Graphic for MPD #0257

MESOSCALE PRECIPITATION DISCUSSION 0257
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
800 PM EDT SUN MAY 21 2017

AREAS AFFECTED...WESTERN PORTIONS OF DEEP SOUTH TX

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

VALID 220000Z - 220400Z

SUMMARY...A COUPLE TRAINING CLUSTERS OF CONVECTION COULD LEAD TO
HOURLY RAIN RATES OF 2.5" AND LOCAL AMOUNTS TO 6".

DISCUSSION...CONVECTION NEAR THE RIO GRANDE HAS BEEN STRUGGLING TO
MOVE EASTWARD TODAY, WEAKENING SIGNIFICANTLY AS IT MOVES INTO A
MORE STABLE ENVIRONMENT.  THE LONGER-LIVED CLUSTERS APPEAR TO BE
MOVING PARALLEL TO AN INSTABILITY GRADIENT AND NEAR A ZONE OF
HORIZONTAL WIND SHEAR WHICH IS ORIENTED NORTHWEST/ SOUTHEAST INTO
DEEP SOUTH TX AND WITHIN A DIFFLUENT AREA OF 1000-500 HPA
THICKNESS.  A SHORTWAVE MOVING ACROSS THE TX BIG BEND IS AIDING
DIVERGENCE ALOFT.  PRECIPITABLE WATER VALUES OF 2-2.25" EXIST
HERE, INFLOW AT 850 HPA IS SOUTH-SOUTHEAST AT 20 KTS.  AT THE
STORM BASE, THIS IS NETTING ABOUT 45 KTS OF INFLOW AS THE STORMS
MOVE SOUTHEAST AT 25 KTS.  EFFECTIVE BULK SHEAR IS 55 KTS.

THE EXPECTATION IS FOR THE PARADE OF STORMS TO CONTINUE MOVING
SOUTHEAST AT 25 KTS.  RADAR INDICATIONS ARE THAT 5-6" OF RAIN FELL
ACROSS NORTHEAST MEXICO WHERE THE CLUSTERS HAVE PASSED BY, AND THE
12Z SPC WRF BEST FITS THIS IDEA.  HOURLY RAIN RATES UP TO 2.5"
WITH LOCAL AMOUNTS UP TO 6" COULD LEAD TO FLASH FLOODING.  THE 12Z
GFS-BASED GALVEZ-DAVISON INDEX INDICATES THAT STORM COVERAGE
SHOULD BEGIN TO WANE IN THIS REGION BEYOND 03Z, SO USED A FOUR
HOUR EXPIRATION.

ROTH

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

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

LAT...LON   27739943 26879844 25979791 25849884 26629957
            27459991


Last Updated: 800 PM EDT SUN MAY 21 2017