Mesoscale Precipitation Discussion 0758
NWS Weather Prediction Center College Park MD
212 AM EDT Tue Sep 04 2018

Areas affected...Middle Rio Grande Valley to the TX Hill Country

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

Valid 040610Z - 041210Z

Summary...Additional heavy showers and thunderstorms are expected
over the next few hours which will locally exacerbate ongoing
flash flooding concerns. New areas of flash flooding will be
likely through the early morning hours.

Discussion...The latest GOES-16 IR satellite imagery has been
showing some significant cloud top cooling over portions of
Dimmitt and Webb counties down along the Rio Grande river over the
last 1 to 2 hours as convection develops within a moderately
unstable airmass and in vicinity of a well-defined and very
slow-moving convectively reinforced outflow boundary. The cold
pool associated with the convection appears to be strengthening
which is fostering a stronger instability gradient given the
persistence of a moist/unstable east-southeast low level flow
regime in off the western Gulf of Mexico. MLCAPE values east of
the cold pool are as much as 1500 to 2000 j/kg and this coupled
with some hints of an MCV near the Rio Grande river will likely
tend to support additional clusters of convective organization.
Meanwhile, the flow aloft remains rather diffluent which will aid
the ongoing convective activity.

PWATS are again very high with GPS-derived values of as much as
2.25 inches. The environment remains conducive to highly efficient
warm rain processes for enhanced rainfall rates that will exceed 3
inches/hr within the stronger convective cores.

Based on the latest HRRR guidance, and considering the satellite
and radar trends, expect locally as much as an additional 3 to 6
inches of rain going through 12Z. Some of this rain could
potentially occur over areas already hard hit be earlier rains,
and especially around the San Antonio metropolitan area. Flash
flooding is ongoing in many areas, and additional flash flooding
is expected, some of which may be particularly significant if the
additional rainfall gets back over the more urbanized areas.

Orrison

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

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

LAT...LON   30509949 30499842 29949789 29129785 27979830
            27389884 27209940 27479984 27749996 28050016
            28820039 29780028