271
AWUS01 KWNH 210112
FFGMPD
MSZ000-LAZ000-ARZ000-TXZ000-210600-

Mesoscale Precipitation Discussion 0964
NWS Weather Prediction Center College Park MD
911 PM EDT Wed Aug 20 2025

Areas affected...Southern Arkansas, western Mississippi, northern
Louisiana

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

Valid 210111Z - 210600Z

Summary...Showers and thunderstorms will continue for several more
hours ahead of a cold front before waning tonight. Rainfall rates
of 2-3"/hr, while generally brief, will produce 2-3" of rain.
Instances of flash flooding are possible.

Discussion...A cold front analyzed by WPC dropping slowly across
southern Arkansas will continue to drift into Louisiana tonight.
The tail end of this front will continue to provide the impetus
for ascent through low-level convergence, aided by a subtle
shortwave dropping concurrently within the general weak northerly
flow regime. Moisture across the region is being channeled ahead
of the front and is analyzed by the SPC RAP to be above 2 inches
even in the pre-convective airmass, with GPS measurements
approaching 2.3 inches within convection. This extreme PW is
overlapping MUCAPE that is still 2000-3000 J/kg despite SBCIN,
supporting the regenerating convection seen over the past two
hours. Rainfall rates have been estimated by local radars to be
above 2.5"/hr, and MRMS 15-min rainfall has been as high as 0.8"
in a few areas (above 3"/hr measured rainfall rates).

The CAMs are struggling to resolve the current activity which is
both much more intense and widespread than progs would suggest.
With the front in place, aided by the weak shortwave energy aloft,
and modest 700mb convergence, it has become more likely that
thunderstorms will persist well after dark. This is despite a lack
of bulk shear keeping storms of the pulse variety, but outflow
boundary collisions and storm mergers in response to the
widespread nature will support additional development. With storm
motions generally slow and chaotic, and with some upstream
development also possible as propagation vectors collapse to 5 kts
and orient obliquely right of the mean wind, these intense
rainfall rates (3"/hr at times continuing) could produce 2-3" of
rain, with isolated higher amounts possible. This is in addition
to rain that has already fallen.

FFG across the area is quite elevated, generally 3-4"/3 hrs and
even 2-3"/1hr. While HREF exceedance probabilities are modest,
less than 10%, this is clearly under-forecasting the true risk.
Additionally, with rainfall rates as intense as they have been,
and with these likely to continue, 15-min rainfall of 0.75-1 inch
in some areas could quickly overwhelm soils to become runoff
despite the elevated FFG. For this reason, any storms that
continue until full convective overturning later tonight could
produce instances of flash flooding.

Weiss

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

ATTN...WFO...JAN...LCH...LIX...LZK...MEG...SHV...

ATTN...RFC...ALR...FWR...ORN...TUA...NWC...

LAT...LON   34798999 34728899 34478866 33938870 33398899
            33018941 32689000 32219065 31629093 31189131
            30879184 30749260 30759312 31209370 31869387
            32399385 33089345 33939271 34399201 34699105