MESOSCALE PRECIPITATION DISCUSSION 0833
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
146 AM EDT WED SEP 27 2017

AREAS AFFECTED...SOUTHWEST TEXAS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 270600Z - 271130Z

SUMMARY...CONTINUED THREAT FOR SMALL BUT HIGHLY EFFICIENT (2.5"/HR
RATES) AND SLOW MOVING CLUSTERS NEAR THE RIO GRANDE OVER ALREADY
SATURATED GROUND CONDITIONS CONTINUING POSSIBLE FLASH FLOOD
THREAT.

DISCUSSION...REGIONAL WSR-88D REFLECTIVITY MOSAIC DENOTES A FEW
IMPORTANT LOWER LEVEL FEATURES THAT WILL REMAIN ORIENTED FAVORABLY
FOR ENHANCED CONVERGENCE OF HIGH THEAT-E AIR (SURFACE TDS TO
75F+).  A WEAK NEAR INVERSION/BOUNDARY LAYER MCV OR APEX TO HIGHLY
AMPLIFIED TROF EXISTS FROM MAVERICK COUNTY SHIFTING EASTWARD WITH
TIME.  EXTENDING SW FROM THE MCV TO CORPUS CHRISTI BAY IS A LOW
LEVEL CONFLUENCE BAND THAT SHOULD REMAIN FAIRLY CONSISTENTLY
LOCATED FOR SHALLOW BUT MODESTLY INTENSE WARM CLOUD/TROPICAL RAIN
SHOWERS FOR ENHANCED RAIN FALL TOTALS OVER THE NEXT FEW
HOURS...THAT IN ALREADY SATURATED AREAS SUCH AS MCMULLEN/N LASALLE
COUNTIES MAY POSE SOME ADDITIONAL FLOODING CONCERNS.

GOES-16 3-CHANNEL WV SUITE DEPICT THE MAIN MID-LEVEL REMNANT
SHORTWAVE OF PILAR LIFTING INTO S COAHUILA.  IN RESPONSE TO THE
WAVE...STRONG SSELY AND SELY ONSHORE FLOW TO 30KTS AT 850MB
PROVIDES AMPLE MST/INSTABILITY FLUX INTO COMPLEX TERRAIN.  AT THE
SAME TIME...THE WV LOOP SHOWS AN INCREASING CONFLUENT FLOW ALONG
THE SPINE OF THE SIERRA MADRE OF NUEVO LEON TO PROVIDE STRONG
CONVECTION AS FAR NORTH AS THE RIO GRANDE/WEBB COUNTY.

10.3UM LOOP DEPICTS THIS AREA OF INCREASED CONVERGENCE LEADING TO
RAPIDLY COOLING TOPS WITH OCCASIONAL -80C OR COOLER OVERSHOOTING
TOPS DENOTED.  GIVEN VERY DEEP MOISTURE AVAILABLE PER CIRA LPW
(.3" EVEN ABOVE 500H) ORIENTED FAVORABLY TO THE REGION INCREASING
EFFICIENCY ALLOWING FOR RATES OF 2.5"/HR WITH SHORTER BURSTS OF
MUCH GREATER MAGNITUDE POSING THE GREATEST RISK FOR FF.  AT THIS
TIME THE GREATEST LIMITING FACTOR IS REMAINING INSTABILITY ACROSS
SW TX...AS MUCAPES ARE AOA 500 J/KG AT BEST TYPICALLY HUGGING THE
RIO GRANDE.  AS SUCH GREATEST FF RISK RESIDES TO THE WEST OF THE
RIVER BUT GIVEN AMOUNT ALREADY FALLEN ESPECIALLY IN WEBB/E DIMMIT
COUNTIES...IT WOULD NOT TAKE MUCH TO REAGGRAVATE FLOODING HERE AS
WELL.

AS BOTH THE MID-LEVEL VORT CENTER OF PILAR LIFTS NORTH...AND THE
MCV IN MAVERICK SHIFT WEST...THERE WILL BE A HIGHER LIKELIHOOD OF
NORTH AND NORTHWEST SHIFT OF THUNDERSTORMS TOWARD 09-12Z.  THIS
WILL INCREASE UPPER LEVEL DIVERGENCE FROM RIGHT ENTRANCE REGION TO
SW-NE ORIENTED 3H JET OVER THE WESTERN PANHANDLE...AS WELL AS
INCREASING SWLY STEERING FLOW TO POSSIBLY DRAG CELLS FURTHER OFF
THE TERRAIN INTO SW TX.  THIS WITH CLOSER PROXIMITY TO SELY
CONFLUENCE BAND FROM CORPUS CHRISTI IS LIKELY TO INCREASE FLUX AS
WELL CONTINUING THE HEAVY RAINFALL THREAT...THOUGH GREATER LACK OF
INSTABILITY IS LIKELY TO LIMIT VERTICAL GROWTH AND OVERALL DEPTH
OF RAINFALL PRODUCTION. 

GALLINA

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

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

LAT...LON   29780066 29689999 29339917 28409844 28279941
            27509931 27479969 27669984 27980018 28370043
            28600067 29430095