Mesoscale Precipitation Discussion 0644
NWS Weather Prediction Center College Park MD
650 PM EDT Sun Aug 12 2018

Areas affected...South-Central Texas

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

Valid 122249Z - 130419Z

Summary...Thunderstorms are increasing in coverage again in
south-central Texas, and this should continue into the early
evening. The storms should initially fill in along the I-35
corridor from the Mexican border to San Antonio, and may expand
further north or northwest with time. Rain rates may reach 2 in/hr
or so with the more organized bands of heavy rain, and this could
produce flash flooding.

Discussion...Convective coverage was on the increase again near
the I-35 corridor in south-central Texas as of 22-23Z. The area
immediately to the southeast of I-35, and to the south of an
outflow boundary stretching from KUVA-KSSF-K3T5, has become quite
hot and unstable with temperatures well into the 90s and dewpoints
in the 70s. A separate outflow boundary was moving north from Deep
South Texas evident as a thin line on GOES-16 visible satellite
and low-level water vapor channels. This sets up the possibility
of a future boundary collision to increase convection, in addition
to the broad S-SE unstable inflow that is currently leading to a
gradual increase in convective coverage as it intercepts the
northern outflow boundary and an existing stationary front.
Therefore, the immediate I-35 corridor is favored for more
concentrated convective activity in the 23-01Z time frame, and may
expand north or northwest later in the evening. General storm
motions were out of the SSW and parallel to the stationary front,
potentially setting up favorable training as convective coverage
increases. Therefore, localized heavy rainfall seems likely, and
flash flooding could develop if this falls in areas more favorable
for flooding (urban areas or locations that have received heavy
rainfall earlier today). Precipitable water values are around 2.0
to 2.1 inches per the latest GPS-PW observations in the area,
which should support heavy rain rates to around 2 in/hr (perhaps
higher on a very local basis).

Lamers

ATTN...WFO...CRP...EWX...SJT...

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

LAT...LON   31219935 30749750 29069816 27479955 28810077
            30190046