AWUS01 KWNH 311440
FFGMPD
LAZ000-TXZ000-312015-

Mesoscale Precipitation Discussion 0883
NWS Weather Prediction Center College Park MD
940 AM EST Thu Dec 31 2020

Areas affected...Southeast TX...

Concerning...Heavy rainfall...Flash flooding possible

Valid 311445Z - 312015Z

SUMMARY....Intersection of greatest rain-rates/totals are
decoupling from areas of lowest FFG/ground saturation; however,
ingredients for isolated flash flooding rain-rates still remain
possible particularly in urban settings and near the coast through
early after-noon.

DISCUSSION...GOES-E WV depicts a very strong winding up closed low
across Northeast Mexico with very impressive baroclinic leaf
across much of TX. The very sharp distinct mid-level drying at
the nose of stronger height-falls/core of the approaching
shortwave rotating around the base/SE side of the cyclone is
advancing through Northern Nuevo Leon. Very strong DPVA
associated with this shortwave, along with favorable placement of
the 120kt 3H jet core just south of the TX coast provides ample
support for low-level wind increases and surface-low level
cyclogenesis. As such, low level wind response is further
strengthening FGEN forcing along the slowly advancing cold front
that extends from the surface wave near ARM due south, while very
strong warm advection is pumping western Gulf moisture/instability
across coastal Southeast TX (south of the stationary front from
said surface low to s of LFK to AQV to ESF to HEZ. Solid MLCAPE
of 500-750 along with 1.5" total PWATs will be increasing due to
the surface/low level response and perhaps some weak solar
insolation by midday reaching 1000-1250 J/kg and 1.75" and
20-25kts of SSE flow converging with nearly due west flow at the
cold front. Combined with frictional convergence at the coast,
thunderstorms capable will start around 1-1.25"/hr rates possibly
becoming 1.5"-1.7/hr through the early afternoon (18-20z).

While the rainfall environment is deteriorating through the
afternoon, the hydrologic situation may be a bit better.
Currently, the cold front/axis of heavy rainfall is along the
eastern edge of the heavy rainfall footprint from over night where
greater than 5" was observed across Jackson to Waller counties
where FFG is compromised to less than .5" through all time-steps
(1/3/6hrs). This axis continues northeast through Trinity and
Polk counties though lower totals were observed with blow-off
showers from the deeper convection. Still, this is aligned with
strong orthogonal intersection with the stationary front to keep
some isolated convective development lingering and compounding
longer duration totals. With the approach of the shortwave,
there remains a hour or so of slow eastward motion before deeper
upper-level steering flow and height-falls support eastward
propagation vectors of the cold front itself. Cells may continue
to train northward along the cold front but overall motions should
reduce totals at a given location or over saturated areas.
However, this also moves the axis through the urban corridor of
the Houston Metro where naturally greater run-off will occur with
hydrophobic ground conditions...so with 2-3" totals to be
expected, flash flooding remains possible here before further
moving west with the strengthening rates by early afternoon.
Currently, the NMMB has been trending the best both in placement
and magnitude likely due to the greater synoptic forcing over the
thermodynamic forcing, though all Hi-Res CAMs are trending a tad
faster on the cold front reducing totals slightly.

A secondary elevated band of convection currently observed at the
leading edge of the 85-7H front/wind shift associated with the
shortwave aloft, lingering mid-level moisture will continue to
support some moderate showers and perhaps an additional .5" totals
which given that lower FFG values may once again yield some
further flooding concerns. There will be a pivot to the TROWAL
across SE TX that may allow for greater duration if at a lower
rain-rate through upglide process rather than vertical/convective
processes.

Gallina

ATTN...WFO...HGX...LCH...

ATTN...RFC...WGRFC...NWC...

LAT...LON 30949474 30609387 29619388 28989489 28429611
29089631 29539633 30179620 30779549
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AWUS01 KWNH 311946
FFGMPD
LAZ000-ARZ000-TXZ000-010100-

Mesoscale Precipitation Discussion 0884
NWS Weather Prediction Center College Park MD
246 PM EST Thu Dec 31 2020

Areas affected...Eastern TX...Western LA...ext Southwest AR...

Concerning...Heavy rainfall...Flash flooding possible

Valid 312000Z - 010100Z

SUMMARY...Multiple bands starting to merge along cold front with
occasional rain-rates up to 1.75"+/hr though with increasingly
progressive cell motions pose possible flash flooding

DISCUSSION...GOES-E WV suite continues to show the inner core of
the closed low shifting eastward with the strengthening eastern
side shortwave. This continues to support solid surface
cyclogenesis near the Houston Metro. This also drawn the
stationary front across LA further south while steepening the
isentropic packing along the front. As such, the very strong
nearly orthogonal boundary layer southeasterly flow and
strengthening 85H southerly to around 45-50kts. This leads to
deeper moisture flux convergence and vertical development and
therefore higher depth of rainfall generation. This also narrows
the TROWAL angling back northwest to the motion of the surface
low. The strong DPVA combined with diffluent 3H flow within the
Left-Exit Region of the 125kt jet should maintain the deepening of
the cyclone and isallobaric component for further rainfall
efficiency as the 850mb front also aligns with the surface
boundary around 22-23z. Given MUCAPES pushing 1000 J/kg and total
PWs around 1.75" and strong flux convergence, rainfall efficiency
of 2"/hr are possible with perhaps some occasional sub-hourly
bursts of higher intensity with merging cells just northeast of
the surface cyclone where streams converge greatest.

