Mesoscale Precipitation Discussion 0756
NWS Weather Prediction Center College Park MD
930 PM EDT Mon Sep 03 2018

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

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

Valid 040130Z - 040545Z

Summary...Heavy showers and thunderstorms will focus over the Hill
Country over the next few hours. Extremely heavy rainfall rates
are expected, and flash flooding will be possible.

Discussion...The latest GOES-16 IR satellite imagery shows cooling
convective tops over areas of the Rio Grande Valley and extending
north up across the TX Hill Country. Showers and thunderstorms
have been developing in response to a moderately unstable boundary
layer with MLCAPE values of as much as 1500 to 2000 j/kg.
Facilitating the convective development has also been proximity of
a differential heating boundary along with relatively diffluent
flow aloft.

Meanwhile, the airmass is very moist as the latest Blended-TPW
data set and GPS-derived data shows PWATS of as much as 2.25
inches. This is allowing for highly efficient rainfall processes
that are contributing to enhanced rainfall rates of as much as 3
inches/hr, and especially within the deeper convective cores where
overshooting tops are occurring.

Over the next few hours, the hires model consensus led especially
by the 12Z ARW/NMMB solutions and also the latest HRRR guidance,
support as much as an additional 3 to 4 inches of rain from the
Hill Country south down to near the middle Rio Grande Valley.
Given the slow cell motions and deep moisture profiles, it is
possible that locally heavier amounts may materialize. These
amounts may result in some flash flooding, with the biggest
concern over any urbanized areas. Will continue to monitor.

Orrison

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

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

LAT...LON   30669838 30549798 29839783 29439792 29099835
            28819840 28479872 28289913 28479964 28290026
            28630061 29110059 29810002 30389925