AWUS01 KWNH 262358
FFGMPD
MSZ000-LAZ000-ARZ000-TXZ000-270500-

Mesoscale Precipitation Discussion 1194
NWS Weather Prediction Center College Park MD
658 PM EST Thu Dec 26 2024

Areas affected...southeastern TX into western LA

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

Valid 262356Z - 270500Z

SUMMARY...Training and repeating of thunderstorms will continue a
flash flood threat from southeastern TX into western LA through
05Z. Rainfall rates of 1-2 in/hr and localized totals of 2-4
inches can be expected.

DISCUSSION...Regional radar imagery at 2330Z across the South
showed a line of thunderstorms extending from northwestern LA into
the Piney Woods of southeastern TX with a second line extending
from Jasper/Newton counties to roughly Matagorda Bay. These storms
were occurring just ahead of a pre-frontal trough/wind shift
located east of a cold front moving through east-central TX. SPC
mesoanalysis data from 23Z showed PWATs of 1.4 to 1.7 inches
coincident with the thunderstorm axis and MLCAPE of 500-2000 J/kg
from the central TX/LA border to the mid TX coast. Favorable shear
profiles ahead of a potent mid-level vorticity max (over north TX)
have supported supercells with enhanced rainfall rates while mean
steering flow from the southwest has resulted in areas of training
and/or repeating rounds of heavy rain from the northern TX/LA
border into the southeastern TX Coastal Plain.

Over the next 3-5 hours, the best upper level support (PVA, upper
diffluence/divergence) will shift eastward from the Arklatex into
northern LA/southern AR. However, RAP forecasts of instability
show only a small pocket of 500-1000 MUCAPE just ahead of the
mid-level low center which should be steadily tracking east
through the first half of tonight. Due to lower instability
profiles, a lower end flash flood threat will exist for these
northern locations where strong forcing will should be balanced by
a fairly progressive translation of heavy rain toward the east and
limited instability.

Farther south, a lack of height falls for the Gulf Coast region
should result in a slower eastward progression of heavy rain. It
is here where greater instability is forecast to reside within the
warm sector (500-1500 J/kg MLCAPE). 850 mb winds of 40-60 kt from
the southwest and similarly oriented mean steering flow will allow
for areas of training and repeating thunderstorms capable of 1-2
in/hr (perhaps locally above 2 in/hr) due to a slower eastward
progression to the axis of thunderstorms. 2-4 inches of rain over
a roughly 2 hour window is expected to result in a couple of areas
of flash flooding through 05Z from southeastern TX into western LA.

Otto

ATTN...WFO...HGX...JAN...LCH...LZK...SHV...

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

LAT...LON   33949177 33509109 32649100 31619139 30769200 
            29699292 29499407 29039513 29219549 30059521 
            31439447 32449392 33769307 

$$
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