northern Rockies on
Tuesday which will bring a surge of Pacific moisture a swath of
locally heavy snow across especially the WA Cascades where locally
over 1 foot of new snow will be possible. Somewhat lesser amounts
are expected for the Oregon Cascades and the Bitterroots.

...Central Rockies into the Central/Southern Plains...

Southern stream shortwave energy ejecting out of the Four Corners
region will amplify as it reaches the southern Plains on Tuesday
and will drive surface low pressure across western and central TX.
The energy over the higher terrain of the CO Rockies should yield
several inches of snow overnight, with low probabilities of
exceeding 4 inches. An increase in warm advection and moisture
transport back into the colder air focused over portions of
southern NE, western KS and northwest OK should allow for a swath
of light to moderate snow to break out across these areas with
locally a few inches of snow possible. Low probabilities of
exceeding 4 inches are denoted.

Days 2/3...

...Western U.S...

A new full-latitude trough and a fairly strong cold front will
arrive across the West Coast by late Wednesday and then traverse
the Intermountain West through Thursday. This will bring a strong
surge of Pacific moisture and strong forcing for heavy snowfall to
occur over the higher terrain of the Cascades, the central and
northern Sierra-Nevada, and also the northern Rockies including
the Sawtooth and Bitterroots. Locally 1 to 2 feet of new snow is
expected for these areas by late Thursday as the upper-level
trough and axis of lower snow levels advances inland.

...Appalachians/Mid-Atlantic to New York and Southern New
England...

The latest forecast continues to advertise a significant winter
storm for portions of the central Appalachians, the Piedmont area
of the Mid-Atlantic, and much of south-central/southeast NY and
southern New England, as a nor'easter develops and lifts up along
the East Coast Wednesday into Thursday. The wave that crosses the
southern Plains Tuesday will eject through the Mid-South Tuesday
night and is forecast to take on a negative tilt as it arrives
across the Appalachians on Wednesday. This will promote
cyclogenesis off the Southeast coast with low pressure then
lifting north and strengthening along the Mid-Atlantic coast and
then arriving offshore of southeast New England by early Thursday.
Meanwhile, a cold, 1036+mb high will be focused over Quebec and
will be nosing south down across the interior of the Northeast and
the Mid-Atlantic with a rather classic cold air-damming event
expected to be in place as the low center moves up the coast.

As strong warm-air advection/isentropic ascent occurs ahead of the
upstream trough along with increasingly moist Atlantic inflow over
the cold air, a large swath of moderate to heavy snow is expected
to develop for portions of the central Appalachians and the
adjacent Piedmont areas of WV/VA/MD and PA on Wednesday with the
focus west of the I-95 urban corridor, and the bigger cities of
Washington D.C., Baltimore and Philadelphia. By Wednesday night
and early Thursday the focus of heavy snow will overspread areas
of southeast NY and southern New England including the New York
City and Boston metropolitan areas. The latest model guidance
favors a rather strong comma-head/deformation zone impacting
especially areas of eastern PA, southeast NY, northern NJ and
southeast New England on Thursday as the low center further
deepens and then begins to edge east-northeast and farther
offshore. Meanwhile, down across areas of northwest SC, western
NC, southwest VA, and southern WV, the cold air will be much more
shallow by comparison, and it appears likely that there will be a
period of freezing rain and locally some sleet as the event ensues
on Wednesday. Some of this may sneak up near or just west of I-95
as well from Washington D.C. to Philadelphia. But the threat of
ice farther north closer to New York City and certainly Boston
will be notably lower.

WPC favored a a multi-model blend approach for the precipitation
forecast with a tad more weighting to the ECMWF, and lesser
weighting to the NAM and GFS. The NAM is farther west and warmer
with the evolution of this system, with the GFS a bit colder and
farther east. While not totally dismissing the NAM and GFS, a
model consensus somewhat closer to the ECMWF and ECENS mean was
favored for QPF and was applied to the thermal profiles.

Based on the latest forecast, the expectation is for storm total
snowfall to be as much as 12 to 18 inches for the interior of
especially the northern Mid-Atlantic, far southeast NY and
southern New England. Where freezing rain occurs, the greatest ice
accretions are expected over northwest NC, southwest to central VA
and over parts of southern WV where locally as much as a tenth to
quarter inch of ice will be possible.

Orrison

$$
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FOUS11 KWBC 160916
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EST Wed Dec 16 2020

Valid 12Z Wed Dec 16 2020 - 12Z Sat Dec 19 2020

...MAJOR WINTER STORM TO IMPACT THE MID-ATLANTIC AND NORTHEAST
INTO THURSDAY...

...Appalachians and Mid-Atlantic and across the Northeast...
(Days 1-2; 12Z Wednesday to 12Z Friday)

A major winter storm develops today and continues into Thursday
for a large swath from the south-central Appalachians, interior
central Mid-Atlantic and nearly all of the northern Mid-Atlantic
and through south-central New England. This storm is expected to
deliver more snow than was observed all of last season (Winter
2019-2020) from northwest Virginia, the eastern West Virginia
Panhandle, central and eastern Pennsylvania, far northern New
Jersey, and at least portions of the New York City metro area.

A southern stream trough shifting east into the Mid-South early
this morning will intensify as it takes on a negative tilt and
swings increasingly to the northeast across the Tennessee Valley
today and the Mid-Atlantic tonight before crossing southern New
England Thursday morning. A weak leading impulse is allowing
surface cyclogenesis off the Georgia coast now which will become
the dominant surface low later today as the mid-level wave
connects with it this afternoon. Energy translates to this
developing coastal low that will track north along the
Mid-Atlantic coast tonight. Another important component to this is
the 1037mb surface high over south-central Quebec that is
supplying ample dry/cold air to the entire Northeast with an
associated surface ridge extending the lee side of the
Appalachians into Georgia. Leading precipitation overrunning this
cold air damming wedge of a surface ridge will start as a mix of
snow and freezing rain with the freezing rain occurs near the Blue
Ridge of NC up through at least central VA this morning (as of 08Z
KGSO in NC is 32F with light freezing rain). As the low
intensifies as it tracks north from Cape Hatteras this afternoon
the baroclinic zone inland over the central Mid-Atlantic
intensifies and low level frontogenesis allows heavier
precipitation and a tighter rain/mix/snow line. Heavy snow is
expected to begin on the cold side in the Potomac Highlands and
northern Shenandoah Valley this afternoon. Of notable importance
is the development of a 700mb low over northern Virginia early
this evening which is an indication of the strength of the system
and forms a good signal for where the heaviest snow will begin and
will continue along and just west of the 700mb low track.

Thermal preferences with this system a blend of the 00Z ECMWF/GFS
which have a similar eastern boundary to snowfall (west/north of
DC then along the I-95 corridor north from Baltimore and most/all
of Long Island and southern New England.

As noted by the main snow line, strong low-mid level warm air
advection ahead of the advancing trough/low will cause any wintry
precip to change to rain over much of the Washington D.C. metro
and east-northeast across southern New Jersey. Additionally, a dry
slot wrapping around the lower mid-level low will track up this
rainy sector tonight. Given a low track off/along the Delmarva
coast tonight, cold air could be retained essentially above the
fall line in the central Mid-Atlantic with accumulating sleet
tracking from northern Virginia this afternoon to north-central
Maryland and southeast Pennsylvania and the NYC metro area
overnight.

There continues to be a slight northward trend in the axis of the
heaviest precip with the 00Z ECMWF a little farther north in
Pennsylvania than the 00Z GFS. The greatest swath of snow is from
south-central to northeast Pennsylvania where Day 1 WPC PWPF is 40
to 60 percent for 18 or more inches of snow. This is 40 percent
for the central Hudson Valley in the Day 1.5 probabilities.
Overall the coverage and intensity of the snowfall forecast is
similar to the previous shift with an increase in interior
southern New York.

A swath of over 8 inches of snow is expected from the eastern West
Virginia Panhandle to the Berkshires of Massachusetts with maximum
amounts possibly reaching two feet in central Pennsylvania. The
risk for 8 or more inches has increased to likely for the rest of Massachusetts. Accumulating ice after 12Z has a 40 to 50 percent
chance of reaching a quarter inch in a swath along/east of the
Blue Ridge from southwest Virginia to north-central Virginia as
well as southern West Virginia west of the Allegheny Highlands.


...Western States...
(Days 1-3; 12Z Wednesday to 12Z Saturday)

An active pattern remains with the northwest CONUS with the next
wave the most meridional for the foreseeable future with trough
axis crossing the central California coast and north late tonight
into Thursday morning. Ample Pacific moisture will produce heavy
snow for higher elevations of much of the central and northern
portions of the Western U.S. over the next few days. The Day 1.5
snow probabilities are moderate to high for 8 or more inches for
the Olympics, Cascades, central and northern Sierra Nevada, and
western slopes of the ID Rockies and Tetons. This shifts to the
northern Wasatch for Day 2. The next wave reaches the Pacific
northwest Friday with Days 2 and 3 snow probabilities moderate to
high for 8 or more inches for the northern Washington Cascades. 72
hour probabilities for 18 or more inches are high in the
Washington Cascades.


