AWUS01 KWNH 231202
FFGMPD
TXZ000-231800-

Mesoscale Precipitation Discussion 0705
NWS Weather Prediction Center College Park MD
801 AM EDT Tue Jul 23 2024

Areas affected...Portions of the TX Hill Country and adjacent
areas of South-Central TX

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

Valid 231200Z - 231800Z

SUMMARY...Clusters of slow-moving showers and thunderstorms with
very heavy rainfall rates this morning will maintain a rather
strong likelihood of flash flooding across portions of
south-central TX, including parts of the TX Hill Country.

DISCUSSION...The latest GOES-E IR satellite imagery shows a rather
impressive convective complex over portions of south-central TX,
including parts of the TX Hill Country. Very slow-moving area of
showers and thunderstorms make up this convective mass with cloud
tops that are as cold as -65C. The environment where the
convection is occurring is quite moist with a weak to moderately
buoyant airmass in place.

MLCAPE values of 1000 to 1500 J/kg are in place with PWs of 2.0 to
2.2 inches based on the early morning GPS-derived PW data. This is
all pooled along a quasi-stationary front draped generally
southwest to northeast across the region which is also helping to
yield an axis of low-level frontal convergence. Coinciding with
this set-up is the proximity of a mid-level trough that is seen in
GOES IR/WV imagery digging southeastward down across the southern
Plains, and this is favoring a rather divergent flow pattern aloft
which is yielding deeper layer ascent and helping to further
support a low-level response with convergent flow into the front.

There has been some subtle increase in the boundary layer
instability over the last few hours with 3-hour MLCAPE
differentials over south-central TX (north and west of Uvalde) of
+200 to + 400 J/kg and this has been coinciding more recently with
additional cloud-top cooling with the convection northeast of Del
Rio.

Expect the ongoing convective mass to persist this morning and
continue to be slow-moving with very heavy rainfall rates that may
reach 2 to 3 inches/hour with the stronger cores. A gradual
movement off to the east-southeast is expected. By midday, a
sufficient cold pool may evolve out of the activity to force some
acceleration of the convection off to the south and east, but
until then, the slow cell-motions will likely yield some very
heavy rainfall totals.

The 06Z HREF and recent runs of the HRRR guidance suggest
additional rainfall totals of 3 to 5 inches at least locally going
through midday. The antecedent conditions across the region are
already rather moist from recent rainfall, and this will encourage
an elevated runoff threat with the additional totals. Additional
flash flooding is likely, and some of it may be locally
considerable to severe. This will include concerns for some of the
urban locations as well.

Orrison

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

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

LAT...LON 31379714 30799676 29989748 29289887 28910034
29250090 29750100 30270060 30799981 31269839

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