Mesoscale Precipitation Discussion 0825
NWS Weather Prediction Center College Park MD
1008 PM EDT Tue Sep 11 2018

Areas affected......South Texas...

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

Valid 120208Z - 120808Z

Summary...Showers and thunderstorms have developed over south
Texas in the vicinity of a low level stationary front. 
Continuation of activity overnight and slow cell motion leads to
local bands of 2-4 inches of rain, resulting in possible flash
flooding.

Discussion...An upper level trough is in place across south Texas
with the RAP analysis at 00z showing a hint of an embedded closed
850 mb circulation.  Showers and storms have developed near the
boundary layer moisture convergence and 850 mb trough.  The models
show slow movement of the upper trough to the east, continuing the
threat overnight.

The 00z Brownsville sounding shows a warm advection profile with
an impressive 2.45 inch PW.  Light winds from the SFC to 700 mb
winds under 10 kt leads to slow cell motions.

The RAP shows boundary layer moisture convergence drifting west,
with the NAM and NSSL WRF showing redevelopment along the coast,
along with coastal convergence as the modest 15 kt inflow
develops.

Heavier rain rates are a result of the
high available moisture and areas of combined upper divergence/low
level convergence.

High-res guidance shows a signal for 2-4" of rainfall across the
threat area.
3-hr FFG guidance in south Texas is variable with the possible
designation due to this variability, including both relatively low
guidance locally under an inch in 3 hours and other clusters
higher with 3-4 inches in 6 hours where it has not rained as much
recently.

With slow movement of the upper trough and continued high
available moisture and areas of low level convergence, this area
will be evaluated later tonight for an extension and possible
inclusion of areas further up the Texas coast towards
Houston/Galveston.

Petersen

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

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

LAT...LON   28609736 28139662 27429654 26029680 25399762
            25759830 27189865 28169830