Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 270851
FFGMPD
LAZ000-TXZ000-271250-

MESOSCALE PRECIPITATION DISCUSSION 0138
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
451 AM EDT WED MAY 27 2015

AREAS AFFECTED...SOUTHEAST TX & SOUTHWEST LA

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

VALID 270850Z - 271250Z

SUMMARY...A BAND OF THUNDERSTORMS IS EXPECTED TO MOVE INTO WESTERN
LA AND SOUTHEAST TX WITH HOURLY RAIN RATES UP TO 2" AN HOUR.
FLASH FLOODING IS POSSIBLE DUE TO SATURATED SOILS.

DISCUSSION...SATELLITE AND RADAR IMAGERY REVEAL A SQUALL LINE
WHICH HAS BECOME OUTFLOW DOMINANT HAS CAUSED ACTIVITY WITHIN ITS
SOUTHERN AND SOUTHWESTERN PERIPHERY TO BECOME MORE SCATTERED AND
BROADER IN AREAL EXTENT.  ITS FORWARD PROGRESSION IS
EAST-SOUTHEAST TO SOUTHEAST AT 25 KTS SINCE 0330Z.  THE INCREASE
IN BREADTH OF THE CONVECTION HAS LED TO A RECENT INCREASE IN
HOURLY RAINFALL RATES, BACK UP TOWARDS 2" LOCALLY.  INFLOW AT 850
HPA IS 25-35 KTS OVER ITS ASSOCIATED OUTFLOW BOUNDARY, IMPORTING
3000-5000 J/KG MUCAPES OVER THE FEATURE.  THE OUTFLOW BOUNDARY IS
STARTING TO PUSH INTO A SOUPY AIRMASS, WITH DEWPOINTS IN THE MID
TO UPPER 70S LOCATED ACROSS SOUTHERN TX AND ALONG THE TX COASTAL
PLAIN.  COMPLICATING THE CONVECTIVE FORECAST IS THE EDGE OF A CAP,
LOCATED FROM NEAR HOUSTON NORTHWESTWARD INTO CENTRAL TX WHICH
APPEARS NEARLY COINCIDENT WITH THE MUCAPE GRADIENT AND 9C 700 HPA
ISOTHERM.

THE MESOSCALE GUIDANCE HAS GENERALLY MISSED THE BOAT CONCERNING
THIS AREA OF CONVECTION, WITH THE 00Z SPCWRF THE BEST OF THE LOT,
ABOUT SEVEN HOURS TOO SLOW ON THE CONVECTIVE SYSTEM`S FORWARD
MOTION.  THE EXPECTATION IS FOR THE AREA OF THUNDERSTORMS TO
CONTINUE MOVING SOUTHEAST AT 25 KTS ACROSS SOUTHEAST TX AND
WESTERN LA AS THUNDERSTORM ACTIVITY CONTRIBUTES TO THE COLD POOL.
THESE SORT OF RAIN RATES CANNOT BE HANDLED IN THE HOUSTON METRO
AREA.  RECENT RAP FORECASTS INDICATE 850 HPA INFLOW INTO THE
REGION COULD INCREASE OVER THE NEXT SEVERAL HOURS, WHICH COULD
AGGRAVATE THE SITUATION.  HOURLY RAINFALL RATES UP TO 2" COULD
LEAD TO FLASH FLOODING OVER SATURATED SOILS.  UNCERTAINTY
REGARDING THE OVERALL FORECAST LED TO THE POSSIBLE CATEGORY BEING
USED.

ROTH

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

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

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

LAT...LON   32579379 32479307 31999229 31299253 30399296
            29759370 29359465 29179518 29879644 30359726
            30979776 31549768 31229722 30719668 30829587
            30719580 31939447


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