MESOSCALE PRECIPITATION DISCUSSION 0126
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
729 PM EDT FRI MAY 04 2018

AREAS AFFECTED...SOUTH TX

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

VALID 042330Z - 050230Z

SUMMARY...SHORT TERM TRAINING AND CELL MERGERS COULD PRODUCE
HOURLY RAINFALL RATES IN EXCESS OF 2.00 INCHES...WHICH COULD POSE
A FLASH FLOOD THREAT INTO THE EVENING HOURS.

DISCUSSION...THE MOST RECENT GOES-16 CLEAN IR LOOP SHOWED AN
INCREASING AREA OF COOLING TOPS ACROSS FAR SOUTH TX INTO
COAHUILA...NUEVO LEON AND TAMAULIPAS IN NEARBY MEXICO (WITH CLOUD
TOPS TEMPERATURES AS COLD AS -75 CELSIUS). THE CONVECTION IS
FORMING IN AN AXIS OF OF 1000-1500 J/KG OF MLCAPE EXTENDING ACROSS
FAR SOUTH TX... AHEAD OF A COLD FRONT DROPPING SOUTH FROM CENTRAL
TX.

THE CONVECTION IS FORMING ALONG A STRONG PRECIPITABLE WATER
GRADIENT...WITH VALUES NEAR 2.00 INCHES NEAR THE RIO GRANDE RIVER.
THUS FAR...THE CONVECTION HAS BEEN FAIRLY SCATTERED ACROSS SOUTH
TX...BUT PER THE KCRP RADAR...THE CONVECTION THAT HAS FORMED HAS
PRODUCED HOURLY RAINFALL RATES OVER 2.00 INCHES OVER DUVALL
COUNTY. THE HCA PRODUCT FROM KCRP SUGGESTS THAT THESE RAINFALL
RATES COULD BE HAIL CONTAMINATED AS RAP SOUNDINGS IN SOUTH TX DID
SHOW SOME DRY AIR IN THE MID LEVELS AWAY FROM THE COAST.

WITH THE BEST FORCING STILL NORTH AND WEST OF THE REGION THROUGH
THE EARLY EVENING HOURS...IT IS STILL UNCLEAR JUST HOW WIDESPREAD
THE CONVECTION BECOMES. THE MOST RECENT HRRR INDICATED CONVECTION
FORMING AHEAD OF THE LINE (WITH PROVIDES LOW LEVEL CONVERGENCE)
AND DROPPING SOUTH TOWARD THE RIO GRANDE RIVER. LOCAL RADARS ALSO
SHOWED OUTFLOW BOUNDARIES BEING PRODUCED...AND THE INTERSECTIONS
OF OUTFLOW BOUNDARIES COULD ALSO PRESENT AN OPPORTUNITY FOR FLASH
FLOODING.

IN THIS SCENARIO...CELLS WITH HOURLY RAINFALL RATES IN EXCESS OF
2.00 INCHES COULD TRAIN OR MERGE. TRAINING OR MERGERS COULD RESULT
IN LOCAL 4.00 INCH RAINFALL AMOUNTS...WHICH COULD OCCUR IN LESS
THAN TWO HOURS. ONE AND THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE
FAIRLY HIGH (GENERALLY ABOVE 3.00 INCHES)...SO TRAINING OR CELL
MERGERS WOULD BE NECESSARY FOR FLASH FLOODING TO INITIATE. THE
MOST RECENT RAP SUGGESTS THAT THE THREAT FOR FLASH FLOODING WILL
WANE DURING THE EVENING HOURS (GENERALLY AFTER 05/03Z)...AS
INSTABILITY IS EITHER EXHAUSTED BY CONVECTION...OR BECOMES
ELEVATED IN FAVOR OF LOW LEVEL CIN.

AT THIS POINT...THE THREAT FOR FLASH FLOODING EXISTS...THOUGH IT
IS NOT CLEAR JUST HOW WIDESPREAD THE THREAT WILL BECOME.
THUS...FLASH FLOODING IS ONLY CONSIDERED POSSIBLE INTO THE EVENING
HOURS.

HAYES

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

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

LAT...LON   28549807 27859790 27059821 26399866 26389908
            26389908 26719929 27459919 28479872