Mesoscale Precipitation Discussion 0879
NWS Weather Prediction Center College Park MD
122 AM EDT Sat Sep 22 2018

Areas affected...South-Central and Central Texas

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

Valid 220521Z - 221100Z

Summary...Scattered thunderstorms were gradually developing ahead
of a steadily advancing cold front across portions of central
Texas. Some storms were beginning to stall in the general vicinity
of the I-35 corridor in central Texas. As storms stall and/or
backbuild, this could lead to locally significant rainfall totals.
Flash flooding would be likely in these areas. Rain rates may
approach or exceed 3 in/hr in the strongest, nearly-stationary
storms.

Discussion...Flash flooding appears likely across parts of
south-central and central Texas into the early morning hours. This
may be somewhat localized, however, as the convective coverage is
more scattered south of the Dallas area, albeit slowly increasing
ahead of the cold front. KGRK and KEWX radars have showed a few
thunderstorm clusters with a propensity to stall, and this
appeared to be occurring in two key areas: (1) a focused area of
low-level moisture flux convergence from KBMQ-KGYB, or just on the
north side of the Austin area, and (2) in an area of weaker flow
closer to San Antonio. The net effect has been localized radar
rain rate estimates in excess of 2 in/hr, and even approaching 3-4
in/hr in some cases. These would be likely to produce flash
flooding, particularly in the typically vulnerable Balcones
Escarpment region near Austin and San Antonio. Hi-res models
almost all show localized 6-hr rainfall maxima in excess of 6
inches, but vary on the placement. This is to be expected given
the lack of a stationary, strong low-level forcing mechanism and
the scattered nature of convection at the moment. However, given
the demonstrated tendency for ongoing convection to stall, these
isolated extreme rainfall amounts seem plausible. GPS-PW
observations in the region are in the 2.1 to 2.4 inch range with
deep tropical moisture promoting highly efficient rain processes.
RAP analysis shows MLCAPE around 1000 j/kg, which should be more
than sufficient to support vigorous convection capable of
producing extreme rain rates given the available moisture.

Lamers

ATTN...WFO...CRP...EWX...FWD...HGX...SJT...

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

LAT...LON   31919701 30499664 28909861 27639968 28540060
            29400057 30389949 31749830