(cont'd)

A Moderate Risk was considered for northern MS/northern AL for Day
2, but given the expected distribution of the instability gradient
and the spread in the placement of the axis of highest rainfall
(which is further north that the 22/12z model suite), that route
was eschewed for this forecast, and this decision was collaborated
with WFOs JAN/MEG/BMX/HUN. Instead, a large Slight Risk was placed
over much of the interior of the Southeast states and near TN
Valley. If there is better agreement with respect to the place of
the best instability, a Moderate Risk could be needed in later
forecasts.

Further south, instability is better across southern MS/southern
AL into the western FL Panhandle, with MUCAPE values between
1000/2000 J/KG. Inflow of 2.00/2.25 inch precipitable water air
within the instability axis could support bands of convection
tracking north into these areas from the northern Gulf of Mexico.
Banding should be fairly progressive as the mid level flow becomes
more westerly at the base of the mid level trough to the north.
Still, hourly rainfall rates here could peaking near 2.00 inches,
with local 3.00+ inch rainfall amounts possible, despite
relatively modest model QPF amounts. Because the ingredients for
flash flooding are expected to be in place, the Slight Risk was
extended southward to cover the threat on Day 2.

...Upper Mississippi Valley/Upper Great Lakes...
Ongoing convection associated with short wave energy tracking
across the Upper MS Valley and Upper Great Lakes during the first
part of Day 2 could still pose a flash flood threat before the
available instability/moisture are shunted to the southeast.
Convection is expected to track along a frontal boundary extending
from central MN into northern WI, where 1000/1500 J/KG of MUCAPE
is expected to remain in place through the morning hours. Moisture
in the column should be sufficient to support convection producing
local hourly rainfall amounts of 1.25 inches as storms cross far
east central MN into northern and central WI before 24/18z.

There was a fair amount of spread concerning the track of the
convection, with the 00z NAM/CONUS taking the convection southeast
more quickly, suggesting the storms become more outflow dominated.
The 00z WRF ARW seemed to possess the best continuity with the
convection from Day 1 into Day 2, and this model was the primary
source for the WPC QPF and Excessive Rainfall. Should the storms
become more outflow dominated (as depicted by the 00z NAM CONUS
Nest), the flash flood threat drops, and the Marginal Risk may no
longer be necessary.

...Pacific Northwest...
The tail end of the first pulse of atmospheric river moisture
crosses WA during the first part of Day 2, as the weakening plume
crosses from WA into the northern Rockies. Hourly rainfall rates
of 0.50 inches are possible across the Olympic and central and
northern Cascades, mainly before 24/18z. Rainfall amounts of 3.00+
inches are possible in the favored upslope areas, before the best
synoptic scale lift exits. When combined with the rainfall
expected during Day 1, there could still be continuing issues with
rivers and streams before the highest rainfall rates drop off
before 25/00z.

After a lull, the next short wave in the flow interacts with the
next piece of the atmospheric river approaching the WA coast
between 25/06z and 25/12z. As IVT values again increase to 750
kg/m/s, and the low level west southwest flow focuses the
1.00/1.25 inch precipitable water air on the terrain, rainfall
rates could again increase to near 0.50 inches toward 25/12z.
There was a fairly good signal for local rainfall amounts between
3.00/4.00 inches during Day 2 between the two shots of moisture,
so a Marginal Risk was placed over the terrain to cover the threat.

Hayes


Day 3
Valid 12Z Fri Sep 25 2020 - 12Z Sat Sep 26 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE MID
ATLANTIC AND SOUTHWEST FLORIDA...

...Mid Atlantic...
As the remnants of Beta are carried by a weakening mid level
trough across the Mid Atlantic during Day 3, deep moisture and
marginal instability could support convection capable of producing
heavy to locally excessive rainfall. There is still some spread
concerning the timing and placement of the remnants, which has
some impact on which area of the Mid Atlantic see the deeper
moisture and instability. The 00z NAM and 00z ECMWF appear to
continue their respective trends of taking the surface system too
far north, allowing instability to extend further north into the
Mid Atlantic states. The 00z GFS also continues its' trend of
being too fast with the mid level trough, and taking the surface
feature offshore too quickly. The 00z UKMET seems as though it is
a fair proxy for the timing and placement of the Beta remains, so
it was used as the basis for the WPC and Excessive Rainfall
Outlook.

