MESOSCALE PRECIPITATION DISCUSSION 0204
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
704 AM EDT SAT JUN 13 2015
 
AREAS AFFECTED...SOUTHWEST & CENTRAL LA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 131102Z - 131602Z
 
SUMMARY...A DEVELOPING MCV IS STARTING TO EXPAND HEAVY RAINFALL
NORTHWARD FROM CAMERON PARISH LA TOWARDS CENTRAL LA.  FLASH
FLOODING IS POSSIBLE OVER THE NEXT SEVERAL HOURS.

DISCUSSION...AN OUTFLOW BOUNDARY FROM EARLY MORNING CONVECTION HAS
SET UP IN THE THERMAL PATTERN FROM THE NORTHWEST GULF OF MEXICO
WHICH EXTENDS INTO SOUTHWEST LA AS A SURFACE CONVERGENCE ZONE.  AN
MCV HAS DEVELOPED OFFSHORE CAMERON LA WHICH IS ACTING TO EXTEND
HEAVY RAINFALL FARTHER INLAND.  PRECIPITABLE WATER VALUES IN THE
REGION ARE 2-2.25" PER GPS INFORMATION.  INFLOW AT 850 HPA IS
30-35 KTS PER VAD WIND PROFILES, EXCEEDING THE 850-400 HPA MEAN
WIND BY 10 KTS.  MUCAPES OF 1000-3000 J/KG LIE ACROSS THE REGION,
WITH THE GRADIENT ALIGNING WITH THE LAND BREEZE NEAR THE COAST. 
HOURLY RAIN RATES UP TO 2.5" HAVE BEEN WITNESSED PER RADAR
ESTIMATES THIS MORNING, WITH LOCAL AMOUNTS OF 5" INDICATED IN
EASTERN CAMERON PARISH LA.

THE EXPECTATION IS FOR THIS RAIN BAND TO EXTEND FARTHER INLAND
WITH TIME, THOUGH THERE IS A NORTHWARD LIMIT OF THE EXTENT OF THE
HEAVY RAINFALL BASED ON THE INITIAL POSITION OF THE LAND BREEZE
BOUNDARY.  SCATTERED THUNDERSTORMS MOVING IN FROM THE NORTHWEST
GULF INTO THE BAND COULD LEAD TO TEMPORARY INCREASES IN RAINFALL
RATES ON A LOCAL BASIS.  BELIEVE THE BAND COULD EXTEND UP INTO
CENTRAL LA OVER THE NEXT 4-5 HOURS.  BY THEN, THE LAND BREEZE
BOUNDARY SHOULD HAVE WEAKENED (15Z), THOUGH SURFACE HEATING TO THE
EAST MIGHT PERPETUATE CONVECTION FOR A LONGER TIME FRAME. 
SUFFICIENT INFLOW INTO THE BAND DOES NOT WEAKEN SUFFICIENTLY (PER
RAP FORECASTS) UNTIL 17Z.  THE FORWARD PROPAGATION VECTORS (NNE 10
KTS) ARE ALIGNED ALONG THE AXIS OF THE THUNDERSTORM ACTIVITY AND
CLOSE IN DIRECTION TO THE MEAN 850-400 HPA WIND, WHICH IS
TROUBLING.  THE 00Z SPCWRF HAS THIS HANDLED THE BEST, THOUGH IT
APPEARS TO BE RETROGRADING THE ACTIVITY TOO QUICKLY -- IT
FORECASTS LOCAL AMOUNTS OF 4-5" THROUGH THE REMAINDER OF THE
MORNING.  WITH HOURLY RAIN RATES OF NEAR 2" AN HOUR, FLASH
FLOODING IS POSSIBLE.


ROTH

ATTN...WFO...LCH...SHV...

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

LAT...LON   31799299 31759231 30189212 29609226 29489323 
            30809345 31549331