MESOSCALE PRECIPITATION DISCUSSION 0246
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
135 PM EDT SAT MAY 20 2017

AREAS AFFECTED...SOUTH-CENTRAL TEXAS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 201735Z - 202130Z

SUMMARY...ADDITIONAL REPEATING SHOWERS AND THUNDERSTORMS WITH VERY
HEAVY RAINFALL IS EXPECTED OVER THE NEXT FEW HOURS. FLASH FLOODING
IS LIKELY.

DISCUSSION...SATELLITE IMAGERY SHOWS A RELATIVELY COMPACT BUT VERY
PERSISTENT CLUSTER OF VERY COLD-TOPPED THUNDERSTORMS WITHIN ABOUT
50 MILES WEST-SOUTHWEST OF SAN ANTONIO TX...INVOLVING ESPECIALLY
UVALDE...MEDINA...ZAVALA AND FRIO COUNTIES OF SOUTH-CENTRAL TX.
THE CLUSTER OF THUNDERSTORMS OVER THE LAST COUPLE OF HOURS HAS
BEEN TENDING TO REPEAT OVER THE SAME AREA AS STRONGER LOW LEVEL
CONVERGENCE ALONG THE TAIL END OF THE FRONT COINCIDES WITH A NOSE
OF VERY MOIST AND UNSTABLE AIR POOLING NORTH UP ACROSS SOUTH
TEXAS. ALSO THERE APPEARS TO BE AT LEAST SOME MODEST UPPER JET
SUPPORT OVER THE AREA AS SEEN WITH THE DIFFLUENT HIGH LEVEL CLOUD
STRUCTURE AND WITH THE LATEST RAP ANALYSIS SHOWING A WEAK JET MAX
CROSSING THE REGION.

THE LATEST HRRR AND HRRRX GUIDANCE STRONGLY SUPPORTS THIS ACTIVITY
TENDING TO REMAIN IN PLACE OVER THE NEXT FEW HOURS WITH SOME
GRADUAL ADVANCE OFF TO THE EAST OR SOUTHEAST EXPECTED AS A
GRADUALLY EXPANDING CONVECTIVE COLD POOL DRIVES INTO THE VERY
MOIST/UNSTABLE LOW LEVEL SOUTHEAST FLOW ACROSS THE REGION.
HOWEVER...CELL REGENERATION OVER THE COLD POOL APPEARS LIKELY.
ADDITIONAL RAINFALL AMOUNTS OF 4 TO 6 INCHES WILL BE POSSIBLE
WHERE THESE CELLS REMAIN ANCHORED OVER THE NEXT FEW HOURS...AND
THE SAN ANTONIO METROPOLITAN AREA WILL NEED TO MONITOR THE
PROGRESS OF THESE CELLS GIVEN THEIR PROLIFIC RAINFALL RATES OF
OVER 2.5 INCHES/HR AS PER RADAR ESTIMATES. FLASH FLOODING WILL BE
LIKELY WHERE THESE CELLS CONTINUE TO CONSOLIDATE.

ORRISON

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

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

LAT...LON   29519866 29329804 28809790 28389842 28359970
            28650032 29090043 29430000