045
AWUS01 KWNH 170930
FFGMPD
TXZ000-171400-

Mesoscale Precipitation Discussion 0280
NWS Weather Prediction Center College Park MD
529 AM EDT Fri May 17 2024

Areas affected...Lower Rio Grande Valley in Southwest Texas...

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

Valid 170930Z - 171400Z

SUMMARY...Slow moving thunderstorms with ample moisture flux;
within a favorable upstream redevelopment environment may support
locally intense thunderstorms capable of 2"/hr rates and possible
flash flooding.

DISCUSSION...GOES-E 3.9um SWIR and local observations denote the
convectively reinforced outflow boundary/cold front has begun to
slow its southward push in proximity to MFE/EBG in Deep South
Texas with a surface wave across the Rio Grande into Old Mexico.
From there, low stratus denotes the cold front is banked up across
the Serranias del Burro though deep moist air continues to flow
over it from two directions.  The cold air is fairly shallow so
925-850mb return south-southeasterly flow continues to pump high
theta-E air along/north of the border while moderately moist
Pacific air over-tops through the 700-500mb per CIRA LPW.  This
supports 1.75-1.9" total PWATs but heavily loaded given upper 70s
moisture is isentropically ascending over the shallow cold front
as far north as NW Maverick county and points west.  MUCAPEs are
reaching 3000 J/kg and with eroding capping.

Aloft, a strong 90+kt 3H jet streak is crossing the area with a
subtle shortwave inflection just upstream just south of the Big
Bend of TX starting to press eastward providing upper-level
favorable ascent.  As such, scattered discrete convective
initiation has occurred across S Dimmitt, Webb and LaSalle
counties with the strongest cell just crossing the River at this
time.  However, high shear environment will support some hail
generation initially, KDP suggest solid mixture with heavy
rainfall as well. Additionally, strong effective bulk shear over
90kts suggest favorable organization and Bunker`s Rgt moving
Propagation vectors suggest slowing forward speed to about
15-20kts for any developing rotating updrafts to allow for
increased duration.  In addition, orientation of convection and
favorable upstream back-building environment suggests flanking
lines may be slow to propagate eastward allowing for additional
updraft towers for repeating.

Given potential for 1.75-2"/hr rates and repeating, localized
spots of 2-4" are possible over the next few hours; particularly
near/along the Rio Grande and points west, due to proximity to
deepest moisture return/unstable mid-level environment that
reduces in effectiveness/stabilizes further east into central and
eastern South Texas.  While area has been fairly dry and FFG
values are naturally higher with 3-4"/3hr range, the potential for
flash flooding will be localized to those locations of 3-4" in
1-3hrs, and so coverage should be scattered to widely scattered
and considered possible.

Gallina

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

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

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

LAT...LON   28959923 28659867 28079838 27609821 26999815
            26629842 26579905 26809944 27159970 27689997
            28040019 28490049 28909997