Jackson


$$
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FOUS11 KWBC 170854
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Thu Dec 17 2020

Valid 12Z Thu Dec 17 2020 - 12Z Sun Dec 20 2020

Day 1...
...Northeast...
Moderate to heavy snow is expected to continue across portions of
the Northeast through early Thursday. Low-to-mid level
frontogenesis along with favorable upper level forcing will
continue to support areas of moderate to heavy snowfall north of a
low moving along the Long Island/southern New England coast today.
Overnight this system has shown the capacity to produce bands
generating snowfall accumulations well in excess of 2 in/hr across
portions of Pennsylvania and Upstate New York. The overnight
HREF, along with recent runs of the HRRR, indicate that banded
snowfall, producing snowfall accumulation rates of over an
inch/hr, is likely to move east from Hudson Valley to the New
England coast. Latest WPC PWPF indicates additional accumulations
of 4-inches or more are likely from the Capital District of New
York to the New England coast, from the Boston Metro to Down East
Maine. Snows are expected to diminish from west to east Thursday
afternoon and evening as the associated upper trough begins to
shear east of the region.

...Western U.S....
A well-defined upper level shortwave and associated frontal band
extending from the northern Rockies to California Thursday morning
will swing east this period, reaching the central Rockies by early
Friday. This system is expected to produce widespread
accumulating snows from the northern Rockies and Sierra, through
the Great Basin and into the central Rockies, with locally heavy
totals across portions of the higher terrain. Meanwhile, onshore
flow ahead of the next system will be sufficient for periods of
snow, with additional heavy accumulations possible across the
northern Cascades.

Days 2-3...
...Pacific Northwest...
A shortwave trough is expected to bring another round of organized precipitation, including heavy snows to portions of the northern
Cascades late Friday. Shortwave ridging is expected to bring a
brief, relatively drier period on Saturday, before strong,
moisture-laden onshore flow begins to settle south into the
region. While heavy precipitation is expected to develop across
western Washington Saturday night into Sunday, snow levels are
forecast to be relatively high at the onset, with some further
increase expected -- confining the threat for heavy snowfall
accumulations largely to the higher terrain on the northern
Cascades.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira

$$
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FOUS11 KWBC 182049
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EST Fri Dec 18 2020

Valid 00Z Sat Dec 19 2020 - 00Z Tue Dec 22 2020


Days 1-3...
...Pacific Northwest to the Northern Rockies...
...Rounds of Moderate to Heavy snow expected through the weekend...
An upper level shortwave is expected move onshore this evening,
supporting another round of mountain snow along the path of the
wave from the WA Olympics/Cascades to the ranges of northern Idaho
and northwest Montana. This system will move quickly east of the
region across the northern Plains.
Heavy precipitation is likely across western Washington late
Saturday into early Sunday as a long fetch of deep,
moisture-laden, westerly flow settles south into the region.
Additional rises in snow levels Saturday night into Sunday morning
will confine heavy snow accumulations to the high peaks of the
northern Cascades.

On Sunday, focus for heavier precipitation briefly shifts farther
south and east along with a frontal boundary settling along the Washington-Oregon border.
The models are now focusing on the development of a wave of low
pressure in the northeast Pacific that moves towards WA that
drives the warm front back north again across WA
Sun night into Mon. This is expected to increase the potential for
locally heavy snow accumulations across the ranges of northern WA
into northern ID and northwest MT.
Here model differences relate to the track of low pressure, with
the ECMWF and GFS clustering well, and the NAM/UKMET tracking the
low precipitation/resultant snow in concert with the low. More
weighting was given to the 00-12z ECMWF/12z GFS/12z Canadian
global QPF/temperature profiles. Strong warm advection in the
preferred solutions keeps snow at higher elevations, stabilizing
as the cold front approaches Monday afternoon or night.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 190913
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EST Sat Dec 19 2020

Valid 12Z Sat Dec 19 2020 - 12Z Tue Dec 22 2020


Days 1-3...
...Pacific Northwest to the Northern Rockies...
Strong westerly flow is expected to direct a plume of anomalously
deep moisture into the Pacific Northwest beginning as early as
late Saturday afternoon -- supporting the development of heavy
precipitation that is expected to continue into the overnight
across portions of western Washington and Oregon. This
precipitation will include snow for the Cascades. However, rising
snow levels late Saturday into early Sunday are expected to
confine heavy accumulations mainly to the higher elevations of the
northern Cascades. With ample moisture spreading east, locally
heavy accumulations are also expected east of the Cascades -- with
areas impacted expected to include the northeastern Oregon
mountains and the northern Rockies -- especially the northern
Idaho and the northwestern Montana ranges west of the Divide. But
here also, rising snow levels are expected to limit heavy
accumulations mostly to the higher peaks.

With onshore flow forecast to ease a bit, expect to see some
decrease in precipitation rates by late Sunday. However, a
lingering frontal boundary and coinciding axis of deep moisture
will continue to support additional locally heavy amounts,
including mountain snows for the Cascades and northern Rockies.

This front is expected to lift north ahead of an amplifying wave
approaching the Washington coast Monday morning. This will
support an additional round of organized precipitation across the
northern Cascades and Rockies as the front lifts back through the
region on Monday. Snow levels are expected to once again increase
ahead of the low, before dropping rapidly as the low and a strong
trailing cold front move across the Northwest into the northern
Rockies late Monday into early Tuesday. Additional heavy snow
accumulations are likely across the northern Cascades. Potential
for heavy snow for the northern Rockies east of the Divide is
expected to increase late Monday. A boundary lifting north ahead
of the wave is expected to support increasing low level
convergence/upslope flow -- raising the threat for heavy
precipitation developing along the Lewis Range in northwestern
Montana by late Monday and continuing into early Tuesday.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira


$$
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FOUS11 KWBC 192044
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EST Sat Dec 19 2020

Valid 00Z Sun Dec 20 2020 - 00Z Wed Dec 23 2020

Days 1-3...
...Pacific Northwest to the Northern Rockies...
Strong deep layer westerly flow is expected to advect deep
moisture towards the Pacific Northwest coast Saturday night,
supporting the development of heavy precipitation across portions
of western Washington and Oregon. This precipitation will include
snow for the higher elevations of the Cascades, and high snow
levels should keep significant accumulations above pass levels.
The moisture surge is expected to be transported well inland and
result in enhanced snowfall for the higher terrain of the Blue
Mountains in eastern Oregon, as well as the northern Rockies.
Rising snow levels in this region are expected to limit heavy
accumulations mostly to the higher peaks.

There will be some decrease in precipitation rates by late Sunday
owing to an abatement of onshore flow and less synoptic scale
forcing. However, a lingering frontal boundary and coinciding
axis of deep moisture will continue to support additional locally
heavy amounts, including mountain snows for the Cascades and
northern Rockies. This front is expected to lift north ahead of
an amplifying wave approaching the Washington coast Monday
morning. This organized storm system will support an additional
round of enhanced precipitation across the northern Cascades and
northern Rockies as the front lifts back across the region on
Monday. Snow levels are expected to once again increase ahead of
the low, before dropping rapidly as the low and a strong trailing
cold front move across the Northwest and then the northern Rockies
by late Monday into early Tuesday. As a result, additional heavy
snow accumulations are likely across the northern Cascades, and
then northern Idaho/western Montana by this time.

...Upper Midwest...
An Alberta Clipper type surface low is expected to track from
North Dakota to Michigan from Sunday night and into Monday. A
swath of mainly light snow, owing to warm air advection and
amplifying 500mb shortwave trough, can be expected from northern
Minnesota to northern Wisconsin, including portions of the Upper
Peninsula of Michigan. Amounts on the order of 1 to 3 inches,
with perhaps some locally higher amounts, are likely across this
region.

...Central Appalachians...
Cold air advection on the backside of the departing low from the
Great Lakes region is expected to produce some lake effect snow
showers downwind of Lake Erie and Lake Ontario, mainly during the
00Z-12Z time period on Tuesday. In addition, upslope flow across
the spine of the central Appalachians in eastern West Virginia
will likely lead to light to moderate snow showers generally
during that same time period, with accumulations generally under 4
inches expected.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Hamrick/Pereira

$$
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FOUS11 KWBC 202034
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Sun Dec 20 2020

Valid 00Z Mon Dec 21 2020 - 00Z Thu Dec 24 2020

Days 1-3...
...Pacific Northwest to the Northern and Central Rockies...

An ongoing atmospheric river event is expected to continue on
Sunday from the Pacific Northwest into the northern Rockies, with
additional locally heavy precipitation likely.
A frontal wave develops over the northeast Pacific and moves
towards northern WA State Monday, northern Idaho Monday night,
followed by Montana on Tuesday.

The approaching low drives the warm front north across OR into WA,
with the heaviest snow on Day 1 north of the front in the higher
peaks of the WA Olympics/northern Cascades and the northern
Rockies, from the northern Idaho and western Montana ranges to
western Wyoming.

