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WPC Met Watch

Mesoscale Precipitation Discussion: #1137
(Issued at 126 PM EST Thu Nov 25 2021 )

MPD Selection

Graphic for MPD #1137

Mesoscale Precipitation Discussion 1137
NWS Weather Prediction Center College Park MD
126 PM EST Thu Nov 25 2021

Areas affected...south Texas

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

Valid 251825Z - 260025Z

Summary...Gradual destabilization is resulting in a few areas of
deepening convection.  These trends should continue through the
afternoon along with a spotty flash flood threat near the Rio
Grande and south Texas coast.

Discussion...A seasonably strong cold front was making steady
southward progress through central/south Texas as of 18Z.  South
of this front, isolation and weak low-level warm advection was
allowing for maintenance of a moist, weakly unstable, and weakly
capped airmass characterized by 1.6-1.8 inch PW values and 1500
J/kg MUCAPE.  A few areas of slow-moving convection were noted and
focused along a couple of surface features: 1) the
southward-moving cold front and 2) along a weak/diffuse trough
oriented parallel to the Texas/northeastern Mexico coastline. 
Based on MRMS analyses, these efficient rainfall producers were
resulting in estimates of 1-2 inch/hr rainfall rates in a few
spots - though these rates were generally below FFG thresholds

The approaching front, weak surface convergence along the front,
and localized convective processes (i.e., outflows) are expected
to maintain slow-moving convection across the discussion area as
long as sufficient surface-based instability can persist inland
ahead of the surging front.  Models/observations suggest that this
potential should exist through at least 22-23Z, with the longest
residence time of unstable area (and greatest flash flood risk)
located along the south Texas coast and Rio Grande areas.  Where
convection is able to persist and/or repeat, a quick 2-3 inches of
rainfall will be possible.  These rates are most likely to
overwhelm hydrologically sensitive (i.e., urbanized) areas
especially near Brownsville Metro - especially if convection near
the coast can develop westward/inland this afternoon.  The overall
flash-flood threat is expected to remain fairly isolated and
spotty, however.




LAT...LON   28439683 28379652 27599702 26939723 26329714
            25969711 25759733 25749805 26239896 26829947
            27389953 27689955 28239899 28299779

Last Updated: 126 PM EST Thu Nov 25 2021

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