Graphic for MPD #0743

MESOSCALE PRECIPITATION DISCUSSION 0743
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
209 PM EDT SUN AUG 27 2017

AREAS AFFECTED...SOUTHEAST TX & SOUTHERN LA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 271808Z - 280008Z

SUMMARY...RAINFALL FROM HARVEY CONTINUES PROGRESSING INTO LA WHILE
PERSISTING IN SOUTHEAST TX.  SIGNIFICANT TO CATASTROPHIC FLASH
FLOODING CONTINUES IN SOUTHEAST TX WHILE IT EXPANDS FARTHER INTO
LA.

DISCUSSION...RADAR IMAGERY HAS SHOWN AS EASTWARD MOVEMENT TO
HARVEY AS OF LATE, WHICH MOVED THE EARLIER BANDS FROM SOUTHEAST TX
INTO SOUTHWEST AND SOUTH-CENTRAL LA.  NEW BANDS CONTINUE TO FORM
NEAR AND WEST OF HOUSTON, BUT REMAIN 50 MILES AWAY FROM ITS
CENTER.  THE INNERMOST BAND IS TRYING TO WRAP AROUND ITS NORTHERN
SIDE AS ELEVATED CONVECTION.  OVERALL, THUNDERSTORM ACTIVITY
REMAINS MAINLY WITHIN THE SYSTEM'S NORTHEAST QUADRANT.  DRIER AIR
AT THE SURFACE AND ALOFT CONTINUES SWEEPING AROUND ITS WESTERN AND
SOUTHERN PERIPHERY WHICH IS GIVING ITS CLOUD PATTERN AN INCREASING
COMMA-LIKE APPEARANCE; DEW POINT FALLS OF 10F WERE SEEN OVER THE
PAST SEVEN HOURS AT BROWNSVILLE TX.  PRECIPITABLE WATER VALUES ARE
2.2-2.7" PER RECENT GPS DATA.  INFLOW AT 850 HPA IS CYCLONIC OUT
OF THE SOUTHEAST AT 20-45 KTS, 50-100% GREATER THAN THE MEAN
850-400 HPA WIND.  THE ORGANIZED THUNDERSTORM BANDS ARE BEING
AIDED BY FAIRLY UNIDIRECTIONAL FLOW WITH HEIGHT IN THE LOW- TO
MID-LEVELS OF THE TROPOSPHERE AND 25-35 KTS OF EFFECTIVE BULK
SHEAR.

THE MESOSCALE GUIDANCE AT 12Z SHOWED A DISTINCT SHIFT TO THE EAST
IN THE RAINFALL PATTERN, WHICH GENERALLY MATCHES RECENT RADAR
TRENDS AND MOTION OF HARVEY.  RAIN BANDS SHOULD CONTINUE RADIATING
EASTWARD OUT OF HARVEY UNTIL AT LEAST SUNSET, WITH THE EASTWARD
LIMIT DEFINED BY THE FLOW AROUND THE BACKSIDE OF A DEEPENING LOW
OFFSHORE GA.  HOURLY RAIN TOTALS UP TO 3" ARE EXPECTED WITHIN THIS
MOIST AIR MASS.  THE MESOSCALE GUIDANCE SHOWS A GOOD SIGNAL FOR 6"
OF RAIN, WHICH IS BELIEVABLE WITHIN THIS CONVECTIVE CYCLONE. 

THE THREAT OF FLASH FLOODING IS INCREASING ACROSS SOUTHERN LA AS
SOILS SATURATE, WHILE SIGNIFICANT TO CATASTROPHIC FLASH FLOODING
CONTINUES ACROSS PORTIONS OF SOUTHEAST TX, INCLUDING THE HOUSTON
METROPOLITAN AREA.  THE AVERAGE RAINFALL WITHIN THE HARRIS COUNTY
EMERGENCY MANAGEMENT NETWORK HAS EXCEEDED THAT OF TROPICAL STORM
ALLISON (2001) IN ALMOST HALF OF THE TIME (2-3 VERSUS 5 DAYS). 
HISTORIC FLOODING IS EXPECTED TO CONTINUE IN THE HOUSTON
METROPOLITAN AREA THROUGH THE FORESEEABLE FUTURE.

ROTH

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

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

LAT...LON   31609465 30549187 29309003 29079015 29019098
            29449196 29459271 29679330 29439441 28879523
            28229658 28749750 29449770 29569797 29689808
            30009803 30699775 30759729 30809662 30919635
           


Last Updated: 209 PM EDT SUN AUG 27 2017