Snow amounts focus where the wave crosses northern ID and
northwest MT. Several additional inches are expected in the Lewis
Range in northwestern Montana. Post frontal steepening lapse
rates and low level ascent in windward facing terrain have the
potential to bring several inches of snow to the WA Cascades and
the OR Cascades. Drying aloft the latter half of Day 2 reduces
snow coverage and intensity.

Models continue to show a well-defined wave with a strong trailing
cold front, driven by an amplifying upper trough, is expected to
plunge southeast across the central Rockies on Tuesday. Post
frontal moist northwest flow with pooled moisture and low level
ascent crosses the ranges of eastern Wyoming and Colorado Tue
night into Wed., producing a period of snow. Several inches of
snow are expected in favored upslope areas. The progressive nature
of the system will help limit the potential for widespread heavy
accumulations. The models have a lot of spread as to where post
frontal lift
produces snow in the central high Plains, with confidence low on
where a focused area of accumulated snow may develop. The GFS is
an outlier, with the UKMET/ECMWF/NAM/Canadian clustering better
with the magnitude of the trough.

...Upper MS Valley to the Central Appalachians...

The models show a progressive clipper system with low pressure
crossing MN tonight leading to low-mid level frontogenesis north
of the low in northern MN. Low level moisture convergence in
northern MN under the frontogenesis max leads to a period of snow.
An area of 2-4 inches of snow is expected before the system
departs into the Great Lakes on Mon.

As the system crosses the upper Oh Valley, a trailing cold front
crosses the central Appalachians.
Post frontal moist west-northwest flow comes into the ridges of
the central Appalachians from southeast PA across West Virginia.
A sustained period of lift in windward terrain in the mountains of
West Virginia should lead to several inches of snow.

The event winds down on Tuesday as a low level ridge approaches
from the Ohio Valley with drying aloft reducing available moisture
and lift. The 12z ECMWF/UKMET/NAM/GFS/Canadian are similar in QPF
amounts and resultant snow totals.

For Days 1-3, the probability of significant icing (0.25 inches or
more) is less than 10 percent.

Petersen

$$
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FOUS11 KWBC 212017
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 PM EST Mon Dec 21 2020

Valid 00Z Tue Dec 22 2020 - 00Z Fri Dec 25 2020


Day 1...
...Central Appalachians...
A vigorous shortwave trough noted in water vapor imagery this
afternoon near Chicago will quickly advance eastward tonight
across portions of the Great Lakes, central Appalachians, and
Northeast. Strong westerly flow in its wake will lead to favorable
upslope snows on the higher terrain areas along the Appalachians.
The latest WPC PWPF show the greatest probabilities for exceeding
4 inch totals along the Allegheny Mountains of West Virginia.
Snowfall rates should taper off Tuesday morning.

...Pacific Northwest to Northern Rockies...
Shortwave troughing pushing onshore the Pacific Northwest later
today will track across Washington State into the northern Rockies
tonight. The combination of large scale favorable forcing for
ascent and moisture will support the development of moderate to
heavy precipitation across the region. The highest snowfall
probabilities remain over the northern Cascades into the far
northern Rockies.

Day 2...
A deepening area of low pressure is expected to track across the
northern Plains late Tuesday night into Wednesday morning. While
the greatest threat for higher accumulating snows will be north in
Canada, minor accumulations are forecast for portions of North
Dakota and northern Minnesota. This combination of light snow with
very strong winds could result in hazardous winter conditions.

Day 3...
...Upper Midwest/Great Lakes...
As the low pressure lifts northeast, backside/wrap around snow
will be likely across portions of northern Minnesota, northern
Wisconsin, and the Upper Peninsula of Michigan. The tight pressure
gradient from a building Canadian high pressure will result in
enhanced lake effect snows off Lake Superior where several inches
could accumulate on Day 3 /00z Thu-00z Fri/.

Additionally, as the strong cold front marches toward the East
Coast, crashing temperatures in the low levels enhanced by a
secondary low pressure lifting northward through southern
Appalachians toward the Mid-Atlantic, could change over
precipitation from rain to snow on the backside before ending
across portions of the Great Lakes, Ohio Valley, and Tennesse
River Valley Thursday afternoon/night. Uncertainty is a bit higher
here with model spread still high with both differences seen in
the timing of the cold air and amount/depth of moisture in place
when thermal profiles do support snow versus rain.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Taylor



$$
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FOUS11 KWBC 222028
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 PM EST Tue Dec 22 2020

Valid 00Z Wed Dec 23 2020 - 00Z Sat Dec 26 2020

...Northern Plains and Upper Midwest...
Days 1-3...

A mid-level trough ejecting from the Northern Rockies will amplify
into a full latitude trough by Thursday, aiding in low-level
cyclogenesis which will deepen in response to the mid-level trough
closing off as the surface low moves across MN and into the
western Great Lakes. While the low is likely to move quickly
eastward limiting the temporal duration of strong ascent and heavy precipitation, a period of moderate to heavy snow is likely NW of
the system in response to intensifying deformation and a
developing TROWAL leading to enhanced comma head precipitation,
especially across the Arrowhead of MN. Additional heavy snow
enhancement is expected south of Lake Superior due to LES. While
guidance continues to show some spread in the exact track of the
low and placement of the best comma precip, the ECMWF remained a
favored solution along with the ECENS/GEFS means. This suggests
the heaviest snow will occur along an axis from far SW MN into far
NE MN, with an additional maxima in NW WI due to LES. WPC
probabilities are high for 4 inches in this region, with isolated
maxima over 10" possible in the Arrowhead or NW WI. Additionally,
very strong winds in excess of 35kts are likely at times as shown
by high probabilities in the CIPS analog guidance. This could lead
to periods of blizzard conditions even during light snow, or after
the snow winds down, due to the high SLR/fluffiness of the snow in
the very cold atmosphere.

As the storm pulls away Thursday into Friday, LES will ramp up
across the U.P. of MI and along the eastern lakeshore of Lake
Michigan. WPC probabilities for 4 inches are generally 10-20% on
Day 3.

...Ohio Valley/Central Appalachians/Great Lakes...
Days 2-3...

An area of low pressure lifting across the Great Lakes and into
Canada will drape an impressive arctic cold front across the
Midwest, racing eastward through the Appalachians and towards the
east coast by Christmas Day. Along this front, secondary low
pressure development is likely across the Southeast/Mid-Atlantic,
with this low lifting quickly northward along the baroclinic zone
into New York by Friday. This low develop and strengthen in
response to PVA/height falls and an increasing upper level
diffluence acting upon the sharp low-level baroclinic zone. There
remains considerable spread in the timing and evolution of this
front/low, and this forecast relied heavily on the
ECMWF/ECENS/GEFS.

The airmass ahead of the front/low is much too warm for snow, and
much of the precipitation will fall as heavy rain. However, as the
low lifts NE the front will shift east behind it, rapidly cooling
the column to cause a p-type transition from rain to snow. Exactly
how quickly this occurs will make all the difference with snow
amounts. Usually, cold air chasing moisture is not favorable for
heavy snow. However, as the front races east, the trough becomes
negatively tilted coincident with at least the modest development
of a TROWAL and sharpening low-level fgen. Persistent mid-level
and upper level divergence for ascent combined with a weak WCB
could throw enough moisture behind the front to allow for several
inches of snowfall to accumulate. This will be aided by what could
be briefly intense rates as the fgen sharpens, so despite hostile
antecedent conditions due to warmth and rain, snow may rapidly
accumulate. This is most likely in the eastern OH VLY due to
enhanced fgen, south of Lake Erie due to LES, or in the central
Appalachians in response to upslope forcing. WPC probabilities are
high for 4 inches across these areas, with locally in excess of 6"
in the WV terrain, or 8" south of Lake Erie, possible.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss



$$
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FOUS11 KWBC 230849
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Wed Dec 23 2020

Valid 12Z Wed Dec 23 2020 - 12Z Sat Dec 26 2020

Day 1...
...Central Plains to the Upper Midwest...
After several runs of struggling to reach a consensus, the 00Z
model runs have moved into better agreement -- offering a strong
signal for moderate to heavy snows developing from eastern
Nebraska and southeastern South Dakota to northwestern Minnesota,
northwestern Wisconsin and the western U.P. of Michigan. A
powerful surface low centered over South Dakota this morning is
forecast to move into the upper Mississippi valley later this
morning. As a well-defined shortwave moving through the base of
the associated upper trough begins to lift northeast from the
central Plains into the upper Mississippi valley, a developing
comma head of moderate to locally heavy snow is expected to
develop on the west side of the low. Low-to-mid level
frontogenesis is expected to help support banded precipitation,
with the 00Z HREF showing several members suggesting snowfall
rates of 0.5-1 in/hr within these bands as they move east across
the region on Wednesday. This snowfall along with strong, gusty
winds is likely to result in reduced visibilities and blizzard
conditions across portions of the region. Latest WPC PWPF
indicates high probabilities for accumulations of 4-inches or more
extending from northeastern Nebraska and eastern South Dakota to
northwestern Minnesota and the western U.P. Within this area,
high probabilities for 8-inches or more extend from south-central
Minnesota to northwestern Minnesota and the western U.P. Some of
the highest probabilities center along the Wisconsin and western
U.P. shores of Lake Superior, where in addition to the
synoptically driven snows, northerly flow on the backside of the
system may provide some lake-enhanced totals.

