MESOSCALE PRECIPITATION DISCUSSION 0230
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
449 AM EDT MON SEP 02 2013
 
AREAS AFFECTED...SOUTHERN TN...NORTHERN AL...NORTHWEST GA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 020848Z - 021148Z
 
SUMMARY...A FORMING BAND OF CONVECTION JUST BEHIND AN OLD POLAR
FRONT/BAROCLINIC TROUGH IS PROPAGATING GENERALLY ALONG ITS AXIS,
LEADING TO THE POSSIBILITY OF REGIONAL FLASH FLOODING.

DISCUSSION...AN AXIS OF WEAK SURFACE CONVERGENCE WITH CAPE VALUES
JUST TO ITS WEST AROUND 2000 J/KG COMBINED WITH LOW-LEVEL INFLOW
OF AROUND 20 KNOTS HAS BEGUN TO FOCUS A FORMING BAND OF
THUNDERSTORMS.  ACTIVITY IS BACKBUILDING ACROSS SOUTHWEST TN,
LIKELY DUE TO AN INCREASE IN LOW-LEVEL INFLOW, HAS HELPED TO
LENGTHEN THIS CONVECTIVE BAND.  DIVERGENCE ALOFT EXISTS BETWEEN A
PAIR OF UPPER LEVEL LOWS NEAR SOUTHEAST GA AND THE MIDDLE TX COAST
AND THE MAIN BRANCH OF THE WESTERLIES ACROSS THE OHIO VALLEY. 
LOW-LEVEL WIND SHEAR OF AROUND 30 KNOTS IS HELPING TO ORGANIZE THE
BAND.  CIN ALONG THE SURFACE BOUNDARY IS FORCING THE CONVECTION TO
BE GENERALLY ELEVATED OVER THE SURFACE BOUNDARY.  THE
CAPE/LOW-LEVEL WIND SHEAR COMBINATION FAVORS A WET MACROBURST
MULTI-CELL PATTERN.  PRECIPITABLE WATER VALUES OF 1.75-2" IN THIS
AREA ARE BETWEEN THE 85TH AND 90TH PERCENTILES FOR EARLY
SEPTEMBER, WHICH SHOULD LEAD TO GREATER THAN NORMAL PRECIPITATION
EFFICIENCY.  RECENT RADAR IMAGERY SUPPORTS THIS OVERALL SCENARIO,
WITH 1-2" HOURLY RAINFALL RATES.  CORFIDI VECTORS AND 850-400 HPA
MEAN LAYER WINDS SUPPORT A MOVEMENT TO THE EAST-SOUTHEAST.

THE 21Z/00Z/03Z RUNS OF THE HRRR HAVE DONE A VERY GOOD JOB
FORECASTING THIS BAND TO DEVELOP, AND THE 00Z SSEO NEIGHBORHOOD
PROBABILITIES INDICATED A 50% CHANCE OF EXCEEDING FLASH FLOOD
GUIDANCE VALUES IN NORTHEAST AL.  FLASH FLOOD GUIDANCE VALUES ARE
FAIRLY HIGH, ON THE ORDER OF 3-4 INCHES IN 3 HOURS.  AS A FEW
HOURS OF TRAINING IS EXPECTED BEFORE CAPE VALUES BEGIN TO EXHAUST,
LOCAL RAINFALL AMOUNTS OF 2-4 INCHES ARE CONSIDERED POSSIBLE,
WHICH COULD CAUSE REGIONAL FLASH FLOODING.  CONFIDENCE IN THIS
OVERALL SCENARIO PLAYING OUT IS AVERAGE.

ROTH

ATTN...WFO...FFC...OHX...BMX...HUN...MEG...

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LAT...LON   33278447 34248775 35028906 35498806 35088606 34078431
            33278447