Mesoscale Precipitation Discussion 0846
NWS Weather Prediction Center College Park MD
1006 PM EDT Sat Sep 15 2018

Areas affected...south-central to southern TX

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

Valid 160205Z - 160805Z

Summary...A threat for localized flash flooding will remain over
south-central to southern TX through 08Z. 2 to 4 inch rainfall
maxima may occur which would generate runoff issues given
saturated soils.

Discussion...00Z RAOBs and VAD wind data showed a surface-850mb
low just north of LRD, drifting slowly toward the northwest as of
02Z. Precipitable water values remained rather high for south TX,
2.1 to 2.5 inches or in the 90-95th percentile for mid-September.
20-30 kt 850 mb flow existed from near CRP into the TX Hill
Country (north of the low center near LRD). These values exceeded
850-300 mb mean flow of similar direction to the 850 mb flow which
would support repeating and training cores of heavy rain given
sufficient lift. Aloft, the presence of a 250 mb anticyclone over
east-central TX will help to support some degree of diffluent flow
over south-central TX over the next few hours.  The greatest
limiting factor is MLCAPE, which ranges from roughly 500 to 1000
J/kg (higher values near the coast) and its forecast weakening
into the night. In addition, 850 mb flow is also forecast to
weaken along the middle to lower TX coast by 06Z, reducing
moisture flux across the higher source region of CAPE near the
coast, which is a change from prior nights as the upper low
continues to move west.

While the flash flood threat does not appear to be very high, and
hi-res model support is spotty overnight, soils are saturated
across much of the region given 600+ percent of average rainfall
over the past week translating into a higher risk for flash
flooding for the area. Several smaller scale meso-lows were
observed in radar imagery over south TX which may not be sampled
well by the hi-res guidance or the HRRR, with these features
acting as possible foci for additional bursts of heavy rain over
the next few hours.

Otto

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

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

LAT...LON   30400035 29969899 29539799 29119749 28719716
            28289691 27789681 26989714 25949751 25949839
            26139900 26449944 26819962 27289979 27700009
            28040041 28420065 28980120 29360142 29820155
            30230127