Day 2...
...Ohio Valley/Central Appalachians/Great Lakes...
As the previously noted low moves north of the Upper Great Lakes,
a sharp trailing cold front is forecast to move east across the
Ohio and Tennessee valleys on Thursday. Rain changing to snow
before precipitation ends is expected to produce mainly light snow accumulations across the lower Ohio and Tennessee valleys through
Thursday. Then by Thursday afternoon, as a well-defined shortwave
begins to move through the base of the associated upper trough,
surface low pressure is forecast to develop and move north along
the front into the central Appalachians -- raising the potential
for significant snows to develop back in the cold air. While
there is some uncertainty in the details given the timing of the
cold air, significant accumulations look likely across portions of
the southern and central Appalachians and Ohio valley Thursday
night into early Friday. As a new upper low begins to develop
over the Ohio valley, surface low pressure will continue to
intensify as it moves north through the upper Ohio valley and
Lower Great Lakes region early Friday. As the low moves north of
the Lakes, northerly flow behind the departing system will support
lake effect snow bands developing off of Lake Erie into
northwestern Pennsylvania and northeastern Ohio Friday morning.

Meanwhile, northwesterly flow across the Upper Great Lakes will
support developing lake effect snow bands, with locally heavy
accumulations likely across portions of the western and central
U.P. of Michigan and along the eastern shores of Lake Michigan.

Day 3...
...California and the Pacific Northwest...
Following a few days of dry weather, precipitation is expected to
return to the region late Friday into early Saturday. Models show
a deepening surface low tracking northeast toward Vancouver Island
with an occluded front moving inland across the Northwest and
Northern California Saturday morning. A well-defined mid level
shortwave/upper jet moving through the base of the associated
trough is expected to support organized heavier precipitation
across Northern California, including significant snows from the
Klamath Mountains to the Sierra.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira


$$
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FOUS11 KWBC 270925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 AM EST Sun Dec 27 2020

Valid 12Z Sun Dec 27 2020 - 12Z Wed Dec 30 2020

...Great Lakes..
Days 1-3...

A wave of low pressure is likely to move from IL to WI and across
Michigan today. As this low moves northeast, a closed 850 mb low
drives a warm front across eastern WI and lower MI. 850 mb
warm/moisture advection and convergence will drive an area of
light snow across east central WI and northern lower MI.
The models forecast a thin band of 0.25 inches liquid equivalent,
so 2-4 inches of snow is likely across portions of east central WI
to northern lower MI.
WPC probabilities are as high as 30% for 4 inches in these areas.

Behind this low, another round of LES is likely for parts of the
U.P., northern L.P. Sunday night into Monday. Amounts should be
modest, as WPC probabilities indicate
low probabilities for 4" in the favored snow belt regions. As the
downstream warm front crosses Lake Ontario into the Tug Hill on
Monday,snow showers develop for a period of several hours before
the front departs. Additional snow showers are expected as the
cold front approaches Tue morning. Several more inches of snow
are possible.

...CA/Great Basin/Rockies...
Days 1-2...

An upper trough moving onshore CA late tonight into early Monday
continues east across the Great Basin Monday and into the central
and southern Rockies Tuesday. This feature will be accompanied by
persistent jet energy crossing central to southern CA inland
across central to southern NV/UT, then CO. Upper level divergence
combines with topographically forced ascent in wind terrain to
produce snows across the central to southern CA Sierra Nevada
mountains, followed by the Transverse Ranges of southern CA. The
jet axis on Day 2/Mon crosses UT and into western CO. The jet
axis crossing the ranges lead to several inches of snow across the
mountains of western CO. The heaviest snow day 2 is expected in
the San Juans due to stronger low level jet that leads to stronger
ascent in windward areas.
WPC probabilities on D2 are high for 8 inches in the San Juans
with maximum amounts over 12" likely.

As the upper trough moves slowly east out of the Rockies and
across the Plains D3/Tue, snow coverage and intensity drops off,
with a few inches of snow expected in the southern CO mountains of
New Mexico before activity ends.

...Central Plains/Upper MS Valley...
Day 3...

As the long wave trough moves east across the Plains, low-mid
level southwest flow increases. The increased low level jet
drives warm/moisture advection with a 850-700 mb front moving
northeast across Nebraska and Kansas through eastern South Dakota,
Iowa, southern Minnesota, Wisconsin and northern Illinois.
The combination of the front and warm advection produces ascent
and snow across northeast Nebraska and southeast South Dakota
across southern Minnesota, northern Iowa, and into southern to
central Wisconsin.

There remains considerable spread in both the placement and timing
of the 700 mb low and resultant QPF and where the precip type
transition zone sets up across the upper MS Valley to the Upper
Great Lakes. The GFS/GEFS differ from the ECMWF/ECMWF Mean,
UKMET, Canadian global, and NAM. A preferred blend included the
GFS/EC, as combination despite the NAM's further north track
reduced QPF amounts and potential snow in eastern Nebraska and
eastern South Dakota.

South of this area, warm advection turns snow to sleet and
freezing rain across northern Kansas to northern Missouri and
Illinois, continuing into Iowa and southern Wisconsin. WPC
probabilities indicate a low risk for 0.25" of freezing rain
accretion in northeast KS to northern MO and southern IA.

The probability of 0.25 inches of icing is less than 10 percent
Days 1-2.

Petersen

$$
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FOUS11 KWBC 272130
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 PM EST Sun Dec 27 2020

Valid 00Z Mon Dec 28 2020 - 00Z Thu Dec 31 2020

...Great Lakes..
Days 1-2...
A wave of low pressure moving across WI and MI tonight will spread
moderate snow across the area. While some moderate snow is likely
across WI this evening, any significant accumulations from this
event are likely from LES developing as the low pulls away. CAA
behind the departing system moving across the lakes on NW flow
will lead to rounds of moderate to heavy snow in the eastern U.P.
and NW L.P. of MI on D1. WPC probabilities are moderate for 4".
Into D2, the associated cold front moving across the area will
lead to redeveloping LES downwind and to the east of Lake Erie and
Ontario, with an effective fetch from Huron transporting
additional moisture. WPC probabilities D1.5 into D2 are high for 4
inches, with amount up to 8" possible in the Tug Hill Plateau.

Additionally, as the arctic front crosses the Northeast brief snow
squalls are possible which are not likely to accumulate much, but
may lead to short duration reduced visibility and hazardous travel.

...Central Plains/Midwest...
Days 2-3...
Longwave trough moving eastward from the Great Basin will send an
intensifying jet streak around its base placing favorable upper
diffluence for surface pressure falls into the lee of the Rockies.
As this occurs, coupling of the new jet streak and the residual
zonal streak to the north will drive deep layer ascent for low
development and strengthening, with a surface low expected to move
northeast into WI and the Great Lakes by Wednesday aftn. Guidance
today has overall weakened due to less northern and southern
stream interaction, leading to better split flow and a more
progressive solution, with phasing only occurring late in the
forecast period. This is agreed upon by the GFS and ECMWF along
with their ensembles which were used heavily for this forecast.

As the low begins to lift northeastward, it will be accompanied by
a band of low-level fgen and intense WAA from western KS into
southern SD. Within this band, intense ascent into the DGZ and an
increasing isothermal layer within model soundings suggest intense
snow rates of >1"/hr for a few hours. However, the WAA will
eventually overwhelm the cold air to cause a p-type change to
freezing rain and eventually rain across KS and parts of NE/IA.
Further north, a band of freezing rain is likely to linger from NE
Kansas into central IA and southern WI. North of there, the column
should stay cold enough for primarily snow. As the two streams
interact Wednesday morning, some enhanced deformation may occur NW
of the surface low causing a local uptick in snowfall across NE IA
into central WI. This is in addition to the leading WAA snowfall.

With a more progressive solution and less energy phasing, WPC
probabilities for heavy snow have trended down just slightly.
However, there is still a high risk for 4 inches from central NE
into southeast SD on D2, expanding into northern IA, southern MN,
and much of WI D3. The heaviest snow, which may exceed 8" is most
likely across NE IA into WI.

South of there, from central KS through northern MO and into
northern IL/southern WI, there is likely to be a longer duration
of freezing rain. Eventually in these areas p-type may also change
to rain before ending, but WPC probabilities indicate a moderate
risk for 0.1" of accretion on D2-3, with isolated pockets to .25"
possible.

...CA/Great Basin/Rockies...
Days 1-2...
An upper trough moving onshore CA early Monday will advect
eastward into the Great Basin Monday and into the central and
southern Rockies Tuesday. This feature will be accompanied by
persistent and robust jet energy crossing central to southern CA
inland across central to southern NV/UT, then CO, providing deep
layer ascent and spreading moisture eastward.

