Mesoscale Precipitation Discussion 0423
NWS Weather Prediction Center College Park MD
355 AM EDT Thu Jun 12 2025

Areas affected...TX Triangle region into the Middle and Upper TX
Coast

Concerning...Heavy rainfall...Flash flooding likely

Valid 120755Z - 121300Z

Summary...Localized hourly rainfall totals as high as 3-4"
expected to continue into mid-morning. Numerous instances of flash
flooding are likely (some life threatening to locally extreme)
with additional localized totals of 6"+ expected (with 8"+ amounts
possible).

Discussion...Clusters of thunderstorms are in the process of
growing upscale into a mesoscale convective system (MCS) across
western portions of the TX Triangle region, from the San Antonio
area northeastward to near Waco. Storms are generally organizing
linearly along the southern and eastern periphery of the
established cold pool, where instability is greatest (1000-3000
J/kg of SBCAPE) with increasing low-level moisture transport (in
association with a strengthening nocturnal low-level jet with
925mb winds of 25-30 kts from the SSE). PWs have also increased to
a very impressive 1.9-2.3 inches, nearing daily max record levels
(using CRP sounding climatology as a proxy). Localized hourly
rainfall totals are estimated to be as high as 3-4" (per MRMS), a
resulting of training (as deep layer steering flow is oriented
nearly parallel to the cold pool boundary, due to the favorable
placement of a mid-upper level low) and warm rain processes
dominating (with wet bulb zero heights around ~13k feet). Storms
training from west-to-east (along the southern edge of the cold
pool) in the vicinity of San Antonio are particularly concerning,
given the flooding sensitivities of the metro area. Meanwhile, a
separate cluster of thunderstorms are growing in strength and
coverage near the Middle TX Coast (where PWs are maximized) due to
the strengthening low-level jet and associated warm-air
advection/isentropic ascent.

Storms should only continue to organize this morning, given the
very favorable aforementioned environment (with differential
divergence only increasing further over the next several hours).
Hi-res models (both the 00z HREF and experimental 18z REFS
ensembles) are in excellent agreement with both the expected
magnitudes and placement of QPF with relatively high probabilities
(40-60%) for both localized 5" exceedance (40-km neighborhood) and
1" exceedance overlap (10-100 km Ensemble Agreement Scale). This
corresponds with high FFG exceedance probabilities (up to 60-70%)
and a decent chance (up to 25-35%) of locally exceeding 100-yr
ARIs. Given this strong ensemble signal, localized 6"+ totals are
expected (and it is notable that hourly HRRR/RRFS runs have been
fairly consistent in depicting localized 8"+ totals, primarily
between the Corpus Christi and Houston metros where repeating
heavy rainfall is most likely). Numerous instances of flash
flooding are likely, some of which are expected to be life
threatening. Given the potential for 100-yr ARI exceedance,
extreme instances of localized flash flooding are possible (with
areas that do not typically flood at risk of experiencing flash
flooding).

Churchill

ATTN...WFO...CRP...EWX...FWD...HGX...SHV...

ATTN...RFC...FWR...NWC...

LAT...LON   32049643 31979551 31329461 29789487 29099531
            28689613 28389719 28619862 29179932 29799937
            30279846 30989817 31929747