250
AWUS01 KWNH 151801
FFGMPD
TXZ000-152300-

Mesoscale Precipitation Discussion 1015
NWS Weather Prediction Center College Park MD
201 PM EDT Sun Sep 15 2024

Areas affected...South Texas

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

Valid 151800Z - 152300Z

SUMMARY...Slow moving showers and thunderstorms will pose a
localized threat of flash flooding for southern TX through the
late evening. Rainfall rates of 1-2 in/hr, but with localized
maxima near 3 in/hr, will be possible along with totals of 3 to 5+
inches.

DISCUSSION...Local radar imagery out of southern TX at 1730Z
showed showers and thunderstorms extending from near Laredo to the
US 281 corridor near Encino along with isolated development near
the Laguna Madre. Cells appear to be forming on an elevated
convergence axis tied to the 850-700 mb remnants of an old frontal
boundary that extended westward through the Gulf of Mexico into
southern TX, as seen on Layered PW Imagery from CIRA and in recent
visible satellite imagery. 850-300 mb mean layer winds of only
5-10 kt from the S or SW were supporting the slow cell movement.
The environment contained PWATs of 2.0 to 2.5 inches (highest to
south) along with 1000-2000 J/kg MLCAPE. Moisture contributions in
the middle/upper levels were tied to former T.S. Ileana in the Sea
of Cortez and low level southeasterly winds funneling moisture
into the region from northeastern Mexico and the adjacent Gulf
waters.

An elongated 700 mb low on the border of Coahuila and Nuevo Leon
is expected to remain roughly in the same place over the next 6
hours per recent runs of the RAP which will keep low level
convergence focused across southern TX. In addition, a forecast
for increasing easterly 850 mb winds by 00Z, reaching 10-15 kt
between CRP and BRO, may support some slow moving cells near the
coast depending on cell evolution with time given slow steering
flow. Some southward propagation into the instability is also
expected in the short term with instability likely diminishing
from west to east over time. Slow cells movement will be capable
of rainfall rates between 1-3 in/hr and localized areas of flash
flooding that may end up being more of an urban flood threat due
to relatively dry antecedent ground conditions, outside of
isolated portions of far south TX.

Otto

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

ATTN...WFO...BRO...CRP...

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

LAT...LON   27879923 27419751 26959713 25959684 25649723
            25809821 26199917 26999959 27479973 27819956