Subj : MESO: Heavy rain/flooding
To   : All
From : Mike Powell
Date : Thu Oct 26 2023 08:43:00

AWUS01 KWNH 260941
FFGMPD
TXZ000-261540-

Mesoscale Precipitation Discussion 1161
NWS Weather Prediction Center College Park MD
540 AM EDT Thu Oct 26 2023

Areas affected...Much of Central and South-Central TX

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

Valid 260940Z - 261540Z

Summary...Rainfall totals of 1-2" may continue through morning,
possibly leading to additional localized totals of 3-5"+.
Scattered instances of flash flooding are possible, and some may
be significant.

Discussion...Showers and thunderstorms continue early this morning
in the vicinity of a surface trough draped across North and
Central TX. Rainfall rates have moderated a bit over the past
several hours but have shown some recovery as of late, still as
high as 1-2"/hr locally. While progression of the convection
eastward has begun to limit the threat for additional flooding
across northeast TX, there is less eastward progression farther
south (into the TX Hill Country and South-Central TX) which is
facilitating localized training and repeating of 1-2"/hr rainfall
rates. This is in a mesoscale environment characterized by PWATs
of 1.4-1.9" (between the 90th percentile and max moving average,
per surrounding sounding climatology), appreciable instability (as
measured by 500-1000 J/kg of ML CAPE), and effective bulk shear of
20-40 kts.

Hi-res guidance has come into far less agreement through 16z,
despite much better agreement among 00z HREF members earlier
overnight. Both the HREF suite and more recent hourly runs of the
HRRR have struggled to depict the intensity of the southern end of
the convection, but the most recent HRRR runs (since 06z) have
begun to depict a more concerning scenario where a narrow band of
training convection sets up near the I-35 corridor (including the
Austin and San Antonio metro areas) and have shown some southerly
trend.  Six hourly totals are indicated to be on the order of 3-9"
in these latest runs, which is certainly plausible given the
highly efficient warm rain processes and potential for training.
That said, the 00z HREF is far less aggressive, generally
suggesting 20-40% probabilities for localized 3" exceedance (per
the 40-km neighborhood method), and negligible (~5%) odds for
localized 5" exceedance.  The above introduces above average
uncertainty.  Given the concerning trends in the HRRR (with
observational trends supporting these concerns), continued flash
flooding is considered possible (as well as locally significant
flooding, given the potential magnitude).

Churchill/Roth

ATTN...WFO...EWX...FWD...HGX...SJT...

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

LAT...LON   32729592 31519596 30249674 28889793 28869937 
            29409997 29800000 30899903 31509830 32229726 
            

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