Upper level divergence combines with topographically forced ascent
in windward terrain to produce snows across the central to
southern CA Sierra Nevada mountains, followed by the Transverse
Ranges of southern CA. The jet axis for ascent will then cross
into UT and into western CO, leading to several inches of snow
across the mountains of western CO. The heaviest snow day 1 is
likely in the Sierra and transverse ranges of CA which are more
orthogonal to the flow and WPC probabilities are high for 8
inches. Heavy snow is also likely in the San Juans D1 due to WAA
snows spreading northward and interacting with the terrain. By D2,
the heaviest snow should shift is expected in the San Juans and CO
Rockies due to stronger low level jet that leads to stronger
ascent in windward areas.
WPC probabilities on D2 are high for 8 inches in the San Juans
with maximum amounts over 12" likely.

Weiss

$$
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FOUS11 KWBC 280940
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
439 AM EST Mon Dec 28 2020

Valid 12Z Mon Dec 28 2020 - 12Z Thu Dec 31 2020

...Great Lakes..

Days 1-3...
A wave of low pressure moving across MI is forecast to cross into
Ontario and then Quebec today. Cold advection behind the departing
system will lead to moderate to heavy snow in the lee shore areas
downstream from lake Superior in the eastern U.P. and NW L.P. of
MI on D1. The warm front south of the low crosses Lake Ontario,
with low level south to southwest winds transporting moisture and
ascent into the areas east of Lake Ontario, including the Tug Hill
and western Adirondacks. This moves downstream until a front
approaches later tonight, with boundary layer winds veering. This
results in the peak low level convergence occurring near the
southeast corner of Lake Ontario tonight, as the 00z GFS has a
bulls eye in ascent there by Tue morning.

On Day 2, the snow downstream from Lake Ontario winds down, with
the snow switching to the upper Lakes within the broad low-mid
level theta-e advection and convergence zone crossing WI, Lake
Michigan, and into the the UP of MI and northwest lower MI.

Light icing is possible in southern lower MI as warm advection
changes precip types quickly from snow to sleet and freezing rain.
Light amounts of each type are expected in southern lower MI.
Several inches of snow are expected in northern lower MI to the
eastern UP of MI on Day 3/Wed due to the combination of synoptic
snow, followed by post-system cold advection and lake effect
snow. Also, on Day 3, the warm/moisture advection zone crosses
Lake Ontario, continuing into the Tug Hill and western
Adirondacks. The continuing warm advection also leads to the
chance of a period of light icing in northern New York.

...Central Plains/Midwest...

Days 1-3...
A longwave trough moving eastward from the Rockies crosses the
northern to central Plains. A coupling of the 2 jet streaks will
drive deep layer ascent for low development and strengthening,
with a surface low expected to move northeast across the mid MS
Valley Tue night to the upper Great lakes Wed.
As the low lifts northeastward, it will be accompanied by a band
of low-mid level frontogenesis and warm advection from KS across
NE into southern SD. Within this band, ascent near the low-mid
level front and warm advection suggest snow rates of 1"-1.5"/hr
for a few hours. Several inches of snow are expected in northern
NE and southern SD to southern MN, northern IA and southern WI.
The higher amounts are expected in portions of northeast IA to
southern WI within the longer duration low level jet and front.

The low level warm advection causes a change from snow to to
freezing rain and eventually rain across most of KS and northern
MO into parts of southern NE/southern IA, followed by northern IL.
The existence of multiple precip types and uncertainty regarding
the duration of each one drive the forecast uncertainty.
The possibility of ice accumulations as much as a quarter inch
exists in northwest MO to southern IA. WPC probabilities indicate
a moderate risk for 0.1" of accretion on Day 2 Tue, with isolated
pockets to .25" possible.

With a more progressive solution and the continued northeast
movement of the low-mid level front, probabilities for heavy snow
have trended down just slightly. However, there is still a high
risk for 4 inches from central NE into southeast SD, northern IA,
southern MN, and much of southern to central WI. The heaviest
snow, which may reach 8", is most likely across NE IA into
southern WI.

...CA/Great Basin/Rockies/Southern Plains to Mid MS Valley...

Days 1-3...
An upper trough moving onshore CA early Monday will advect
eastward into the Great Basin today and into the central and
southern Rockies Tue. This feature will be accompanied by
persistent and robust jet energy crossing central to southern CA
inland across central to southern NV/UT, then CO, providing deep
layer ascent and spreading moisture eastward. Upper level
divergence combines with topographically forced ascent in windward
terrain to produce snows across the southern CA Sierra Nevada
mountains, and Transverse Ranges of southern CA. Heavy snow is
also likely in the San Juans of CO/adjacent northern NM today into
early Tue due to WAA snows spreading northward and interacting
with the terrain. As upper divergence maxima persist in the ranges
of western CO, locally heavy snow are expected in windward areas.
WPC probabilities on D2 are high for 8 inches in the San Juans
with maximum amounts over 12" likely.

On Day 3, the upper trough moves across the southern Plains. An
increasing number of forecasts show a 700-500 mb low developing
over the southern Plains.
The cool near the mid-upper low may allow snow to develop on the
western edge of the precip shield over portions of west Texas
north into Missouri.
The low level cold air also leads to a band of mixed sleet and
freezing rain in the AR Ozarks across southern MO, where the
greatest threat of significant icing exists on Day 3/Wed.

...Pacific Northwest...

...Day 3...
The models are indicating a resurgence of deep layer moisture and
ascent, triggered initially by an upper level jet streak with
embedded divergence maxima crossing western WA early Wed, then
continuing inland across ID and western MT Wed night. This leads
to widespread moderate to heavy precipitation across the northwest
and then spreading into the ranges of northern ID and northeast OR
Wed afternoon as the surge of moisture and ascent moves east. The
stronger ascent and longer duration of precipitation favors the WA
and OR Cascades received the heavier snow. 1 to 2 feet of snow are
expected across most of the WA Cascades, and up to a foot in the
OR Cascades and ranges of northeast WA and northern ID.

Petersen

$$
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FOUS11 KWBC 290923
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EST Tue Dec 29 2020

Valid 12Z Tue Dec 29 2020 - 12Z Fri Jan 01 2021

Days 1 to 3...

...Central Plains and Upper Midwest and Great Lakes...

A shortwave associated with the broader upper trough is expected
to eject out across the northern to central Plains and upper
Midwest.
Snow has developed in western SD and central Nebraska, and this
area should continue to grow as a broad and strong warm advection
pattern across the broader Plains and Midwest. The stronger
ascent occurs on the nose of the 850-700 mb jet crossing eastern
KS across northern MO and eastern Iowa. A band of heavy snow is
expected across eastern Iowa to southern and eastern WI where
temperature remain cold enough for snow. 6 to 10 inches of snow is
likely in this area.

South of this area, a mixed precip event occurs from northern KS
across northern MO and adjacent southeast NE, far southern Iowa,
and northern IL/southern MI.
The expectation is for sleet and freezing rain, as a strong warm
nose aloft moves in. The sub-freezing boundary layer temperatures
will likely result in areas of a 0.10 to 0.25+" of ice accretion
from northeast KS through northwest MO, southern IA and into
northwest IL. Lighter freezing rain amounts are expected into
southern lower MI and northwest IN/central IL. The primary
uncertainty is tight gradients exist in northeast KS and northern
MO, with the high res windows showing potential for locally higher
sleet/icing accumulations in northeast KS vs. the lighter QPF and
resultant ice accumulations in the global models.

...Southern Plains to the Lower/Middle MS Valley along with the OH
Valley and Northeast...

Cold high pressure will be building across large areas of the
Midwest and Great Lakes region by Thursday. The upper trough and
closed low over the
southwest will be drifting east and should reach the southern
Plains on Thursday. This will in turn generate strengthening
surface low pressure over southeast Texas
with increasingly strong warm-air advection and Gulf of Mexico
moisture transport moving inland. Coupled upper divergence/lower
convergence maxima in conjunction with the moisture fluxes lead to
an expanding area of moderate to heavy precipitation.

The northwest periphery of the precipitation shield is likely to
encounter sufficient cold air from the high center to the north
for an axis of sleet and freezing rain, with concerns for locally
significant ice accretion possible for areas of northwest AR,
southern MO and southern IL on Day 2. As much as a 0.25+" of ice
accretion will be possible. On Day 3, the axis of icing moves
north into northern MO, eastern KS and much of central OK and into north-central TX.
Light icing is possible in northern portions of the OH Valley.
Farther to the northeast across the interior of PA/NY and New
England, some light icing from freezing rain will be possible on
the warm-air advection side of the aforementioned cold front as it
arrives across the Northeast on Thursday.

West of the 700 mb closed low, accumulating snowfall is forecast
down across areas of southwest TX including parts of the Edwards
Plateau and adjacent areas of the TX Big Bend. This will be tied
into the track of the upper-level low center which is expected to
foster locally strong dynamic cooling and enough of it in
conjunction with low-level cold-air advection for rain to mix with
or change over to snow. Higher accumulations on Day 2 would be in
the Davis Mountains of southwest TX, then extending into west
central Texas on Day 3. As the closed mid level circulation
moves north out of TX, precip in central to western OK and
southwest MO eventually changes over to snow, so light
accumulations are expected near the 700 mb low as it arrives.

