Mesoscale Precipitation Discussion 0827
NWS Weather Prediction Center College Park MD
1254 PM EDT Wed Sep 12 2018

Areas affected...Central and Northern Texas Coast

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

Valid 121649Z - 122200Z

Summary...Showers and thunderstorms will increase across coastal
Texas with periods of very heavy rainfall. 2-4" of rain with
isolated higher amounts may produce flash flooding.

Discussion...Cooling cloud tops noted on GOES-16 IR imagery just
off the coast of Texas are indicative of strengthening convection.
This is developing SE of a stationary front just offshore. At the
same time, clearing noted on GOES-16 visible imagery is coincident
with increasing MLCape of up to 1000 J/kg on latest RAP analysis.
Thunderstorms have started to develop in this more unstable air,
while at the same time intensifying offshore as evidenced by HGX
radar estimated hourly rain rates of up to 2"/hr.

A weak low pressure evident in low-level wind fields will drift
across the lower Rio Grande Valley while mean 0-6km flow will
persist out of the SE at around 10 kts. Backed flow on the east
side of this low pressure will help to enhance confluence and
increase moist advection onshore over the next several hours. This
will drive strong lift atop the stationary boundary which will be
further enhanced by frictional convergence. Additionally, MLCape
is forecast to increase towards 1500 J/kg inland from the coast
suggesting convection will maintain intensity as it lifts onshore.
In an environment with PWATs over 2.2 inches, this suggests very
heavy rainfall potential with rain rates over 2"/hr possible.

Corfidi vectors are at large angles to the mean wind, so
backbuilding of thunderstorms is possible. Redevelopment along
convergence boundaries and outflows is likely in a pulse
environment with 0-6km environmental shear less than 15 kts.
High-res guidance has a clear signal for 2-4" of rainfall, with
locally higher amounts likely along these boundaries. This area
has received significant rainfall in the past 24 hours, so 3-hr
FFG values are low, around 2-2.5 inches, so flash flooding is
possible.

Weiss

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

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

LAT...LON   30229429 30159380 30039375 29949367 29709382
            29519420 29399441 29229482 29019511 28909531
            28769551 28559583 28359626 28199659 28099668
            27969688 27789701 27559717 27399738 27429772
            27559785 27939789 28369783 28699775 28939765
            29219739 29449705 29669658 29869607 30029544
            30169489