MESOSCALE PRECIPITATION DISCUSSION 0742
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
809 AM EDT SUN AUG 27 2017

AREAS AFFECTED...SOUTHEAST TX & SOUTHWEST LA

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

VALID 271208Z - 271808Z

SUMMARY...CATASTROPHIC FLOODING IN THE HOUSTON METROPOLITAN AREA
IS EXPECTED TO WORSEN AND COULD BECOME HISTORIC IN ASSOCIATION
WITH HARVEY, WITH POTENTIALLY SIGNIFICANT FLOODING ALSO EXPECTED
IN OTHER SATURATED AREAS OF SOUTHEAST TX.  HOURLY RAIN TOTALS UP
TO 3" WITH LOCAL AMOUNTS TO 7" ARE EXPECTED, BRINGING LOCAL STORM
TOTAL ACCUMULATIONS TOWARD 35" LOCALLY BY 18Z.

DISCUSSION...A COMBINATION OF DRIER/STABLE LOW LEVEL AIR HAS
INFILTRATED THE CENTER AND FORCED THE CONVECTIVE PATTERN AROUND
THE SYSTEM TO RESEMBLE A CONVECTIVE T-BONE STRUCTURE.  A
SIGNIFICANT DEW POINT GRADIENT IS SHOWING UP SOUTH OF ITS CENTER
NEAR THE COAST OF MAINLAND TX -- WEST OF PADRE ISLAND, EXTENDING
RIGHT UP THE COAST BEFORE CURLING NORTHWARD WEST OF ANGLETON TX. 
MID-LEVEL DRY AIR IS IMPINGING ON ITS WESTERN SIDE -- THERE IS THE
APPEARANCE OF SOME WESTERLY VERTICAL WIND SHEAR OVER HARVEY. 
BANDS OF DEEP CONVECTION CONTINUE TO ROTATE INTO GALVESTON,
HOUSTON, AND AREAS NORTHEAST OF PALACIOS TX OVER 100 MILES FROM
THE CENTER OF HARVEY, WHOSE CENTER IS SLOWLY EDGING AWAY FROM THE
AREA.  HOURLY RAIN TOTALS PER RECENT RADAR ESTIMATES HAVE FALLEN
TO ABOUT 3", WHICH CORRESPONDS TO HARRIS COUNTY EMERGENCY
MANAGEMENT RAIN GAGE TOTALS.  RAIN TOTALS OVER THE PAST 48 HOURS
HAVE REACHED 27" IN SOUTHEAST HARRIS COUNTY, WITH AMOUNTS CLOSER
TO 12" ACROSS NORTHERN AND WESTERN HARRIS COUNTY.  VOLUME-WISE,
THIS HAS LIKELY REACHED THE RAINFALL THAT FELL DURING ALLISON IN
JUNE 2001, AND IT CONTINUES TO RAIN.  BOTH THE ML CAPES AND MU
CAPES HAVE BEEN SLOWLY DECLINING, WHICH LIKELY EXPLAINS THE
REDUCING HOURLY RAIN RATES, WITH VALUES NEAR 1000 J/KG ALONG THE
UPPER TX COAST.  INFLOW AT 850 HPA IS CYCLONIC, OUT OF THE
SOUTHEAST AT 35-45 KTS PER VAD WIND PROFILES.  PRECIPITABLE WATER
VALUES ARE ~ 2.75".

SPIRAL BANDS TEND TO MOVE AWAY FROM THE CENTER OF CONVECTIVE LOWS,
SO IT IS POSSIBLE THAT THE BANDING IN THE GALVESTON/HOUSTON AREA
COULD CONTINUE FOR SEVERAL MORE HOURS -- FOR AS LONG AS THE STORM
IS DRIFTING SOUTHWARD.  ANY EASTWARD DRIFT WOULD POTENTIALLY MOVE
THE HEAVIEST RAIN AXIS EASTWARD.  MEANWHILE, TO THE NORTHEAST,
ISSUES ARE EXPECTED ACROSS THE GOLDEN TRIANGLE OF SOUTHEAST TX AND
IMPERIAL CALCASIEU AS RECENT RAINS HAVE LOWERED FLASH FLOOD
GUIDANCE VALUES AND CONVECTION FROM THE SLOWLY INCOMING BAND
OCCASIONALLY MERGES IN WITH THUNDERSTORMS ALONG AN EFFECTIVE
STATIONARY FRONT TO ITS EAST-NORTHEAST.  RAINFALL HAS BEEN WELL
ABOVE AVERAGE IN SOUTHWEST LA AND DAM ISSUES REMAIN IN PORT ARTHUR
TX.  THE MESOSCALE GUIDANCE SHOWS A SLOW RETREAT IN RAINFALL
RATES/AMOUNTS WHEN COMPARED TO OVERNIGHT AND EARLIER THIS MORNING,
BUT THE DAMAGE HAS BEEN DONE.  THE MESOSCALE GUIDANCE INDICATES
LOCAL AMOUNTS TO 7", WITH HOURLY RAIN TOTALS UP TO 3" EXPECTED. 
THIS WOULD ELEVATE THE CATASTROPHIC FLOOD EVENT INTO RECORD
TERRITORY FOR THE HOUSTON METROPOLITAN AREA.

ROTH

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

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

LAT...LON   31369488 30229234 29519258 29709352 29179477
            28299649 28299715 29459862 30919765