Flash Flood Guidance
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000
AWUS01 KWNH 222222
FFGMPD
MSZ000-LAZ000-230420-

MESOSCALE PRECIPITATION DISCUSSION 0261
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
621 PM EDT MON MAY 22 2017

AREAS AFFECTED...SOUTHERN LA & VICINITY

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

VALID 222220Z - 230420Z

SUMMARY...THUNDERSTORMS ARE BEGINNING TO DEVELOP ACROSS SOUTHWEST
LA.  HOURLY RAIN RATES UP TO 2.5" AND LOCAL AMOUNTS TO 5" ARE
FORECAST FOR SOUTHERN LA THROUGH 04Z; UNCERTAINTY IS GREATER THAN
USUAL.

DISCUSSION...BREAKS IN THE CLOUDINESS ACROSS THE UPPER TX COAST
AND ABOVE IMPERIAL CALCASIEU LA COMBINED WITH AN EASTWARD RETREAT
OF CONVECTION WITHIN THE NORTHWEST GULF OF MEXICO HAVE LED TO
INCREASED INSTABILITY AS OF LATE, WITH THE LATEST MLCAPES UP TO
500-1000 J/KG.  THUNDERSTORMS HAVE BEEN SLOWLY DEVELOPING ACROSS
SOUTHWEST LA DURING THE PAST HOUR.  THIS IS OCCURING NEAR A
SECTION OF A HARD TO FIND FRONTAL ZONE (SHOWING UP BEST IN THE DEW
POINT FIELD) TO THE EAST OF A MESOSCALE WAVE OF LOW PRESSURE.
DIVERGENCE ALOFT IS BEING AIDED BY A SHORTWAVE MOVING INTO THE
ARKLATEX.  PRECIPITABLE WATER VALUES OF 2"+ EXIST HERE.  INFLOW AT
850 HPA IS 25-30 KTS OUT OF THE SOUTHWEST.  EFFECTIVE BULK SHEAR
IS 30 KTS PER SPC MESOANALYSES.

CONVECTION SHOULD EXPAND IN COVERAGE, WHICH WOULD BE AIDED BY CIN
AFTER SUNSET, ALLOWING INSTABILITY ALOFT/HIGHER MUCAPE VALUES TO
BE TAPPED.  THE 12Z GFS-BASED GALVEZ-DAVISON INDEX SHOWS
INCREASING VALUES THROUGH 03Z WHICH IMPLIES GREATER CONVECTIVE
COVERAGE WITH TIME.  CONVECTIVE MOVEMENT IS EXPECTED TO BE THE
GREATEST HURDLE TO HIGHER AMOUNTS, AS THUNDERSTORMS SHOULD MOVE
EAST-NORTHEAST AT 30-40 KTS, THOUGH ORGANIZED CONVECTION WOULD
MOVE MORE EASTWARD.  CELL TRAINING AND MERGERS WOULD ENHANCE RAIN
RATES.  THE AVAILABLE MOISTURE AND FORECAST INSTABILITY SHOULD
SUPPORT HOURLY RAIN RATES TO 2.5".  THERE IS A GOOD SIGNAL FOR
LOCAL AMOUNTS TO 5", THOUGH SOME OF THE GUIDANCE (LIKE THE 12Z
ARW) INDICATED CONVECTIVE INITIATION HOURS AGO, INCREASING
FORECAST UNCERTAINTY.

ROTH

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

ATTN...WFO...LCH...LIX...

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

LAT...LON   31129021 30308967 29258987 29049099 29489202
            29489263 29549279 29769375 30319364 30879188




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