Mesoscale Precipitation Discussion 0639
NWS Weather Prediction Center College Park MD
619 AM EDT Sun Aug 12 2018

Areas affected...SW TX...CENTRAL TX

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

Valid 121018Z - 121518Z

Summary...Convective heavy rains developing as of 10Z had the look
of a likely flash flood event, at least on a local scale to the
north and west of San Antonio.

Discussion...A WPC Moderate Risk of excessive rainfall covers
parts of southwest and central Texas. The first of what could be
several slow-moving thunderstorm clusters was seen developing in
radar and satellite imagery this morning just north of Uvalde,
with other echoes percolating north of San Antonio at 10Z.

Several ingredients for flash flooding appeared to be coming into
place. Satellite loops of total precipitable water followed a
plume of rich tropical moisture marked by surface dewpoints 74 F
or greater that had originated over the western Gulf and was
nearing the developing convection. Both radar and satellite loops
were also indicative of a strengthening inflection in the upper
level flow near the base of the trough from west Texas to near the
Big Bend - placing difluence, and in particular upper speed
divergence, over south-central Texas and the Hill Country.

The tandem of upper support and low level flow drifting uphill
into the terrain should foster growth of a small MCS or several
clusters of organized storms this morning. The 00Z WRF-ARW had
nailed the timing and location of the development. Several other
models were clustered nearby, with the HREF mean even signaling
this small scale event. The QPF from these models suggests
isolated 4 inch rainfall through 15Z. These estimates could even
be low. The early storms already appeared to be anchored and were
very deep, with echo tops over 50 kft from KEWX. Given the
tropical thermodynamic environment with tall, skinny CAPE, high
web-bulb zero values, and PW approaching 2.10 inches, rainfall may
add up very quickly. The ground in some areas had been wetted by
rains over the past day or two. Isolated flash flooding appears
likely this morning. It is also worth noting that Corfidi vectors
for backward propagation would take activity slowly to the
southeast over time, possibly putting more population centers at
risk by mid to late morning. Expect a continuing threat as the
pattern evolves later this afternoon.

Burke

ATTN...WFO...EWX...MAF...SJT...

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

LAT...LON   30929790 30059741 29609777 29209934 28600063
            28620082 29740211 30300054 30469891