MESOSCALE PRECIPITATION DISCUSSION 0587
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
900 PM EDT SAT AUG 20 2016
 
AREAS AFFECTED...PORTIONS OF SOUTH-CENTRAL AND SOUTHEAST TX 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 210100Z - 210700Z
 
SUMMARY...EXTREMELY HEAVY RAINFALL WILL BE LIKELY THIS EVENING
INTO THE OVERNIGHT HOURS...WITH SIGNIFICANT AND POSSIBLY
LIFE-THREATENING FLASH FLOODING EXPECTED.

DISCUSSION...LATE-DAY VISIBLE SATELLITE IMAGERY SHOWS A VERY
WELL-DEFINED AND COMPACT MCV ACTUALLY MOVING SOMEWHAT
PROGRESSIVELY OFF TO THE EAST-NORTHEAST OUT OF THE TX HILL COUNTRY
AND TOWARD SOUTHEAST TX. HOWEVER...WHILE THE MCV IS GENERATING A
CONCENTRATED AREA OF CONVECTION WITH VERY INTENSE RAINFALL
RATES...THERE IS AN EXTENDED AXIS OF STRONG THUNDERSTORMS WITH
VERY INTENSE RAINFALL THAT HAS SET UP AROUND THE SOUTHWEST FLANK
OF THE MCV...EXTENDING FROM JUST WEST OF BRENHAM TO FAYETTE AND
THEN DOWN TO THE SOUTHERN SUBURBS OF SAN ANTONIO. THIS SW/NE
ORIENTED BAND OF CONVECTION COINCIDES WITH A WELL-DEFINED
CONVECTIVELY REINFORCED BOUNDARY THAT IS INTERCEPTING VERY MOIST
AND UNSTABLE AIR POOLING NORTH OUT OF SOUTH TX...WITH PWATS OF 2.2
TO 2.4 INCHES AND MLCAPE VALUES OF 2500 J/KG.

THE SET-UP OVERNIGHT FAVORS THE CONTINUATION OF A SW/NE ORIENTED
AXIS OF EXTREMELY HEAVY CONVECTIVE RAINFALL AS THIS SURFACE
BOUNDARY IS EXPECTED TO STALL OUT WHILE COINCIDING WITH A
RELATIVELY DIFFLUENT UPPER FLOW PATTERN AND PERSISTENTLY
MOIST/UNSTABLE BOUNDARY LAYER FOR CONVECTION TO DEVELOP AND REPEAT
OVER THE SAME AREA.

HIRES GUIDANCE GOING BACK TO 12Z/SAT...INCLUDING RUNS OF THE
ARW/NMMB AND NSSL-WRF HAVE BEEN ALL SHOWING A VERY STRONG SIGNAL
FOR EXTREME RAINFALL OVERNIGHT ACROSS PORTIONS OF THE TX HILL
COUNTRY IN PARTICULAR. HOWEVER...RECENT CONVECTIVE TRENDS VIA
SATELLITE AND RADAR SHOW THAT THE MCV HAS BECOME NOTABLY MORE
PROGRESSIVE...AND HAS ALLOWED A LOW LEVEL FOCUS FOR VERY HEAVY
RAINS A BIT FARTHER SOUTH AND EAST OF THESE EARLIER HIRES MODELS.

THE LATEST NCEP HRRR/HRRRP AND ESRL HRRR GUIDANCE SUPPORTS A
SOUTHEASTWARD SHIFT WITH A STRONG FOCUS FROM NEAR AND JUST SOUTH
OF SAN ANTONIO AND NORTHEAST TO NEAR AND JUST EAST OF FAYETTE.
ADDITIONAL RAINFALL AMOUNTS OF AS MUCH AS 4 TO 8 INCHES ARE
EXPECTED THROUGH THE EARLY MORNING HOURS IN VICINITY OF THESE
AREAS...AND WILL BE CAPABLE OF PRODUCING SIGNIFICANT AND POSSIBLY
LIFE-THREATENING FLASH FLOODING. 

ADDITIONAL HEAVY RAINS THOUGH WILL REMAIN FOCUSED NEAR THE MCV OFF
TO THE NORTHEAST FOR THE NEXT FEW HOURS WHICH WILL ALSO FOCUS
FLASH FLOODING POTENTIAL FOR AREAS OF SOUTHEAST TX NORTHWEST OF
HOUSTON TX. WILL CONTINUE TO MONITOR.

ORRISON

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

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

LAT...LON   31629468 30959415 30339442 29729527 29049642 
            28409769 27889872 27849924 28109966 28509969 
            29099924 29729859 30249780 30739692 31199603