The mid level trough tracking from the OH Valley to the Mid
Atlantic states during Day 3 weakens as it moves eastward. Though
the remnants of Beta could be almost indistinguishable by Day 3, a
20/30 knot low level southwest flow is expected to transport 1.75
inch precipitable water ahead of the trough from GA into NC.
Within the moisture plume, 500 J/KG of MUCAPE is expected to
support low topped storms. Deep moisture in the column could
support hourly rainfall rates of 1.25 inches, particularly over
central NC into far northeast SC. Local rainfall amounts could top
2.50 inches in spots, but the relatively progressive nature of the
system could limit the threat of training, effectively capping
rainfall amounts here.

For now, a Marginal Risk was placed over a portion of the Mid
Atlantic (mainly NC/SC) in deference to the moisture into the
column, as well as the low level forcing. Should the faster 00z
GFS prove more correct, the threat for flash flooding would drop,
and it is possible that the Marginal Risk could be removed in
subsequent forecasts if there is a faster trend in later model
guidance.

...Southwest Florida...
Deepening moisture and instability focused on a developing frontal
boundary across South FL could support convection producing heavy
to excessive rainfall, particularly over southwest FL. While much
of the guidance suggests that a frontal boundary in the vicinity
of South FL could focus the deep moisture and instability, there
is a fair amount of spread in not only the placement of higher
rainfall amounts, but the maximum amounts as well. The 00z
ECMWF/NAM are more indicative of expected conditions over South FL
during Day 3, so these models were used as a template for the WPC
QPF and Excessive Rainfall Outlook.

Low level convergence along the front extending from the Upper
Keys into southwest FL during the first part of Day 3 should be
sufficient to focus the deep moisture plume (with precipitable
water values near 2.25 inches, which is between two and three
standard deviations above the mean) and instability (as MUCAPE
values reach 1000/1500 J/KG) to support developing storms between
25/12z and 25/18z. Given the deep moisture in the column, and
increasing warm cloud heights, hourly rainfall rates could exceed
2.00 inches, though this is not explicitly forecast by the 00z
regional/global guidance.

Since South FL is expected to remain under the flat mid level
ridge, storm motions are expected to remain light (generally under
10 knots), and storms that develop over the waters should tend to
track over southwest FL. Short term training is possible where
storms come ashore, and where this occurs, the 00z NAM shows the
potential for local 4.00 inch rainfall amounts. The main threat
should end sometime near 26/00z, as the low level convergence
along the front relaxes, and instability wanes. It is possible
that the storms remain largely offshore, as some of the 00z
regional/global models indicate, but since the ingredients for
flash flooding are expected to be in place, a Marginal Risk was
placed over southwest FL on Day 3 to over the threat.

...Pacific Northwest...
After the initial moisture plume pushes across WA during the first
of of Day 3, there could be another lull in the deep moisture
until late in the period. During the lull, the upslope flow
weakens to a degree, and moisture values drop as the best of the
early plume heads for the Northern Rockies. Rainfall amounts
during Day 3 appear to be fairly modest, with the favored upslope
areas receiving 2.50 inches or less. Though these areas should
still be wet from heavy rainfall during Day 1 and a portion of Day
2, it is unclear where the additional rainfall will aggravate any
potential flooding from these days. Given the lack of deeper
forcing, no excessive was assigned for Day 3 in this forecast.

It appears as though the next slug of deeper moisture (with
precipitable water values increasing once again to 1.00/1.25
inches) waits for the next short wave in the flow, which appears
to impact the Pacific Northwest after 26/12z. Should later model
solutions indicate as faster timing with the next plume of
significant moisture, a Marginal Risk could be needed for Day 3 in
subsequent forecasts.

Hayes

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt

$$
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