Mesoscale Precipitation Discussion 0818
NWS Weather Prediction Center College Park MD
1205 PM EDT Mon Sep 10 2018

Areas affected...Central and Northeast Texas Coast

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

Valid 101604Z - 102100Z

Summary...Showers and thunderstorms will increase in coverage and
lift onshore through the afternoon. Rainfall of 2-4" is possible
which has the potential to produce flash flooding.

Discussion...A cluster of thunderstorms noted by cooling cloud
tops on GOES-16 IR imagery and regional radar mosaic is expanding
in coverage across the NW Gulf of Mexico. An MCV is implied by
rotation in the radar SW of San Antonio, while a strong shortwave
is digging southward along the NM/TX border. These features in
tandem are helping to drive mid and upper level diffluence across
E TX, which will be further enhanced by a 50kt 300mb jet lifting
NE this afternoon, placing E TX in the favorable right entrance
region to this feature. Although low-level winds are weak, latest
RAP analyzed MLCape is around 1500 J/kg, more than sufficient to
lift surface parcels, which will then be lifted upward by the
synoptic lift implied by the increasing diffluence aloft.

Morning PWATs from upper air soundings at CRP as well as RAP
analysis shows a large area of 2.1 to 2.3 inches flooding into the
TX coast. While these are not extreme values, they are above the
90th percentile for the date which combined with forecast warm
cloud depth approaching 15kft is suggestive of efficient warm rain
processes even in convective elements. This is echoed by HGX
estimated rainfall rates of 1 to 1.5 inches per hour lifting onto
the TX coast, coincident with KDP values of 2-3 deg/Km coincident
with high reflectivity. The convection offshore is likely to
increase in coverage during the next few hours and lift northward.
Although FFG is not exceedingly low, 3-4" in 3 hours, a stripe of
heavier rainfall is likely along and just inland from the coast
where frictional convergence and a low-level theta-e axis merge.
Rainfall of 2-4" is possible across this area as storms move
slowly on 0-6km mean wind of less than 10 kts. This may be
sufficient to produce flash flooding across the area.

Weiss

ATTN...WFO...CRP...HGX...LCH...

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

LAT...LON   30389437 30389411 30289389 30099375 29849381
            29669393 29559403 29399438 29229469 29079495
            28869527 28709559 28569593 28399624 28289649
            28219677 28409704 28759700 28949684 29319647
            29599619 29879572 30049533 30209498 30299460