MESOSCALE PRECIPITATION DISCUSSION 0333
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1141 PM EDT FRI AUG 29 2014
 
AREAS AFFECTED...SRN LA...CENTRAL LA...FAR SE TX 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 300340Z - 300940Z
 
SUMMARY...MODELS PREDICT THE PLAUSIBLE ONSET OF MESOSCALE FORCING
WHICH PRECEDES A HEAVY RAIN SIGNAL IN THEIR QPF. RADAR AND MODEL
TRENDS GIVE US CONFIDENCE IN RAIN INITIATING NEAR THE LA/TX COAST
BY 06-08Z...WITH EXPANSION OF HEAVY RAIN INLAND THEREAFTER. LOCAL
RATES OF 3"/HOUR COULD LEAD TO FLASH FLOODING.

DISCUSSION...MODELS ANALYZED A LOW JUST OFFSHORE WHICH HAD A CORE
SLIGHTLY WARMER THAN THE AMBIENT ENVIRONMENT. THIS IS FORECAST TO
STRETCH AND BECOME PART OF A MID LEVEL TROUGH AXIS THAT WILL DRIFT
NORTHEAST...SWEEPING UP UNUSUALLY MOIST AIR. PRE-CONVECTIVE PW
VALUES OF MORE THAN 2.25 INCHES...AND POST-CONVECTIVE VALUES OF
2.5 INCHES ARE 2.5 STANDARD DEVIATIONS FROM CLIMATOLOGY...AN
IMPRESSIVE FEAT AT THIS LATITUDE. SIMULTANEOUS WITH THE FORMATION
OF THE MID LEVEL TROUGH THE RAP PREDICTS A DIURNAL ACCELERATION OF
THE LOW LEVEL FLOW OFF THE GULF TO AROUND 35 KT AT 900 MB. BY
06-08Z THIS YIELDS INCREASINGLY CONVERGENT FLOW COINCIDENT WITH
HEAVY QPF FROM A NUMBER OF HIRES MODELS SUCH AS THE 12Z WRF
ARW...NMM...AND SEVERAL HRRR RUNS. THE MODEL TREND IS TO FAVOR
SOUTHWEST LA AND FAR SE TX FOR HEAVY RAIN...WHEREAS CONVECTION
FARTHER EAST IN LA MAY LACK A FOCUSING MECHANISM.

EARLY ON...IMMEDIATE COASTAL AREAS STRADDLING THE LA/TX LINE WILL
BE FAVORED FOR VERY HEAVY RAIN. BUT WITH THE TROPICAL AIRMASS
PRECLUDING STRONG COLD POOLS THE EVENT MAY BE ABLE TO DEVELOP
INLAND PAST I-10 INTO CENTRAL LOUISIANA BY MORNING...AS DEEP LAYER
FORCING SPREADS IN THAT DIRECTION. ISOLATED RATES UP TO 3"/HOUR
ARE LIKELY...WHICH MAKES THE HIGH FFG NUMBERS IN THIS REGION
ACHIEVABLE...AND WOULD BE ESPECIALLY OF CONCERN FOR THOSE IN URBAN
AREAS OR THOSE TRAVELING THROUGH OUTLYING AREAS.

BURKE

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

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

LAT...LON   29649464 30799425 31379365 31749288 31759227 31439180
            30749147 29939135 28939195 28639321 29009421 29649464