MESOSCALE PRECIPITATION DISCUSSION 0249
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
503 PM EDT SAT MAY 20 2017

AREAS AFFECTED...SOUTH TX

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

VALID 202103Z - 210103Z

SUMMARY...THUNDERSTORMS WITH HIGH RAIN RATES CONTINUE TO DROP
SOUTH TO THE EAST OF THE RIO GRANDE.  HOURLY RAIN RATES TO 2.5"
WITH LOCAL AMOUNTS TO 6" ARE ANTICIPATED.

DISCUSSION...A CLUSTER OF THUNDERSTORMS SOUTHEAST OF DEL RIO HAS
BEEN DROPPING SOUTH AT ~10 KTS OVER THE PAST SEVERAL HOURS.  THE
STORMS HAVE BEEN SLIGHTLY ELEVATED AND FOCUSED ALONG AN OUTFLOW
BOUNDARY WHICH HAS LATELY BEEN REINFORCED BY A SYNOPTIC SCALE COLD
FRONT.  THE IMPORTANT ENVIRONMENTAL FACTORS FOR HEAVY RAINFALL
SEEM TO BE PRESENT -- PRECIPITABLE WATER VALUES NEAR 2", 850 HPA
INFLOW OF 25-30 KTS, 0-6 KM BULK SHEAR OF ~50 KTS, AND MU CAPES OF
2000-4000 J/KG.  THE LOW-LEVEL INFLOW APPEARS TO BE COUNTERING THE
NORMAL TENDENCY OF THE CONVECTION TO MOVE EASTWARD WITHIN A
DIFFLUENT THICKNESS PATTERN, WHICH HAS MAINTAINED RAINFALL
EFFICIENCY.

THE MESOSCALE GUIDANCE IS FAIRLY UNANIMOUS ON EXPECTATIONS IN THIS
REGION, SHOWING A SIGNAL FOR LOCAL AMOUNTS OF 6" DURING THE NEXT
SEVERAL HOURS.  STORMS MOVING IN FROM THE SOUTH COULD MERGE WITH
THIS CLUSTER, FURTHER ENHANCING RAIN RATES/TOTALS.  THE AVAILABLE
MOISTURE AND INSTABILITY SUPPORTS HOURLY RAIN RATES TO 2.5".  THE
UNCERTAINTY HERE LIES IN COVERAGE AND THE POSSIBILITY THAT THIS
CONVECTION COULD TRY TO MOVE SOUTHWEST ACROSS THE RIO GRANDE OVER
THE NEXT SEVERAL HOURS AS THE 850 HPA INFLOW ATTEMPTS TO BACK,
WHICH WOULD SUPPORT BACK BUILDING INTO THE INSTABILITY POOL WHICH
IS STRONGEST IN NORTHEAST MEXICO.

ROTH

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

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

LAT...LON   29059926 28559867 27379838 26589847 26659950
            27940007