...Pacific Northwest and Northern Rockies...

A strong upper trough and associated closed low will advance
inland across the Pacific Northwest by late Wednesday. Strong
onshore flow into the higher terrain of the Cascades will yield
robust Pacific moisture transport, which should combine with mid
level ascent to bring snow to the WA Olympics and Cascades on Day
1. As the upper trough moves onshore Wed, cooling aloft leads to
lowering snow level and greater coverage of snow in the ranges of
WA/OR into the northern Rockies.
Heavy snowfall accumulations of 1 to 2 feet of snow are expected
with the heaviest totals likely in the WA Cascades. Locally heavy
snow though is expected downstream across portions of the northern
Rockies Wed in the ranges of northeast WA across northern ID as
the forward branch of the 300 mb crosses the region, accompanied
by upper divergence maxima. On Thu., the upper trough moves east
of the Rockies, so snow coverage/intensity should taper in the
Pacific northwest to northern Rockies.


Petersen

$$
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FOUS11 KWBC 302131
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EST Wed Dec 30 2020

Valid 00Z Thu Dec 31 2020 - 00Z Sun Jan 03 2021

Day 1...
...Pacific Northwest to the Northern Rockies...
As a deep, compact low moves toward the northern tip of Vancouver
Island, a well-defined mid-level shortwave/upper jet moving to its
south is expected to move inland near the California-Oregon border
this evening. This will help support the development of organized
heavier precipitation farther south across Oregon into
northwestern California. As this shortwave drops quickly to the
southeast overnight, the previously noted low is expected to
quickly weaken and drop southeast across the Pacific Northwest on
Thursday -- supporting additional heavy snows from portions of the
northern Cascades to the northern Rockies. WPC PWPF shows high
probabilities for additional snow accumulations of 8-inches or
more for portions of the Washington and Oregon Cascades, the Blue
and Wallowa mountain region in southeastern
Washington/northeastern Oregon, and the Idaho Panhandle ranges.

...Southern Plains...
An upper low will continue to deepen as it moves across northern
Mexico Wednesday night into Thursday morning, assuming a negative
tilt as it lifts into South Texas on Thursday. The depth of the
cold air is expected to remain sufficient for snow on the
northwest side of the developing comma head -- with several inches
of accumulation likely across portions of West Texas. In addition
to strong upper forcing, low-to-mid level frontogenesis is
expected to enhance the potential for heavy snowfall rates.
Several members of the 12Z HREF show the threat for snowfall bands
producing over an inch/hr drifting slowly across the Davis
Mountains to the Stockton Plateau region. WPC PWPF shows very
high probabilities for accumulations of 8-inches or more across
portions of this region, with lower probabilities extending
farther east into the western Edwards Plateau and Concho Valley.
An icy mix is more likely farther to the east, with accumulating
ice expected from the Texas Hill Country northward to eastern
Oklahoma and the Ozark region.

Day 2...
...Southern and Central Plains to the Mid Mississippi Valley and
Southern Great Lakes...
A deep low centered over North Texas Thursday evening is forecast
to lift north-northeast across the Ozark region and into the mid
Mississippi valley on Friday -- drawing plenty of moisture and
warm air aloft into the eastern Plains, mid Mississippi valley and
Great Lakes. Strong upper forcing along with low-to-mid level
frontogenesis is expected to produce widespread precipitation
moving north across Oklahoma, eastern Kansas and the lower
Missouri and mid Mississippi valleys Thursday night into Friday
morning, with shallow cold air supporting a wintry mix and
accumulating ice. While differences in model thermal profiles and
run-to-run differences in the track of the system have limited
forecast confidence in the details, there is still a fair amount
of certainty that by Friday morning, significant ice accumulations
can be expected across portions of eastern Kansas and across
northern and central Missouri, including the KC metro. WPC PWPF
continues to show high probabilities for ice accumulations of
0.10-inch or more, with some areas within this area likely to
receive over a 0.25-inch. As the northern edge the
precipitation shield continues to move north on Friday, the
potential for accumulating is likely to decrease some with the
introduction of daytime heating. However, it is expected that the
cold air will be sufficient for a wintry mix, including some
freezing rain, lifting north across central to northern Illinois,
northern Indiana and southern Lower Michigan during the day on
Friday. Meanwhile, deepening cold air on the backside of the
system is expected to support a changeover to snow before
precipitation ends. While widespread heavy snow accumulations are
not expected, at least an inch or two can be expected from the
eastern Texas Panhandle and Northwest Texas to northwest Missouri
and southern Iowa.

...Central Appalachians...
As the area of low pressure approaching the Great Lakes continues
to direct moisture and warm air aloft from the central into the
eastern U.S., models show high pressure over the central U.S.,
shifting into the Northeast, with a cold air damming signature
setting up along and east of the central Appalachians Friday
morning. This is likely to produce wintry precipitation, with
areas of accumulating ice. Guidance shows the biggest threat for
significant ice accumulations centered from western Maryland into
central Pennsylvania.

Day 3...
...Pacific Northwest...
Another upper level shortwave and its associated frontal band are
expected to bring another round of wet weather, including heavy
mountain snow, into the region Friday night. While this system is
expected to weaken and move progressively east on Saturday. Deep
southwesterly flow ahead of the next approaching system will
continue to channel deeper moisture and the potential for heavy
precipitation into northwestern Washington through late Saturday.
Heavy snowfall totals on the order of a foot or more can be
expected across the higher elevations of the Olympics and northern
Cascades.

...Northern Mid-Atlantic/Northeast...
As low pressure continues to move northeast across the Great
Lakes, a wintry mix will continue to spread north from the central
Appalachians and northern Mid-Atlantic to the Northeast late
Friday into Saturday. A retreating cold-air wedge is expected to
support freezing rain at the onset, with accumulating ice expected
from central Pennsylvania northeastward to western New England.
High probabilities for 0.10-inch of ice or more are shown from
Catskills to the Adirondacks, Berkshires and southern Greens. A
secondary low developing along the northern New England coast on
Saturday, will help maintain the cold air across interior Maine,
where snow is expected to remain the predominate precipitation
type through the event. WPC PWPF shows high probabilities for
snow accumulations of 4-inches or more across much of central and
northern Maine.

Pereira


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FOUS11 KWBC 310918
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EST Thu Dec 31 2020

Valid 12Z Thu Dec 31 2020 - 12Z Sun Jan 03 2021

...Southern Plains/Upper Midwest/Northeast...
Days 1-3...

...A significant and large scale winter storm will is expected
across the eastern half of the CONUS into early 2021...

A deep closed low positioned over the Sierra Madre of Mexico this
morning will deepen as it moves into Texas tonight. This feature
will then gradually begin to fill as it lifts northeast across the
Central Plains and towards the Great Lakes Friday night, and then
into New England Saturday. Beneath this mid-level feature, a
surface low pressure is likely to lift northeast from TX to MO on
D1, and then more rapidly to the northeast to be off the coast of
Maine by Saturday aftn. This low will be accompanied by a leading
warm front, and robust antecedent WAA spreading significant and
widespread precipitation across the eastern half of the CONUS.

Guidance this morning has trended a bit south and slower with the
primary system, and the mass fields of the ECMWF/GFS were used in
the blend, with the NAM thermals preferred for its low-level
thickness evolution. Cool surface high pressure ahead of this low
will be reinforced by mid-level confluence ahead of the upper
feature, and as WAA spreads precipitation northward, an expansive
area of freezing rain is likely. There are still considerable
uncertainties into how much freezing rain will accumulate due to
model uncertainty in the strength of the low-level warm nose and
northward extent of precipitation. The WAA is likely to be quite
strong so many areas from eastern OK through the Ohio Valley and
into parts of the Northeast will start as freezing rain but then
change to rain. Further north, a band of significant freezing rain
is likely, and WPC probabilities for 0.25" are greater than 50% on
D1 in eastern KS and much of central MO, lifting into northern MO,
IL, and IN/MI D2, as well as spreading into the higher terrain of
PA ahead of the expansive warm front where a CAD setup is likely.
Isolated amounts to 0.5" are not out of the question, but heavy
rates should somewhat limit accretion efficiency and the
antecedent surface airmass is not extremely cold. Although these
regions demarcate the area of most likely heavy freezing rain,
there is likely to be rather expansive coverage of at least a few
hundredths of freezing rain accretion extending from West Texas
northeast all the way into Maine, with pockets of 0.1-0.2" likely
in central TX, the Midwest, and the terrain of New York and
western New England.

