Mesoscale Precipitation Discussion 0085
NWS Weather Prediction Center College Park MD
115 PM EDT Wed Mar 26 2025

Areas affected...South-Central and Deep South TX

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

Valid 261715Z - 262315Z

SUMMARY...Developing and expanding clusters of heavy showers and
thunderstorms are expected through this afternoon. Very heavy
rainfall rates and locally slow cell-motions will support an
increasing flash flood threat.

DISCUSSION...The midday GOES-E satellite imagery shows a
well-defined mid-level shortwave trough ejecting east across
northwest mainland Mexico which will begin to cross the Rio Grande
Valley by later today. Increasingly divergent flow aloft ahead of
these height falls along with some smaller scale vort energy will
be gradually overspreading the lower Rio Grande Valley and
adjacent areas of south-central TX and will be interacting with an
increasingly moist and unstable airmass.

MLCAPE values over much of the region are already locally on the
order of 1500 to 2000 J/kg, with PWs of 1.5 to 1.75 inches.
Persistent low-level south-southeast flow will continue to bring
moisture up across much of southern TX in general going through
the afternoon hours, and the latest CIRA-ALPW and MIMIC-TPW data
sets show notable concentrations of deeper and more anomalous
moisture advecting into the lower Rio Grande Valley ahead of the
upstream mid-level height falls/shortwave energy.

This will set the stage for developing and expanding clusters of
heavy showers and thunderstorms going through the afternoon hours.
The convective cells in this moistening environment should become
increasingly efficient and will be capable of producing very high
rainfall rates considering the level of instability and some
uptick in updraft helicity parameters.

The 12Z HREF guidance suggests the stronger and more organized
convective cells may be capable of producing 2 to 3 inch/hour
rainfall rates. A combination of multicell and some isolated
supercell structures are generally expected to evolve over time,
and some of the cells that become more deeply rooted are expected
to potentially become very slow-moving with cell-motions a bit to
the right of the deeper layer mean flow. This will support some
locally enhanced rainfall totals, with some rainfall totals by
early this evening of 3 to 5+ inches possible.

Antecedent conditions are very dry, but with such high rainfall
rates and potential for slow cell-motions, the threat for flash
flooding will be increasing with time. Additional MPDs will likely
be required this evening to further address the excessive rainfall
threat going into the overnight hours.

Orrison

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

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

LAT...LON   29289749 28919687 28229668 27529713 27149726
            26709731 26009720 25929757 26109820 26369895
            26729931 27479964 27839989 28240022 28800007
            29279882