MESOSCALE PRECIPITATION DISCUSSION 0021
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
329 AM EDT SUN APR 28 2013
 
AREAS AFFECTED...SOUTHERN TENNESSEE...NORTHERN ALABAMA...FAR
NORTHWESTERN GEORGIA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 280728Z - 281128Z
 
SUMMARY...INCREASING STORM MOTIONS AND A SHIFT OF
INSTABILITY/FORCING AWAY FROM SATURATED AREAS WILL LIMIT THE
POTENTIAL FOR ADDITIONAL FLASH FLOODING THIS MORNING.

DISCUSSION...EARLIER CONVECTIVE SYSTEM WHICH LED TO HEAVY RAIN AND
FLOODING IN TENNESSEE OCCURRED DURING FAVORABLE COUPLING OF A
BROAD LOW LEVEL JET WITH AN UPPER LEVEL SHORTWAVE TROUGH. MODELS
TEND TO BRING PRECIPITATION EASTWARD THROUGH THIS
MORNING...RAISING CONCERNS ABOUT THE REGION OF LOW FLASH FLOOD
GUIDANCE BETWEEN NASHVILLE AND KNOXVILLE WHICH RECEIVED 2-3 INCHES
OF RAIN ON SATURDAY.

MESO-ANALYSIS...HOWEVER...SUGGESTS THAT THE REGION WILL SEE
WIDESPREAD RAIN...BUT THAT THE POTENTIAL FOR PERSISTENT HEAVY RAIN
OVER ANY GIVEN LOCATION WILL BE LESSENED BY A FEW FACTORS. A WEDGE
OF DRIER MID LEVEL AIR AND MUCAPE VALUES LESS THAN 500 J/KG WERE
ANALYZED ACROSS EAST-CENTRAL TN. THIS WOULD SUGGEST PREFERENTIAL
REDEVELOPMENT ALONG THE TAIL END OF ONGOING CONVECTIVE
SYSTEM...WITH NEW DEVELOPMENT TOWARD THE SOUTH OF MURFREESBORO TN
AND INTO NORTHERN ALABAMA...AWAY FROM THE MOST SUSCEPTIBLE AREAS
INDICATED BY FLASH FLOOD GUIDANCE. OBSERVATIONS AND RAP FORECASTS
ALSO INDICATED AN UPPER LEVEL SHORTWAVE AND ASSOCIATED JET MAX
LIFTING AWAY FROM TN...RESULTING IN DIVERGENCE OF 850 MB WINDS
OVER THE REGION OF INTEREST. A SECOND UPPER LEVEL JET MAX AND
ASSOCIATED DIFLUENCE WAS MOVING INTO NORTHERN ALABAMA WHERE THE
NSSL WRF...SPC WRF...AND HIRES WRF-NMM INITIATE AN AXIS OF
MODERATE QPF. VEERED LOW LEVEL FLOW WILL FAVOR PROGRESSIVE
MOTION...AND THE HIRES MODEL QPF AMOUNTS...WHICH TEND TO RUN ON
THE HIGH SIDE...SUGGEST ONLY 1-2 INCHES OF RAIN AT MOST.

BURKE


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

LAT...LON   33748763 33748653 33838578 34078512 34588454 35208423
            35768424 36268471 36328560 36028672 35568728 35048759
            34358827 34018869 33758860 33748763