In addition to the freezing rain, areas of heavy snow are also
likely on the northern and western edge of this system. On D1,
heavy snow is likely in West Texas beneath the upper low where
lapse rates and cold air aloft combine to drive instability and
heavy snow rates. The strong height falls, intense upper
diffluence, and steep lapse rates will likely combine with
mid-level fgen to drive a corridor of heavy snowfall, and WPC
probabilities are high for 4 inches, with locally as much as 8"
possible. During D2 as the low shifts northeast, a TROWAL is
likely to develop throwing rich theta-e air NW within the WCB and
combining with an axis of deformation as northern stream energy
interacts with the primary trough. This should lead to a band of
heavy snow NW of the center from north-central OK into eastern KS
and far NW MO. WPC probabilities for this area have increased as
the HREF snowband probabilities indicate a high chance for 1"/hr
snow rates as the low pulls off to the northeast. There is a
moderate risk for 4 inches across this area on D2. As the low
moves further northeast and begins to spread precipitation across
New England D2-3, heavy snow is likely to overspread much of
upstate NY and northern New England which will be cold enough for
primarily snow due to a southern shift in the guidance overnight.
Eventually the WAA may still cause a changeover to freezing rain
in some places, but WPC probabilities for 4" of snow are high in
the Adirondacks, Greens, Northeast Kingdom of Vermont, and much of
northern NH and ME.

...Pacific Northwest to Northern Rockies...
Days 1-3...

A series of shortwave troughs, separated by brief shortwave
ridging will move onshore the PacNW through the weekend, each one
accompanied by a surge of moisture on increasing Pacific jet
energy. The first is a potent wave moving onshore this morning
which will drive ascent through jet level diffluence combined with
mid-level PVA and height falls. Periods of heavy snow are likely
in the terrain of the WA Cascades, Blue Mountains of OR, and
eastward into the Northern Rockies. This shortwave will advect
quickly eastward shutting off the heavy precipitation by this
evening. Before snowfall ends, heavy accumulations are likely and
WPC probabilities are high for 6" in these ranges, with more than
12" possible in isolated parts of the WA Cascades.

Late D2 into D3, another shortwave moves onshore bringing rapid
height falls accompanied by a more robust just streak to provide
deep layer ascent. The overall column is a bit colder with this
second, and mid-level WAA/moist advection ahead of the shortwave
will provide plenty of moisture for heavy precipitation noted by
IVT above the 90th percentile according to the NAEFS tables moving
onshore Washington Saturday evening. Heavy snow is likely in the
Olympics and Cascades of WA, and spreading into the OR Cascades
southward, and towards the Northern Rockies eastward. WPC
probabilities are high for 8 inches in these ranges D3, with
locally more than 2 ft possible in the highest terrain of the WA
Cascades and Olympics where upslope flow will contribute to
enhanced snowfall.



Weiss


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FOUS11 KWBC 312052
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EST Thu Dec 31 2020

Valid 00Z Fri Jan 01 2021 - 00Z Mon Jan 04 2021

Day 1...
...Southern and Central Plains to the Mid Mississippi Valley and
Southern Great Lakes...
A deep low currently moving into southern Texas is forecast to
lift north-northeast across the Ozark region and into the mid
Mississippi valley on Friday -- drawing plenty of moisture and
warm air aloft into the eastern Plains, mid Mississippi valley and
the Great Lakes. Strong upper forcing along with low-to-mid level frontogenesis is expected to produce widespread precipitation
moving north across Texas, Oklahoma, eastern Kansas and the lower
Missouri and mid Mississippi valleys Thursday night into Friday
morning, with shallow cold air supporting a wintry mix and
accumulating ice ahead of the approaching low. The 12Z model
consensus showed an additional southern shift in the axis of
heavier ice accumulations, but overall significant ice
accumulations still appear likely from eastern Kansas, and across
much of central and northern Missouri, into central Illinois.
WPC PWPF continues to show high probabilities for ice
accumulations of 0.10-inch or more across this region, with some
areas within this area likely to receive over a 0.25-inch.
Meanwhile, deepening cold air on the backside of the system is
expected to support a changeover to snow before precipitation
ends, with low-to-mid level frontogenesis supporting the potential
for a narrow stripe of heavier amounts moving north from Northwest
Texas through central Oklahoma into southeastern Kansas overnight.
The 12Z models showed a tightening of the western gradient along
with an easterly shift in the axis of accumulating snows across
this region.

As the northern edge the precipitation shield continues to move
north on Friday, the potential for accumulating ice is likely to
decrease some with the introduction of daytime heating. However,
it is expected that the cold air will be sufficient for a wintry
mix, including some freezing rain at the onset, lifting north
across southern Iowa, central to northern Illinois, northern
Indiana and southern Lower Michigan during the day on Friday.
These areas too should see a changeover to snow, with a few inches
possible before ending. Most models agreed on a slight increase
in snow accumulations extending from southeastern Iowa to Lower
Michigan.

...Central Appalachians...
As the area of low pressure approaching the Great Lakes continues
to direct moisture and warm air aloft from the central into the
eastern U.S., models show high pressure over the central U.S.,
shifting into the Northeast, with a cold air damming signature
setting up along and east of the central Appalachians Friday
morning. This is likely to produce wintry precipitation, with
areas of accumulating ice. Guidance shows the biggest threat for
significant ice accumulations centered from the Eastern West
Virginia Panhandle and western Maryland into central Pennsylvania.

Day 2...
...Pacific Northwest...
Another upper level shortwave and its associated frontal band are
expected to bring another round of wet weather, including heavy
mountain snow, into the region Friday night. While this system is
expected to weaken and move progressively east on Saturday. Deep
southwesterly flow ahead of the next approaching system will
continue to channel deeper moisture and the potential for heavy
precipitation into northwestern Washington through late Saturday.
Heavy snowfall totals on the order of a foot or more can be
expected across the higher elevations of the Olympics and northern
Cascades.

...Southern Great Lakes...
Precipitation is expected to continue across the region as low
pressure moves from the lower Ohio valley into the lower Great
Lakes Friday evening. Additional snow is expected along the
western edge of the associated comma head as it moves across
northern Illinois and southern Wisconsin before ending Friday
evening, with a wintry mix continuing across southern and central
Lower Michigan before ending as snow Friday night. Impacted areas
are likely to see no more than an additional inch or two of snow,
with up to a 0.1 inch of ice across portions of southern Lower
Michigan.

...Northern Mid-Atlantic/Northeast...
As low pressure continues to move northeast across the Great
Lakes, a wintry mix will continue to spread north from the central
Appalachians and northern Mid-Atlantic to the Northeast late
Friday into Saturday. A retreating cold-air wedge is expected to
support freezing rain at the onset, with accumulating ice expected
from central Pennsylvania northeastward to western New England.
High probabilities for 0.10-inch of ice or more are shown from the
Catskills to the Adirondacks, Berkshires and southern Greens. A
secondary low developing along the northern New England coast on
Saturday, will help maintain the cold air across northern New
Hampshire and interior Maine, where snow is expected to remain the
predominate precipitation type through much of the event. WPC
PWPF shows high probabilities for snow accumulations of 4-inches
or more across much of northern New Hampshire and central to
northern Maine.

Day 3...
...Pacific Northwest to the Northern Rockies...
A well-defined mid level shortwave and upper jet, accompanied by a
plume of deep moisture, will move into the region late Saturday.
This will support additional heavy mountain snows from the
Olympics and northern Cascades to the northern and central Idaho,
and northwestern Montana ranges -- with accumulations of a foot or
more likely across a large portion of the Washington Cascades.

Pereira

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FOUS11 KWBC 010855
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EST Fri Jan 01 2021

Valid 12Z Fri Jan 01 2021 - 12Z Mon Jan 04 2021

...Midwest/Great Lakes...
Day 1...

Anomalously strong closed low pivoting atop Oklahoma this morning
will gradually lift northeast while filling through today, before
becoming absorbed into the westerlies Saturday morning and
shifting off towards New England. This closed mid-level low will
be accompanied by a robust subtropical jet streak angling across
TX/Deep South and coupling with a robust zonally oriented jet
streak across Quebec to drive upper ventilation and surface low
strengthening from OK to IN before shifting into New England
Saturday. Deep layer ascent through mid-level divergence
downstream of the closed low, the aforementioned upper divergence,
and convergence invof surface low will produce precipitation, with
strong WAA ahead of the main system lifting isentropically atop a
surface high to expand precip north and east through the day. This
WAA will be the primary driver of precipitation today, and as the
850mb flow increases to 30-50kts from the south, an expansive area
of freezing rain is likely. There is some uncertainty continuing
into how much freezing rain will accrete due to A) wet bulbs that
are currently a bit below guidance, and B) precip rate which can
have an impact on accretion efficiency. Some sleet on the north
side of the expanding precip shield may reduce freezing rain
accretion, however, the strong of the WAA should quickly
transition much of this to freezing rain, so the bigger concern is
precip rate which could become intense at times through weak
elevated instability and pockets of high PV anomaly rotating
around the low. Despite these uncertainties, WPC probabilities for

0.25" of freezing rain are high for a narrow swath from northeast

MO through northern IL (just south of Chicago) and into southern
MI. Local maxima approaching 0.5" is possible. This echoes the WSE
mean and HREF mean values for this area.

