000

AWUS01 KWNH 271209

FFGMPD

LAZ000-TXZ000-271808-



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



...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...



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



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



LAT...LON   31369488 30229234 29519258 29709352 29179477 

            28299649 28299715 29459862 30919765 


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