MESOSCALE PRECIPITATION DISCUSSION 0458
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
712 AM EDT THU AUG 20 2015
 
AREAS AFFECTED...SOUTHEAST TX & VICINITY 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 201110Z - 201510Z
 
SUMMARY...STORMS ACROSS THE UPPER TX COAST SHOULD HAVE HOURLY RAIN
RATES UP TO 2.5", WHICH COULD LEAD TO FLASH FLOODING.

DISCUSSION...THUNDERSTORMS WITH HEAVY RAINFALL ARE DEVELOPING
BETWEEN GALVESTON AND PORT ARTHUR.  THESE ARE ELEVATED NORTH OF AN
OUTFLOW BOUNDARY, WHICH HAS A WAVE SOUTH OF GALVESTON.  INFLOW AT
850 HPA IS ABOUT 20 KTS OUT OF THE SOUTH-SOUTHWEST, SLIGHTLY
EXCEEDING THE MEAN 850-400 HPA WIND.  MUCAPES UPSTREAM NEAR THE
MIDDLE TX COAST ARE IMPRESSIVE, WITH VALUES OF 3000-6000 J/KG. 
PRECIPITABLE WATER VALUES OF 2.1-2.2" ARE REPORTED VIA GPS.

THE 06Z SSEO NEIGHBORHOOD PROBABILITIES SUGGEST THE PEAK OF THE
HEAVY RAIN EVENT WILL BE IN THE 12-15Z TIME FRAME.  SINCE THIS IS
EARLY MORNING ACTIVITY, BELIEVE THE THREAT WILL BE CONSTRAINED TO
THE COASTAL TWO TIERS OF COUNTIES/PARISHES OF SOUTHEAST TX AND FAR
SOUTHWEST LA, AND CONFINED THE THREAT TO PLACES TO THE NORTHEAST
OF THE DIVERGENCE FORECAST AT 850 HPA NEAR THE MIDDLE TX COAST. 
AFTER 15Z, INFLOW AT 850 HPA INTO THE REGION WEAKENS, WHICH SHOULD
REDUCE THE BROADER FLASH FLOOD THREAT.  OUTSIDE OF THE 09Z HRRR,
THE CAM GUIDANCE APPEARS TO HAVE A HANDLE ON THIS ACTIVITY. 
MESOSCALE QPF GUIDANCE ADVERTISES LOCAL AMOUNTS OF 4-7" IN AND
NEAR THIS AREA, WHICH WOULD EXCEED FLASH FLOOD GUIDANCE, EVEN
OUTSIDE OF THE HOUSTON METROPOLITAN AREA.  HOURLY RAIN RATES UP TO
2.5" ARE POSSIBLE, WHICH SHOULD ALLOW THE RAIN TO FALL QUICKLY. 
USED THE POSSIBLE CATEGORY HERE, SINCE SOME OF THE AREA INCLUDED
IS SWAMPLAND.

ROTH

ATTN...WFO...HGX...LCH...

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

LAT...LON   30789364 30369293 29669368 29539424 28669579 
            28749646 29619617 30639467 
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