MESOSCALE PRECIPITATION DISCUSSION 0215
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
942 AM EDT WED AUG 14 2013
 
AREAS AFFECTED...PORTIONS OF EASTERN LA AND SOUTHERN MS 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 141341Z - 141741Z
 
SUMMARY...THUNDERSTORM CLUSTERS WITH RAINFALL RATES AROUND 2" PER
HOUR ARE EXPECTED OVER THE NEXT SEVERAL HOURS, WHICH COULD CAUSE
REGIONAL FLASH FLOODING ACROSS PORTIONS OF EASTERN LA AND SOUTHERN
MS.

DISCUSSION...THE 13Z SURFACE OBSERVATIONS INDICATE THAT A WEAK
SURFACE LOW HAS FORMED ACROSS NORTH-CENTRAL LA NEAR WINN PARISH
LA, WITH A PAIR OF THUNDERSTORM CLUSTERS FORMING A NW-SE ORIENTED
OUTFLOW BOUNDARY FROM CENTRAL AND SOUTHEAST LA AND SOUTHERN MS. 
THE BEST DIVERGENCE IN THE COUNTRY EXISTS IN THIS REGION, BETWEEN
THE SOUTHERN EDGE OF THE WESTERLIES AND AN UPPER LEVEL LOW IN THE
EASTERN GULF OF MEXICO.  CAPE VALUES, PER THE 12Z SOUNDING OUT OF
SLIDELL LA AND JACKSON MS ARE AROUND 2000 J/KG.  THE MESOSCALE
GUIDANCE HAS HAD A HARD TIME WITH THE ONGOING CAPE
PATTERN/PRECIPITATION PATTERN, WHICH LED THE 09Z HRRR OUTPUT
ASTRAY.  AT THE CURRENT TIME, THE PIECES OF GUIDANCE BEST HANDLING
THE THUNDERSTORM ACTIVITY ARE THE 06Z NAM CONEST AND THE 00Z ECMWF
MODELS.

THE OUTFLOW BOUNDARY IS ORIENTED AT A SIGNIFICANT ANGLE TO 850 HPA
INFLOW OF AROUND 25 KNOTS PER VAD WIND PROFILES AND THE LATEST RAP
RUN, WHICH IS HELPING TO GROW A CLUSTER OF THUNDERSTORMS ACROSS
EAST-CENTRAL LA.  THE BEST LOW-LEVEL INFLOW IS EXPECTED TO SHIFT
SOUTHWARD WITH TIME, NEARLY IN TANDEM WITH THE OUTFLOW BOUNDARY. 
PRECIPITABLE WATER VALUES OF 2-2.5 INCHES EXIST ACROSS THE REGION,
EXCEEDING THE 90TH PERCENTILE FOR MID-AUGUST, WHICH IS AIDING
PRECIPITATION EFFICIENCY.  THE STORMS WHICH DROPPED OUT OF
SOUTH-CENTRAL MS INTO SOUTHEAST LA DROPPED A QUICK 2.27 INCHES OF
RAIN AT BROOKHAVEN MS.  

THIS MAIN LA CLUSTER IS EXPECTED TO MOVE GENERALLY SOUTHEASTWARD,
WITH ORGANIZED ACTIVITY MOVING MORE SOUTHWARD AND LESS ORGANIZED
STORMS MOVING MORE EAST-SOUTHEASTWARD WITH TIME.  COMBINED WITH
THE CURRENT RAINFALL ACROSS AREAS ASSOCIATED WITH A SECOND BATCH
OF THUNDERSTORMS NORTH OF SLIDELL LA, AND NEW SCATTERED
THUNDERSTORMS BETWEEN THE TWO CLUSTERS, REGIONAL FLASH FLOODING IS
POSSIBLE.  RADAR ESTIMATES SHOW THE STRONGER STORMS ARE GENERATING
HOURLY RAIN RATES OF TWO INCHES, WHICH WOULD EXCEED HOURLY
GUIDANCE VALUES.  CELL TRAINING OVER 2-3 HOURS IS POSSIBLE AS
STORMS CONTINUE TO BACKBUILD TOWARDS A STATIONARY FRONT ACROSS
CENTRAL LA.

ROTH

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

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

LAT...LON   32019268 32259150 31488931 30648858 30198853 30158879
            29758874 29388907 29138886 28848933 29068978 28959054
            29249150 30839257 32019268