NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
618 PM EDT SAT MAY 23 2015
AREAS AFFECTED...MIDDLE RIO GRANDE VALLEY...CENTRAL AND NORTHEAST
TX
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY
VALID 232205Z - 240415Z
SUMMARY...VERY INTENSE RAINFALL RATES ARE ANTICIPATED WITH MERGING
AND LOCALLY TRAINING CLUSTERS OF INCREASINGLY ORGANIZED
CONVECTION. FLASH FLOODING IS LIKELY THIS EVENING.
DISCUSSION...THE LATEST IR SATELLITE IMAGERY IS SHOWING NUMEROUS
CLUSTERS OF RAPIDLY COOLING CONVECTIVE TOPS ACROSS THE TX HILL
COUNTRY AND DOWN TOWARD THE RIO GRANDE RIVER AS INCREASINGLY
DIVERGENT FLOW ALOFT AHEAD OF AN UPPER TROUGH INTERACTS WITH A
DEEP POOL OF VERY MOIST AND UNSTABLE AIR.
THE LATEST GOES-SOUNDER LI/S WITH THE AID OF ROBUST DIURNAL
HEATING ARE AS LOW AS -6 TO -8 AND WITH GPS-DERIVED PWATS CLOSE TO
2 INCHES. SBCAPE VALUES ARE NOW RUNNING AS HIGH AS 3000 TO 4000
J/KG...AND ALL OF THIS FAVORABLE THERMODYNAMIC ENERGY IS EXPECTED
TO COUPLE THROUGH THE EVENING WITH AN UPTICK IN DEEP LAYER ASCENT
FOR INCREASINGLY MORE ORGANIZED BANDS OF CONVECTION.
INDIVIDUAL CELL MOTION OVER THE HILL COUNTRY IS GENERALLY TO THE
NORTH...BUT THERE IS EXPECTED TO BE SUFFICIENT CONVECTIVE
ORGANIZATION AND DEVELOPMENT OF A COLD POOL WITH TIME SUCH THAT
CONVECTION WILL ORIENT ITSELF MORE SW/NE WITH THE MEAN FLOW AND
THEN LOCALLY BEGIN GRADUALLY BOWING OFF TO THE EAST AS
ORGANIZATION INCREASES. STRONGER WIND FIELDS ALOFT DOWN TOWARD THE
MIDDLE RIO GRANDE VALLEY ARE ALREADY ALLOWING FOR AN EASTWARD BOW
TO THE CONVECTION HERE.
THE 12Z ARW AND 12Z NMMB ARE BOTH EXPECTED TO BE TOO SLOW WITH THE
EASTWARD CONVECTIVE EVOLUTION DOWN BY THE RIO GRANDE RIVER.
FARTHER NORTH OVER THE HILL COUNTRY AND ESPECIALLY UP INTO
PORTIONS OF NORTH-CENTRAL TX...THE ARW AND NMMB ARE BOTH EXPECTED
TO BE UNDERDONE. THE LATEST RADAR AND SATELLITE TRENDS WOULD FAVOR
A NOTABLY MORE EXPANSIVE AXIS.
THE SET-UP THIS EVENING FAVORS A SW/NE AXIS OF VERY HEAVY AND
POSSIBLY LIFE-THREATENING RAINFALL AMOUNTS GIVEN THE LIKELIHOOD OF
CELL-MERGERS...TRAINING CELLS AND OVERSHOOTING TOPS. ALL
TOGETHER...RAINFALL RATES MAY EXCEED 3 INCHES/HR LOCALLY...AND
THUS VERY IMPRESSIVE STORM TOTAL RAINFALL AMOUNTS WILL BE
POSSIBLE. IN FACT...THE 20Z HRRR SUGGESTS THE POTENTIAL FOR
LOCALLY AS MUCH AS 4 TO 8 INCHES THROUGH MID-EVENING...WITH SOME
OF THE HEAVIEST ACTIVITY FOCUSING JUST NORTH AND WEST OF A LINE
FROM SAN ANTONIO TO AUSTIN TO WACO. WILL CONTINUE TO MONITOR.
ORRISON
ATTN...WFO...CRP...EWX...FWD...SHV...SJT...
