MESOSCALE PRECIPITATION DISCUSSION 0125
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1227 AM EDT FRI MAY 04 2018

AREAS AFFECTED...SOUTH-CENTRAL TEXAS AND THE EDWARDS PLATEAU

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

VALID 040426Z - 041015Z

SUMMARY...THUNDERSTORMS WILL BECOME MORE NUMEROUS OVERNIGHT IN
SOUTH-CENTRAL TEXAS...SPREADING FROM THE RIO GRANDE VALLEY TO THE
NORTHEAST WITH TIME. SLOW STORM MOTIONS AND HEAVY RAIN COULD LEAD
TO RAIN RATES TO AROUND 2.0 TO 2.5 IN/HR...WHICH COULD LEAD TO
LOCALIZED FLASH FLOODING.

DISCUSSION...AS OF 04Z GOES-16 IR SATELLITE LOOPS AND KDFX RADAR
SHOWED A CONSIDERABLE INCREASE IN CONVECTIVE COVERAGE AND
INTENSITY OVER THE RIO GRANDE VALLEY. HI-RES MODELS WERE
STRUGGLING TO MATCH THE PLACEMENT OF ONGOING
CONVECTION...GENERALLY FOCUSED TO THE NORTHWEST OF DEL RIO ALONG
THE RETREATING DRYLINE. INSTEAD THE DEVELOPING THUNDERSTORMS
APPEARED TO BE CONCENTRATED NEAR A MAXIMUM IN LOW-LEVEL MOISTURE
FURTHER TO THE SOUTHEAST. CIRA BLENDED TPW PRODUCT INDICATES
PRECIPITABLE WATER VALUES IN THE VICINITY OF DEVELOPING CONVECTION
APPROACHING 2 INCHES...AND THIS IS SUPPORTED BY GPS-PW
OBSERVATIONS AROUND 1.8 TO 1.9 INCHES IN THE SAME REGION. THESE
VALUES ARE ABOVE THE 90TH PERCENTILE IN THE DRT SOUNDING
CLIMATOLOGY FOR EARLY MAY AND NEAR RECORD VALUES.

CONVECTIVE COVERAGE IS EXPECTED TO INCREASE IN THIS MOIST UNSTABLE
ENVIRONMENT (MUCAPE AROUND 2000 J/KG PER RAP ANALYSIS) DURING THE
OVERNIGHT HOURS. THIS IS ALSO SHOWN BY MOST CONVECTION ALLOWING
MODELS...BUT THE PLACEMENT IS SLIGHTLY DIFFERENT THAN REALITY. THE
CONVECTION MAY BEGIN TO CONGEAL INTO CLUSTERS WITH RELATIVELY SLOW
AND VARIED STORM MOTIONS...PARTIALLY DUE TO SOME SPLITTING
SUPERCELLS...WHICH WOULD THEN TEND TO MOVE TO THE NORTHEAST OR
EAST WITH TIME ALONG THE LEADING EDGE OF BROAD MOIST SOUTHEASTERLY
LOW-LEVEL INFLOW  OVER SOUTH TEXAS. THIS WOULD POTENTIALLY PLACE
THE HEAVIEST RAINFALL FROM NEAR DEL RIO AND THE US-90 CORRIDOR
EAST TO THE SAN ANTONIO AND BALCONES ESCARPMENT REGION. THE
NEAR-RECORD PWATS WOULD AND ORGANIZED NATURE OF THE CONVECTION
COULD SUPPORT HOURLY RAIN RATES POSSIBLY IN EXCESS OF 2 IN/HR WITH
RAINFALL TOTALS IN THE LOCALIZED MAXIMA IN EXCESS OF 5 INCHES.
THESE AMOUNTS COULD LEAD TO FLASH FLOODING...PARTICULARLY IN MORE
URBANIZED AREAS GIVEN THE RELATIVELY HIGH FFG ACROSS MUCH OF THE
REGION OVERALL. THIS DISCUSSION AREA IS ALSO OUTLINED FURTHER TO
THE NORTHWEST INTO THE EDWARDS PLATEAU AND CONCHO VALLEY TO
ACCOUNT FOR THE HI-RES MODEL SIGNAL FOR ADDITIONAL DEVELOPMENT
ALONG THE RETREATING DRYLINE. CONFIDENCE IS LOWER IN SIMILARLY
ROBUST DEVELOPMENT THAT FAR NORTHWEST GIVEN ONGOING RADAR AND
SATELLITE TRENDS...BUT FFG IS LOWER IN THOSE AREAS WITH RECENT
HEAVY RAINFALL.

LAMERS

ATTN...WFO...CRP...EWX...MAF...SJT...

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

LAT...LON   31260005 31239897 30719824 29579801 28639845
            28099910 27809986 28120032 28630071 29150103
            29680180 30060201 30690140