MESOSCALE PRECIPITATION DISCUSSION 0756
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
912 PM EDT MON AUG 28 2017

AREAS AFFECTED...UPPER TX COAST INTO SOUTHERN AND CENTRAL LA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 290130Z - 290600Z

SUMMARY...SIGNIFICANT FLASH FLOODING IS EXPECTED INTO THE EARLY
MORNING HOURS... PARTICULARLY ACROSS THE UPPER TX COAST AND
PORTIONS OF SOUTHERN LA.

DISCUSSION...THE GOES-16 CLEAN IR LOOP SHOWED SLOWLY COOLING TOPS
IN THE VICINITY OF KIAH EARLY THIS EVENING...WITH COLDER TOPS
CLOSER TO THE COAST. THE RADAR PRESENTATION WITH THE EXPANDING
AREA OF CONVECTION IS MORE REMINISCENT OF MID LEVEL DEFORMATION
NORTH OF THE CENTER OF HARVEY. HOURLY RAINFALL RATES IN THE
DEFORMATION ZONE HAVE GENERALLY BEEN BETWEEN 1.00 AND 1.25
INCHES...ESPECIALLY OVER BRAZORIA AND GALVESTON COUNTIES.

THE GOES-16 SHORT WAVE IR LOOP SHOWED BANDING FEATURE DEVELOPING
SOUTH OF HARVEY...AND REGIONAL RADARS ARE STARTING TO SHOW BANDS
OF CONVECTION FEEDING INTO THE DEFORMATION ZONE OVER THE UPPER TX
COAST. THE BANDS ARE EXPECTED TO FEED INTO THE MID LEVEL FORCING
OVER THE UPPER TX COAST...INCREASING RAINFALL RATES THROUGH
06Z....FOCUSED BY THE LOW LEVEL CONVERGENT FLOW ON THE EAST SIDE
OF HARVEY.

THERE IS A HIGH RESOLUTION MODEL SIGNAL FOR THE AREA NORTH OF THE
CENTER OF HARVEY TO RECEIVE AN ADDITIONAL 4 TO 8 INCHES OF
RAINFALL...WITH THE BEST SIGNAL EXTENDING FROM GALVESTON ACROSS TO
NEAR HOUSTON THROUGH 06Z. HOWEVER...THE MOST RECENT HRRR SUGGESTS
THAT THE HIGHEST RAINFALL MAXIMUM MAY OCCUR CLOSER TO THE COAST
FROM EASTERN BRAZORIA INTO SOUTHERN CHAMBERS COUNTY TX. THE
ADDITIONAL RAINFALL COULD RESULT IN SIGNIFICANT FLASH FLOODING IN
AREAS THAT AREA ALREADY FLOODED....EXACERBATING ONGOING FLOODING. 

FURTHER EAST ACROSS MUCH OF SOUTHERN LA... THE PRECIPITATION
PROCESS IS MORE CONVECTIVELY DRIVEN IN THE WARM SECTOR ASSOCIATED
WITH HARVEY. A POOL OF 1000-1500 J/KG OF MLCAPE LIES ALONG AND
SOUTH OF A FRONTAL BOUNDARY EXTENDING ACROSS THE LA COAST.
CONVECTION FORMING AND MOVING NORTH ACROSS SOUTHERN LA CAN PRODUCE
HOURLY RAINFALL RATES IN EXCESS OF 2.00 INCHES.

THE BANDED FEATURES COULD PRODUCE LOCAL 3 TO 6 INCHES OF RAINFALL
OVER SOUTHERN LA (AND THESE VALUES ARE SUPPORTED BY THE MOST
RECENT HRRR...AS WELL AS THE 12Z WRF ARW). THIS MUCH RAINFALL
COULD EXACERBATE ONGOING FLOODING...OR INITIATE NEW FLASH FLOODING
IN AREAS WITH SATURATED SOILS.

ADDITIONAL FLOODING IS EXPECTED AFTER 06Z ACROSS PORTIONS OF THE
UPPER TX COAST INTO SOUTHERN LA.

HAYES

ATTN...WFO...HGX...JAN...LCH...LIX...SHV...

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

LAT...LON   31659353 31619247 30909068 30269019 29629062
            29709349 29539398 29159462 28899517 28799564
            28979620 29529632 30369601 31239507