MESOSCALE PRECIPITATION DISCUSSION 0633
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
229 PM EDT SAT OCT 31 2015
 
AREAS AFFECTED...SOUTHEAST LA & SOUTHERN MS 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 311828Z - 010028Z
 
SUMMARY...AN OUTFLOW BOUNDARY SHOULD LIE DOWN NEAR THE LA/MS
COAST, ACTING AS A FOCUS FOR TRAINING CONVECTION.  HOURLY RAIN
RATES TO 2.5" COULD LEAD TO FLASH FLOODING.

DISCUSSION...A PROGRESSIVE CONVECTIVE LINE/OUTFLOW BOUNDARY HAS
BEEN PUSHING STEADILY EASTWARD AT ~30 KTS, ALONG THE LINES OF THE
MAGNITUDE OF THE FORWARD PROPAGATING CONVECTIVE VECTORS AND
PARALLEL TO THE 1000-500 HPA THICKNESS PATTERN.  THE TAIL END OF
THE BOUNDARY HAS LAID DOWN NEAR 29.5N LATITUDE JUST OFFSHORE
SOUTHWEST LA, WHICH COULD BE AN INDICATION OF THE LATITUDE WHERE
THE BOUNDARY ORIENTS WITHIN THE NEXT SEVERAL HOURS NEAR THE LA/MS
COASTS.  PRECIPITABLE WATER VALUES ARE IN THE 2.2-2.3" RANGE IN
THIS AREA, WHILE 850 HPA INFLOW 35-45 KTS IS IMPORTING INSTABILITY
FROM A POOL OF MLCAPES OF 1000-2000 J/KG WHICH EXTENDS FROM
SOUTHERN LA SOUTHWARD INTO THE GULF OF MEXICO.

THE CAM GUIDANCE IS IN GENERAL AGREEMENT FOR HEAVY RAINFALL NEAR
THIS REGION, WITH LOCAL AMOUNTS OF 3-5" INDICATED OVER SEVERAL
HOURS.  THIS COULD FALL QUICKLY, AS HOURLY RAIN RATES UP TO 2.5"
ARE SUPPORTED BY THE AVAILABLE MOISTURE.  WHILE LOW-LEVEL INFLOW
IS EXPECTED TO SLOWLY WEAKEN OVER THE NEXT SEVERAL HOURS, IT
SHOULD REMAIN NEAR/ABOVE 25 KTS THROUGH 00Z, WHICH SHOULD ALLOW
FOR ORGANIZED CONVECTION THROUGH THE PERIOD.  THE MAIN HEAVY
RAIN/FLASH FLOOD THREATS HERE ARE TWOFOLD: CELL MERGERS WITHIN THE
NEXT FEW HOURS AS SCATTERED SHOWERS/THUNDERSTORMS MERGE WITH THE
INITIAL CONVECTIVE LINE, THEN BROAD OVERRUNNING CONVECTION WHICH
THEN TRAINS FROM WEST TO EAST NORTH OF THE OUTFLOW BOUNDARY WHICH
ORIENTS NEAR 29.5N LATITUDE.  THE ALIGNMENT OF THE OUTFLOW
BOUNDARY NEAR 29.5N COULD TAKE SEVERAL HOURS IN AREAS AS FAR EAST
AS THE MS/AL BORDER.  THE 12Z ARW, 12Z WRF4NSSL, AND 17Z HRRR
APPEAR BEST FOR HEAVY RAINFALL PLACEMENT.  FLASH FLOOD GUIDANCE
VALUES ARE HIGH HERE, SO USED THE POSSIBLE CATEGORY.

ROTH

ATTN...WFO...JAN...LCH...LIX...MOB...

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

LAT...LON   31668955 31628880 31338849 30268834 29608858 
            28978908 28898941 29218967 29138992 28899042 
            29069100 29409147 29609188 30279191 31569114