Mesoscale Precipitation Discussion 0816
NWS Weather Prediction Center College Park MD
1126 PM EDT Sun Sep 09 2018

Areas affected...South Texas...

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

Valid 100325Z - 100900Z

SUMMARY...Slow moving, moisture rich cells continue to pose flash
flooding risk with further development expected later overnight.

DISCUSSION...Current mesoanalysis/RADAR Mosaic and GOES-16 WV/IR
suite depict a mature MCV from earlier thunderstorm cluster south
of the Rio Grande has sharpened some surface boundaries along the
northwest and northern quadrants providing solid moisture
convergence to develop new cells.  Enhanced ESEly low level flow
is confluent but also provides moderate moisture flux with 850mb
Tds in the 16C range and mean mixing ratios near 17-18C.  Based on
blended TPWs/GPS network available moisture is about 2.25"-2.4"
across much of South Texas.   Given proximity to MCV and slight
westward tilt from 850mb center near LRD and 5H vort center over
the northern Sierra Madre, mean cloud bearing flow is very weak
and downstream (to the east) from the updraft cores pressing
eastward toward the convergence lines.  This will provide a
broader area for prolonged or areal coverage of heavy rainfall
across S Zapata into S Starr county. 

Later this evening, as the boundary layer further decouples, low
level easterly flow becomes less chaotic and supports 10-15kts of
flow off the Gulf and generally convergent along the 99th meridian
at the apex of the 850mb trof.  Multiple Hi-Res CAM signals
suggest a 6-7z initiation for which will be slow moving and
suggest WNW propagation toward saturated areas of Zavala and
LaSalle counties.  Rates of 2"/hr and slow motions suggest
additional totals of 2-4" are likely presenting additional flash
flooding threat further north though continued convergence toward
the 850mb continues through 07-08z in Deep South TX. 

Gallina

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

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

LAT...LON   29399915 28809864 28209834 27009766 26049771
            25959844 26469922 27089957 27679986 28010019
            28180023 28820060 29180026