Mesoscale Precipitation Discussion 0835
NWS Weather Prediction Center College Park MD
929 AM EDT Fri Sep 14 2018

Areas affected...TX Coast & Vicinity

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

Valid 141328Z - 141858Z

Summary...A tropical disturbance is poised to make its TX
landfall.  Hourly rain totals to 2" with local amounts to 4" are
expected over the next several hours.

Discussion...Radar imagery and polar orbiting satellite estimates
show a broad area of rain with embedded showers and thunderstorms
which should soon make a TX landfall.  This is in association with
a tropical disturbance that the National Hurricane Center has been
monitoring for some time.  Precipitable water values are 2.3-2.6"
per recent GPS data, exceedingly high for any time of the year. 
Inflow at 850 hPa is cyclonic at 25-30 knots per VAD wind
profiles, which combined with surface observations indicate a low
level center/1008 hPa low pressure system somewhat east-southeast
of Rockport TX.  There is no CIN per SPC mesoanalyses due to the
high degree of moisture.  ML CAPE values of 1000-2500 J/kg lurk
near and just offshore the coast.

The expectation is for this low/inverted trough to move ashore in
2-3 hours, which should lead to and increase in rain rates/totals
from there on out, as instability from the warm Gulf of Mexico is
ushered in within the wake of the tropical disturbance.  The
mesoscale guidance is not unified in its messaging, though
seemingly low in convective coverage, showing a scattershot signal
for local amounts in the 3-4" range through 19z.  Hourly rain
totals to 2" are expected, though cell mergers and long enough
bouts of training could exceed this number.  The expectation is
that the rainfall would lead to an instability gradient remaining
near the coast during the MPD period as skies are cloudy farther
inland.  Considering the wet conditions across the region during
the past couple weeks depressing flash flood guidance, heavy rain
issues are expected, particularly within urban areas.

Roth

ATTN...WFO...BRO...CRP...EWX...HGX...LCH...

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

LAT...LON   30489530 30389401 29749329 29589387 29369446
            28949515 28699554 28099666 27039727 26089706
            25809717 25919798 26139851 27249820 28629740
            29039712 30149605