MESOSCALE PRECIPITATION DISCUSSION 0124
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1200 AM EDT TUE APR 18 2017

AREAS AFFECTED...SOUTHEAST TX

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

VALID 180400Z - 181000Z

SUMMARY...CLUSTERS OF VERY SLOW-MOVING SHOWERS AND THUNDERSTORMS
WITH INTENSE RAINFALL IS EXPECTED OVERNIGHT. POCKETS OF FLASH
FLOODING WILL BE POSSIBLE.

DISCUSSION...A RATHER WELL-DEFINED MCV IS DRIFTING VERY SLOWLY
EASTWARD ACROSS SOUTHEAST TX AND IS EXPECTED TO HELP FOCUS
MULTIPLE CLUSTERS OF HEAVY SHOWERS AND THUNDERSTORMS OVERNIGHT.
SOME OF THE DEVELOPMENT IS EXPECTED TO BE NEAR THE VORT CENTER
ITSELF WHICH IS CURRENTLY NEAR FAYETTE TX AND THE CONVECTION IS
LIKELY TO BE RATHER CONCENTRATED GIVEN THE FOCUSED NATURE OF THE
VORT DYNAMICS AND INTERACTION WITH A RELATIVELY DEEP POOL OF
MOISTURE. PWATS OF 1.6 TO 1.7 INCHES ARE NOTED IN THE LATEST
BLENDED-TPW DATA...WITH CIRA-LPW DATA SHOWING CONCENTRATED
MOISTURE IN THE 850/700MB AND 700/500MB LAYERS. THIS SUGGESTS
RATHER HIGHLY EFFICIENT RAINFALL AND ENHANCED RAINFALL RATES. THE
ONE MITIGATING FACTOR IS A LACK OF INSTABILITY...BUT THE LATEST
RAP ANALYSIS IS SHOWING SOME PARTICULARLY CONCENTRATED FORCING
NEAR THE VORT CENTER ALONG WITH A SURFACE MOISTURE CONVERGENCE
MAXIMA.

CONVECTIVE DEVELOPMENT IS ALSO LIKELY TO OCCUR CLOSER TO THE
MIDDLE AND UPPER TX COASTS OVER TIME AS LOW LEVEL MOISTURE AND
INSTABILITY TRANSPORT FOCUSES INTO A SURFACE TROUGH JUST INLAND
OVER THE COASTAL PLAIN. THE LATEST RAP ANALYSIS SHOWS A STRONG
INSTABILITY GRADIENT IN VICINITY OF THE TROUGH AXIS WITH A POOL OF
MORE THAN 2000 J/KG OF MUCAPE OUT OVER THE NORTHWEST GULF OF
MEXICO. LOW LEVEL SOUTHEAST FLOW IN THE BOUNDARY LAYER WILL HELP
TRANSPORT AND FOCUS THIS ENERGY INTO THE TROUGH AXIS AND SHOULD
HELP STRONGLY FACILITATE CONVECTIVE DEVELOPMENT AS THE UPSTREAM
VORT CENTER APPROACHES. CLUSTERS/BANDS OF SLOW-MOVING AND LOCALLY
TRAINING SHOWERS AND THUNDERSTORMS ARE EXPECTED WITH INTENSE
RAINFALL. EXPECT RAINFALL RATES OF 2 TO 3 INCHES/HR GIVEN THE PWAT
ENVIRONMENT.

HIRES MODELS LED BY CONSECUTIVE HRRR RUNS AND ALSO THE 12Z
ARW/NMMB SOLUTIONS AND 00Z NAM-CONEST SHOW SIGNALS FOR AS MUCH AS
3 TO 6 INCHES OF RAIN OVERNIGHT. PARTS OF THE REGION HAVE ALREADY
SEEN HEAVY RAINFALL LEADING TO WET ANTECEDENT CONDITIONS. SO THE
ADDITIONAL RAINFALL MAY RESULT IN SOME ENHANCED RUNOFF CONCERNS
AND FLASH FLOODING.

ORRISON

ATTN...WFO...CRP...EWX...FWD...HGX...LCH...SHV...

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

LAT...LON   32219564 31379445 30119427 29459459 29049509
            28569607 28349638 27659724 27839809 28399846
            29049793 29809739 30539721 31599685