MESOSCALE PRECIPITATION DISCUSSION 0730
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
729 PM EDT FRI AUG 25 2017

AREAS AFFECTED...UPPER TX COAST...SOUTHWEST LA

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

VALID 252330Z - 260430Z

SUMMARY...OUTER BANDS FROM HURRICANE HARVEY WILL CONTINUE TO CROSS
THE UPPER TX COAST AND NEARBY SOUTHWEST LA INTO THE LATE EVENING
HOURS.

DISCUSSION...THE GOES-16 CLEAN IR LOOP SHOWED BURSTS OF DEEP
CONVECTION IN THE OUTER BANDS OF HURRICANE HARVEY SOUTH OF THE
CENTRAL LA COAST (WITH TOPS AS COLD AS -80 CELSIUS). OTHER BURSTS
OF CONVECTION IN THE BANDING HAVE BEEN NOTED OVER THE UPPER TX
COAST (NORTHEAST OF HOUSTON)...THOUGH THE TOPS WERE NOT NEARLY AS
COLD.

THE LCH RADAR SHOWED HOURLY RAINFALL RATES NEAR 2.50 INCHES WITH
THE DEEP CONVECTION SOUTH OF THE CENTRAL LA COAST. AS THE BANDING
HAS APPROACHED THE COAST... INSTABILITY DECREASED NEAR A FRONTAL
BOUNDARY EXTENDING FROM THE MIDDLE TX COAST THROUGH SOUTHERN MS.
THE BEST INSTABILITY LIES SOUTH OF THE FRONT...AND AS THE
CONVECTION APPROACHES THE COAST... ISENTROPIC LIFT OVER THE FRONT
HAS BEEN SPREADING OUT THE ELEVATED INSTABILITY... RESULTING IN A
WEAKENING TREND FOR MOST OF THE OUTER BANDS.

THE HOURLY RATES OVER THE UPPER TX COAST WERE CLOSER TO 1.50/1.75
INCHES... WHERE INSTABILITY IS LOWER. THE LOW LEVEL CONVERGENCE
ALONG THE FRONT IS FOCUSING THE INSTABILITY AND 2.50 INCH
PRECIPITABLE WATER AIR INTO CONVECTION AS IT CROSSES THE UPPER TX
COAST...RESULTING IN ENHANCED RAINFALL RATES.

THE BANDING FEATURES ARE EXPECTED TO CONTINUE COMING ASHORE OVER
THE UPPER TX COAST AND NEARBY SOUTHWEST LA THROUGH THE LATE
EVENING HOURS. THE CONVECTION IS EXPECTED WEAKEN AS IT REACHES THE
COAST...BUT HOURLY RAINFALL RATES BETWEEN 1.50 AND 2.00 INCHES
COULD RESULT IN LOCAL 2.50 TO 5.00 INCHES OF QPF...ESPECIALLY
WHERE THE BANDING ALLOWS SHORT TERM TRAINING.

THE MOST RECENT HRRR SUPPORTS THE IDEA OF LOCAL 5.00 INCH
MAXIMA...MAINLY OVER THE UPPER TX COAST BETWEEN KGLS AND KBPT.
WHILE IS POSSIBLE THAT THE HRRR IS OVERESTIMATING THE AVAILABLE
INSTABILITY...THE CONTINUED HEAVY RAINFALL WITH THE BANDING
FEATURES WILL EXTEND THE FLASH FLOOD THREAT INTO THE LATE EVENING
HOURS. 

HAYES

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

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

LAT...LON   30579242 30379153 29869147 29609219 29249375
            29159485 29879505 30469396