Flash Flood Guidance
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000
AWUS01 KWNH 180237
FFGMPD
LAZ000-TXZ000-180636-

MESOSCALE PRECIPITATION DISCUSSION 0047
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1036 PM EDT FRI APR 17 2015

AREAS AFFECTED...SOUTHEAST TX AND SOUTHWEST LA

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

VALID 180236Z - 180636Z

SUMMARY...AN MCS IS PRODUCING HEAVY RAINFALL RATES OF 2" AN HOUR
AS IT MOVES QUICKLY THROUGH SOUTHEAST TX.  SHOULD THESE RAINFALL
RATES CONTINUE, FLASH FLOODING IS POSSIBLE ACROSS FAR SOUTHEAST TX
AND SOUTHWEST LA.

DISCUSSION...A LONG-LIVED MCS HAS BEEN MOVING 35 KTS TO THE
EAST-NORTHEAST ACROSS SOUTHERN TX OVER THE PAST SEVERAL HOURS,
SIMILAR TO WHAT FORWARD PROPAGATION VECTORS FROM THE 18Z GFS
SUGGEST.  IT IS GENERATING ITS OWN STORM-RELATIVE INFLOW.  MUCAPE
VALUES NEAR AND IN ADVANCE OF THE SYSTEM ARE 1000-2000 J/KG.
PRECIPITABLE WATER VALUES ARE BETWEEN 1.8-2" PER GPS DATA.  THE
ENVIRONMENT IS IN PLACE FOR WET MULTICELLS, WITH 0-6 KM BULK SHEAR
VALUES OF 35-40 KTS SUFFICIENT FOR ORGANIZED CONVECTION.

THE FORECAST IS A TRICKY ONE FOR THIS REGION, HENCE THE POSSIBLE
WORDING.  AN OUTFLOW BOUNDARY TO THE SOUTH IS INCREASINGLY
OFFSHORE THE LA COAST AS YOU MOVE EASTWARD, WHICH IS ALSO SEEN IN
REAL-TIME SPC MESOANALYSIS MUCAPE DATA.  DESPITE A FAVORED
EAST-NORTHEAST MOTION AT 35 KTS, THE HEAVIER RAIN RATES AROUND THE
MCS COULD CONCEIVABLY SLIP SOUTHWARD INTO THE GULF OF MEXICO
RATHER THAN GO INLAND ACROSS LA.  IT APPEARS THE MCS HAS MATURED
PER RECENT SATELLITE IMAGERY, WHICH SHOULD ALLOW FOR A FEW MORE
HOURS OF HEAVY RAIN RATES, WHEREVER IT TRACKS.  RECENT HRRR RUNS,
12Z SPC WRF, 12Z NSSL4WRF, AND 18Z CANADIAN REGIONAL MODEL HAVE
THE BEST GRASP ON THIS SYSTEM`S EVOLUTION SO FAR, AND THEY SHOW
THE STRIPE OF HEAVIEST RAINFALL JUST INLAND ACROSS SOUTHEAST TX
AND SOUTHERN LA.  A THIN BAND OF THUNDERSTORMS TRAILING THE MCS
COULD ALLOW HEAVY RAINFALL AND FLASH FLOODING CONCERNS TO PERSIST
FOR A FEW MORE HOURS BEFORE ANOTHER POSSIBLE CONVECTIVE COMPLEX
TRACKS THROUGH SOUTHERN TX.  HOURLY RAIN RATES UP TO 2" AN HOUR
FOR OVER AN HOUR COULD CAUSE FLASH FLOODING FOR AS LONG AS THE MCS
STAYS ASHORE.

ROTH

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

ATTN...WFO...CRP...EWX...HGX...LCH...

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

LAT...LON   30229473 30199224 29639169 29429182 29369277
            29299347 29189438 29139513 29059701 29739648




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