On the far NW side of this system, a band of precipitation is
likely to dynamically cool to produce heavy snowfall across
eastern KS and into far NW MO. While precip will initially be
mixed/freezing rain, a strengthening TROWAL aloft to drive
moisture into the column coincident with intense deformation
should produce a narrow band of heavy snow with rates in excess of
1"/hr. This band is likely to be transient to the NE and will
occur after a period of warmer antecedent conditions, but WPC
probabilities are moderate for 4 inches, with locally 6" possible.


...Mid-Atlantic/Northeast...
Days 1-3...

Two separate systems will impact the Mid-Atlantic and Northeast
through the first weekend of 2021. The first is currently moving
across Oklahoma, and as this lifts northeast it will weaken and
become absorbed into the mean flow to race across the Northeast on
Saturday. Well ahead of this system, which is composed of a strong
but weakening upper trough and a modest surface low, robust WAA
coincident with mid-level divergence and modest diffluence through
a coupled upper jet structure will drive expanding precipitation
northward beginning this aftn. A cold high pressure in place ahead
of this system will be reinforced both through mid-level
confluence downstream of the approaching upper low, and also
through precipitation falling into the high driving a modest wedge
development. As WAA begins in earnest on a LLJ progged to approach
50 kts, precipitation primarily in the form of rain and freezing
rain will expand from the central Appalachians and into the
Northeast. Further north, especially across northern NY state and
into central New England, snow will be the initial p-type. While
the WAA may eventually overwhelm the wedge to turn all precip to
rain across the Mid-Atlantic and SNE, WPC probabilities indicate a
high chance for 0.25" or more of freezing rain across the Laurel
Highlands of PA, into the Poconos, and even the southern Catskills
of NY. Locally freezing rain may exceed 1/3" of an inch before
changeover. Further north and east and into some of the lower
terrain as far east as the Worcester Hills of MA, freezing rain
may accrete to 0.1".

As the surface low moves quickly eastward, the 700mb low will drag
across NNE. Points along and north of this low will remain all
snow, and this could accumulate heavily, especially across parts
of Northern NH and much of central/northern ME where persistent
WAA and intense isentropic ascent will drive a long duration
moderate to heavy snowfall. WPC probabilities for 4 inches are
high on D2 across much of northern NH and ME, and into the terrain
of upstate NY and VT. Isolated locations, especially across Maine,
may reach 10" of accumulation before the low pulls away Saturday
night.

Very quickly behind this first system, another low pressure is
likely to lift northeastward and impact the Northeast with wintry
precipitation again on Sunday. There remains considerable
uncertainty into this second system as its evolution will depend
somewhat on the outcome of the first wave. The first low will
leave a cold front/baroclinic zone across the Mid-Atlantic and
Southeast. A tough emerging from Mexico on Saturday will tilt
negatively and close off, interacting with this baroclinic zone to
drive cyclogenesis across the Southeast. As this low lifts
northeast, mid-level divergence and height falls combined with WAA
will spread precipitation northward, while the upper low
intensifies and may try to pull the surface low further to the NW.
While guidance differs on the track and hence amount of wintry
precipitation that may impact the region on Sunday, WPC
probabilities are already showing a low-mdt risk for 4" of
snowfall, highest in the terrain of the Catskills and Berkshires.


...Pacific Northwest...
Days 1-3...

A series of three distinct shortwaves embedded within confluent
mid-level flow and a modest atmospheric river will drive nearly
continuous moist advection with periods of enhanced ascent through
the weekend. The individual shortwaves are progged to arrive on
the coast of WA/OR tonight, Sunday morning, and Monday morning.
Each of these impulses will be accompanied by modest height falls
and briefly backed mid-level flow to the SW for robust moist
advection, as well as cold front which should decay as they push
inland but will aid in low-level convergence. Additionally, the
persistent westerly and confluent 700-500mb flow will produce IVT
in excess of 500 kg/m/s, highest D3, providing plentiful moisture
for rounds of heavy precipitation, including snow in the terrain
above 6000 ft early D1, falling to 3000 ft behind the first cold
front. Snow levels are progged to climb to around 5000 ft ahead of
each subsequent impulse/cold front, before crashing back to 3000
ft in their wakes. This suggests that the heaviest snow through
the 3 days will remain above 5000 ft, but with moderate to heavy
snow falling to 3000 ft at times. The ranges most impacted will be
from the Shastas/Siskiyous of northern CA, to the WA and OR
Cascades, the Olympics, and eastward through the Bitterroots,
Sawtooth, Northern Rockies, and into the Tetons. Probabilities for
6 inches from WPC increase in coverage each day, and there will
likely be some higher terrain of the Olympics and Cascades that
receive in excess of 1 ft each of the 3 days. 3-day total snowfall
is likely to reach 2-3 ft in the OR Cascades and parts of the
Northern Rockies, with isolated maxima above 5 ft possible in the
WA Cascades.


Weiss



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FOUS11 KWBC 012054
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Fri Jan 01 2021

Valid 00Z Sat Jan 02 2021 - 00Z Tue Jan 05 2021

Day 1...
...Pacific Northwest to the Northern Rockies...
An upper level shortwave and its associated frontal band are
expected to support a round of organized precipitation, including
mountain snow, as they move across the Northwest into the northern
Rockies late today into the overnight. Even though this system is
expected to quickly weaken and move east, unsettled weather is
likely to continue as strong westerly flow ahead of the next
system begins to channel deeper moisture into the region.
Guidance shows organized heavier precipitation returning to the
Northwest as this next system's associated warm front moves into
the region late Saturday. One day snow totals are likely to
exceed a foot across the higher elevations of the Olympics and
Washington Cascades.

...Northeast...
The low currently centered over the mid Mississippi valley this
afternoon will continue to move northeast toward the Great Lakes
overnight. Meanwhile, warm advection precipitation ahead of the
system will continue to interact with a cold high moving off of
the Northeast coast. This will support a wintry mix with a period
of snow, changing to sleet and freezing rain across much of
Upstate New York and western New England, with mainly snow farther
east across most of Maine. While some areas, especially portions
of the Catskills to the southern Adirondacks, and the Berkshires
to the southern Green Mountains, could see some significant ice
accumulations, the overall trend in the models has been toward a
colder solution, with a longer period of snow especially over the
Adirondack region into northern Vermont overnight. As the parent
low moves east of Lake Ontario, energy is expected to quickly
transfer to a coastal low that is forecast to quickly develop and
move from southern New England into the Gulf of Maine Saturday
morning. Low-to-mid level frontogenesis is likely to support
moderate to heavy snowfall within the deformation band setting up
north of the low. The the 12Z HREF guidance shows several members
suggesting snowfall rates of 0.5-1 in/hr across an area stretching
the North Country region of New York to western Maine Saturday
morning, with rates increasing as the band shifts farther east
across Maine during the afternoon. The latest WPC PWPF, shows
high probabilities for snow accumulations of 4-inches or more
covering a large portion of the North Country, northern Vermont,
northern New Hampshire and most of interior Maine. Probabilities
for 8-inches or more have increased across portions of Maine, with probabilities of 50 percent or greater covering a large portion of
central Maine.

Days 2-3...
...Pacific Northwest/California to the Northern Rockies...
Heavy precipitation is likely to continue, with more widespread
potential for heavy snow as the previously noted system moves
across the region Saturday night and Sunday. Additional heavy
snows are likely for the Washington Cascades, with snow levels
dropping below pass level Saturday night. Heavy accumulations are
also likely from the Wallowa Mountains in northeastern Oregon to
the central Idaho ranges, as well the Idaho Panhandle to the
northwestern Montana ranges.

As a front lingering near the Oregon-California border lifts back
to the north, this is expected to bring additional organized
precipitation, including additional heavy snows back into the
Olympics and northern Cascades early Monday. Then by late Monday,
an amplifying shortwave approaching the coast is expected to
support heavier precipitation farther south -- increasing the
potential for heavy snows across the southern Cascades into the
northern Sierra.

...Ohio Valley/Great Lakes to the Northeast...
An upper low moving into the southern Plains on Saturday is
forecast to follow a track similar to its predecessor -- lifting
across the mid Mississippi valley into the lower Ohio valley and
then the southern Great Lakes region Sunday morning. This is
likely to produce some light precipitation, including light snows
across the Ohio valley into the Great Lakes Sunday morning. At
this time, heavy amounts do not appear likely. Meanwhile, as the
system moves through the Ohio valley, a secondary low is expected
to develop over the Southeast and track to the Mid Atlantic coast
by midday Sunday. Precipitation developing between the two
centers will spread north through the northern Mid Atlantic into
the Northeast on Sunday, likely beginning as a period of snow with
light accumulations from Pennsylvania into New York and southern
New England. While the coastal low is likely to continue to
deepen as moves away from the Mid Atlantic coast, its track and
potential impacts on the Northeast are far from certain. While
the 12Z GFS takes the low closer to the coast -- showing some
potential for significant snows developing across New England on
Monday -- the general consensus of the remaining models is farther
east, with lighter snow accumulations through late Monday.

Pereira

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