Greater veering flow aloft and increasing steering flow as the mid
to upper level jet streams northeast, the low level warm
conveyor/TROWAL should allow slightly elevated remaining
convective elements to stream well north into East-Central TX and
Northwest LA. However, increased speeds will also reduce duration
of the 2"+/hr rates with expectedtotals of 2-3" genearlly along a
line from LFK to JAS to between IER/POE where cells may linger a
bit longer along/just north of the front before upgliding
producing scattered streaks extending northward. This is best
represented by recent HRRR and HRRRv4 solutions. After 00z, the
mid-level forcing wave may begin to overtake the lower-level
surface boundary and start to reduce best deeper vertical ascent
and reduce overall coverage as cells expand into central LA, and
across areas of drier ground conditions, making flash flooding a
bit less possible, though lower FFG values near SHV and into AR
will need to monitored closely.

Gallina


ATTN...WFO...FWD...HGX...LCH...SHV...

ATTN...RFC...LMRFC...WGRFC...NWC...

LAT...LON 33429362 32869264 31129287 29749312 29479443
29969478 30569513 31219526 31779547 32449556
33039542 33389472
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AWUS01 KWNH 021453
FFGMPD
FLZ000-GAZ000-022000-

Mesoscale Precipitation Discussion 0002
NWS Weather Prediction Center College Park MD
953 AM EST Sat Jan 02 2021

Areas affected...FL Panhandle... Southern GA

Concerning...Heavy rainfall...Flash flooding likely

Valid 021445Z - 022000Z

SUMMARY... Training band of heavy rain and embedded thunderstorms
to support a growing threat of flash flooding this morning and
into the afternoon hours.

DISCUSSION... GOES 16 IR imagery shows cold cloud tops associated
with deep convection over the Gulf of Mexico. This area of
convection is focused along a quasi-stationary frontal boundary
positioned from South Carolina and southern Georgia extending
southwest through the the Florida Panhandle and into the Gulf of
Mexico. The front is located between a deep upper trough over the
south-central U.S. and a subtropical high over the northern
Bahamas. 12Z sounding out of TLH sampled an environment favorable
for heavy rainfall with a deep warm cloud layer that extended more
than 10 kft up into the column. In addition, 850-300mb mean winds
are out of the SW, oriented parallel to the frontal boundary.
Ample atmospheric moisture is available with PWs of 1.78" and
skinny MUCAPE was also present. This environment is also expected
to translate farther north and east into south-central Georgia
this morning.

As the day progresses, the slow moving nature of the frontal
boundary will remain a trigger for additional heavy rain and
thunderstorms. Available MUCAPE may vary at times but generally
should hover close to 500 J/kg into the early afternoon hours.
Area averaged HRRR soundings early this afternoon suggest PWATs
hover around the 1.8-1.9" range with some guidance indicating PWs
up to 2" possible later in the day. Over the last several hours
some locations have already reported over 2" of rainfall with
hourly rates over 1" within heavier cells. The training of deep
moist convection over these areas are likely to produce additional
rainfall totals between 2-4" inches over the next five hours with
locally higher amounts possible where the most intense
thunderstorms take shape. There are already reports of flash
flooding and with overly saturated antecedent conditions having
already occurred, additional areas flash flooding are likely to
transpire throughout the day.

Mullinax

ATTN...WFO...CHS...FFC...JAX...TAE...

ATTN...RFC...SERFC...NWC...

LAT...LON 32358213 32128172 31768162 31158234 30288360
29708456 29438524 30038557 31308416 32028295
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AWUS01 KWNH 251934
FFGMPD
TNZ000-KYZ000-ILZ000-260003-

Mesoscale Precipitation Discussion 0012
NWS Weather Prediction Center College Park MD
233 PM EST Mon Jan 25 2021

Areas affected...Southern Kentucky

Concerning...Heavy rainfall...Flash flooding possible

Valid 251933Z - 260003Z

Summary...Showers and thunderstorms this afternoon will be capable
of producing hourly totals up to 1" with totals through early
evening up to 2-2.5". This rain falling over some areas that have
already picked up 1-2" (locally higher) since early this morning
could cause additional runoff and flooding problems, particularly
for portions of southern/south-central Kentucky.

Discussion...Low pressure analyzed over northwest Arkansas
recently with its wavy boundary/warm front now situated near the
KY/TN state line as of 18Z. Waves of overrunning moderate to
embedded heavy rain have been ongoing since this morning where
12-hour estimated QPF shows widespread 1-2" totals with some
localized amounts near 3" for portions of southwest KY.

Through 00Z, the warm front over Tennessee should lift further
north into southern/south-central Kentucky ahead of the
approaching low and cold front passage, allowing for a brief wedge
of better instability (RAP suggests upwards of 500 to perhaps 750
J/kg by 22Z) to accompany the expected showers and thunderstorms
aided by a persistent low level jet of 50-60 kts at 850 mb. The
much above normal moisture in place (PWs 1 to 1.3 inches) and
sufficient low level convergence will provide the necessary
forcing and moisture.

There is fairly good agreement in the latest hi-res guidance with
the most recent runs of the HRRR performing well with the totals
so far. Stripes of higher totals across central Kentucky are
expected through 00Z with totals of 1-1.5" likely coming from
hourly totals 0.5-0.75" at times, supported by the 12Z HREF.

The most recent MRMS QPE to FFG ratios are approaching 1 mainly
south of the KY Parkways across western/central Kentucky while
areas around Hopkinsville are currently between 1-1.3. Soil
saturation is up to 85-95 percent across much of the area. As
such, if the additional rain does line up over areas saturated
from this morning, then further runoff/flooding problems will
develop, especially across southwest into south-central Kentucky.

Taylor


ATTN...WFO...LMK...MEG...OHX...PAH...

ATTN...RFC...LMRFC...OHRFC...NWC...

LAT...LON 37608517 37198492 36708645 36618728 36438857
37148861 37528719
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