857
AWUS01 KWNH 220126
FFGMPD
LAZ000-TXZ000-220630-

Mesoscale Precipitation Discussion 0970
NWS Weather Prediction Center College Park MD
925 PM EDT Thu Aug 21 2025

Areas affected...Coastal Plain from the Middle TX Coast through
South-Central LA

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

Valid 220125Z - 220630Z

Summary...Showers and thunderstorms will continue to develop and
move slowly along the TX/LA coasts this evening. Rainfall rates
may at times reach 4"/hr, which could cause 2-3" of rain with
locally higher amounts and instances of flash flooding.

Discussion...The regional radar mosaic this evening shows an
expansion of very slow moving thunderstorms aligned near the Gulf
Coast from far SW LA through near San Antonio Bay, TX. This
activity is blossoming in response to convergent ascent as
low-level north winds over the land intersect east winds on the
coast. This convergence is occurring beneath an elongated
mid-level trough axis with subtly steeper lapse rates, providing
additional ascent to the area. PWs measured on the KCRP and KLCH
were around 2.1 inches, near the 90th percentile for the date,
which is overlapping with MUCAPE of more than 2000 J/kg to produce
an impressive thermodynamic environment. This environment is
supporting radar-estimated rainfall rates from KHGX that have been
2.5-3"/hr this evening.

Although the CAMs feature a high variability in placement of heavy
rain, there is good agreement that heavy rain amounts will occur
during the next few hours. Moisture convergence along the coast
may elevate PWs to 2.25 to 2.5 inches, supporting even more
intense rain rates as forcing for ascent persists. This suggests
that even with loss of daytime instability, the elevated MUCAPE
will support intense rainfall, for which the HREF neighborhood
probabilities forecast a 40-60% chance of exceeding 2"/hr, with
brief rates reaching 4"/hr possible as suggested by the HRRR
15-min rain accumulation product showing up to 1".

While these rates themselves could be sufficient to overwhelm
soils and lead to rapid runoff, the storms containing this intense
rain will also be very slow moving to potentially prolong this
prolific rainfall. 0-6km mean winds of just 5 kts will cause
storms to drift generally to the east, but a lack of bulk shear
indicating pulse-type convection will also result in chaotic
motion during storm mergers and boundary collisions. This could
lengthen the duration of heavy rain, or cause multiple rounds, in
a few areas, leading to amounts that could reach 2-3" or locally
higher (HREF and REFS probabilities for 3"/6hrs over 30%).

FFG across the region is elevated at generally 3-4"/3hrs for which
exceedance probabilities are modest. Despite this, at least
isolated instances of flash flooding appear possible due to the
intensity of this rain, with the most likely impacts occurring
should any storm linger over more urban areas.


Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...CRP...EWX...HGX...LCH...

ATTN...RFC...FWR...ORN...NWC...

LAT...LON   31139339 30959251 30359179 29809173 29599191
            29529221 29499282 29319376 29049457 28719532
            28259625 27869679 27679716 27719746 27879754
            28379763 28789746 29379713 29819673 30589581
            30819514 30999448