Mesoscale Precipitation Discussion 0841
NWS Weather Prediction Center College Park MD
221 AM EDT Sat Sep 15 2018

Areas affected...south-central to southern TX

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

Valid 150620Z - 151215Z

Summary...The threat for flash flooding should increase through
the morning across portions of south-central to southern TX. Local
totals of 3-5 inches will be possible through 12Z.

Discussion...GOES 16 infrared satellite combined with base
velocity data from KCRP and VAD wind data showed an 850-700 mb low
over northern Brooks county at 06Z. A smaller scale meso-low was
observed farther north near Goliad county, moving off toward the
northwest. Radar imagery showed disorganized convection in the
vicinity of the 850-700 mb low but with the highest rain rates
near the coast, more specifically near Corpus Christi and Aransas
Bays. The SPC mesoanalysis agreed with the RAP, showing the
highest MLCAPE (~2000 J/kg) just offshore of the lower TX coast
with 1000 J/kg just inland and parallel to the coastline.
Precipitable water values were 2.3-2.5 inches over the region per
00Z soundings and recent GPS measurements.

The 850-700 mb low is forecast to slowly drift toward the
northwest over the next 6 hours, with this motion allowing 850 mb
flow to become better positioned to tap into offshore instability
and feed convection into the southern Texas Coastal Plain. Corfidi
Vectors show near zero magnitude here through 12Z. In addition, an
upper level anticyclone, currently centered about 100 miles
offshore the southern coastline, is forecast to translate west to
near the coast which should help to enhanced lift.

Closer to the low center itself, despite lower available
instability, slow to nearly stationary storm motions may allow for
some locally higher totals along and west of Route 281. Since much
of southern TX has experienced 400 to 600+ percent of average
rainfall over the past week, flash flooding will be possible.

Otto

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

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

LAT...LON   29639975 29549734 28479648 27549672 26699691
            25749713 26539936 28370050