MESOSCALE PRECIPITATION DISCUSSION 0489
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
700 PM EDT THU SEP 10 2015
 
AREAS AFFECTED...ERN TX...NRN LA...WRN MS 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 102300Z - 110200Z
 
SUMMARY...VERY SLOW MOVING THUNDERSTORMS WILL PRODUCE RAPID
ACCUMULATIONS OF 2-3 INCHES OF RAIN BEFORE LOSING INTENSITY TOWARD
SUNSET. FLASH FLOODING IS POSSIBLE.

DISCUSSION...EVENTS ACROSS TX/LA/MS MAY BE NEAR THEIR PEAK AS OF
2250Z...BUT POTENTIAL FOR ADDITIONAL FLASH FLOODING DOES LOOK TO
REMAIN THROUGH ABOUT 02Z PER 1-HOUR RAIN RATES IN THE HRRR AND
SIMULATED REFLECTIVITY FROM THE NCEP HI-RES MODELS. FORCING ALONG
A 700 MB TROUGH AXIS HAD COMBINED WITH OUTFLOW FROM EARLY MORNING
COASTAL CONVECTION TO PRODUCE A SIZABLE...ALTHOUGH WEAK...COLD
POOL STRETCHED BENEATH THE TROUGH AXIS. THE NORTH SIDE OF THE COLD
POOL WILL REMAIN FAVORED FOR NEW CELL DEVELOPMENT...WITH NEW CELLS
ESSENTIALLY PULSING FOR 30-60 MINUTES AND THEN REFORMING A SHORT
DISTANCE NORTHWARD...AS HAD BEEN SEEN ALREADY THIS AFTERNOON. WITH
LIGHT STEERING FLOW FROM THE NORTHWEST THIS WILL ALLOW FOR PERIODS
OF TRAINING ON SMALL SPACE SCALES...AND GIVEN VERY DEEP MOISTURE
THE THREE INCH PER HOUR RATES ESTIMATED BY RADAR ARE LIKELY
ACCURATE. THERE WAS A MESONET OBSERVATION OF 2.4 INCHES IN ONE
HOUR NEAR MONTEREY LA...AND AN UNOFFICIAL REPORT OF 2 INCHES IN 30
MINUTES IN EAST TEXAS.

THE CONVECTION HAD DEVELOPED NORTHWARD ACROSS THE GRADIENT TOWARD
SLIGHTLY DRIER AIR...BUT WITH PRECIPITABLE WATER VALUES STILL
ABOVE 1.75 INCHES. THE DRIER MID LEVEL AIR ACTED ALSO TO MAINTAIN
MODERATE INSTABILITY...AND ALTHOUGH LOW LEVEL CUMULUS IN NORTHERN
LA HAD BEGUN TO FADE...THE AIR WAS LIKELY UNSTABLE ENOUGH TO
SUPPORT ANOTHER 2-3 HOURS OF SPOTTY HEAVY RAINFALL ALONG THE
OUTFLOW BOUNDARY FROM TX TO MS...PERHAPS GETTING NEAR I-20.

BURKE

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

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

LAT...LON   32639161 32349068 31529072 30959142 30779267 
            30719386 30719548 31169631 31929611 32409458