MESOSCALE PRECIPITATION DISCUSSION 0689
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
308 AM EST THU DEC 24 2015
 
AREAS AFFECTED...CENTRAL/NORTHEAST AL...NORTHERN GA...EASTERN
TN...WESTERN NC...EXTREME WESTERN SC 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 240800Z - 241400Z
 
SUMMARY...VERY HEAVY SHOWERS AND THUNDERSTORMS WILL IMPACT THE TN
VALLEY AND AREAS OF THE SOUTHERN APPALACHIANS AND ADJACENT
FOOTHILLS THROUGH MID MORNING.

DISCUSSION...IR SATELLITE IMAGERY SHOWS COOLING CLOUD TOPS AND A
FEW OVERSHOOTING TOPS IN ASSOCIATION WITH THE SW/NE ORIENTED BANDS
OF CONVECTION MOVING ACROSS CENTRAL AND NORTHEAST AL AND WELL INTO
EASTERN TN. SOME OF THE CELLS ARE BEGINNING TO TRAIN OVER THE SAME
AREA...AND MEANWHILE ADDITIONAL CONVECTIVE TOPS ARE SHOWING SIGNS
OF INTENSIFICATION OVER THE LAST HOUR ALSO OVER NORTHERN GA.

THE ACTIVITY IS BEING FOSTERED BY A SHORTWAVE TROUGH THAT IS
EJECTING NORTHEAST ACROSS THE LOWER AND MID MS VALLEY REGION. THE
UPPER AIR FLOW IS QUITE DIVERGENT DOWNSTREAM OF THIS ACROSS THE TN
VALLEY AND THROUGHOUT THE SOUTHERN APPALACHIANS AND ADJACENT AREAS
OF THE SOUTHEAST. MEANWHILE...A DEEP LAYER SOUTHWESTERLY FETCH OF
MOISTURE INVOLVING THE SUBTROPICAL JET IS HELPING TO PROVIDE
ANOMALOUSLY HIGH PWATS ACROSS MUCH OF THE EASTERN U.S. EARLY THIS
MORNING...AS HIGH AS 1.6 TO 1.8 INCHES...AND IN SOME CASES RUNNING
2 TO 4 STD DEV ABOVE NORMAL. IN FACT...THE CIRA LAYERED PW PRODUCT
SHOWS A DISTINCT CONNECTION OF THE MID AND HIGH LEVEL MOISTURE TO
EASTERN TROPICAL PACIFIC OCEAN.

THE CONVECTION AHEAD OF THE AFOREMENTIONED SHORTWAVE TROUGH AND
ASSOCIATED COLD FRONT IS LIKELY TO PERSIST THROUGH MID MORNING
GIVEN THE DEGREE OF LARGER SCALE ASCENT AND PERSISTENCE OF A VERY
MOIST AND AT LEAST MODESTLY UNSTABLE AIRMASS. AREA VWPS SHOW A
SOUTHWEST LOW LEVEL JET OF 50 TO 60 KTS AND THIS WILL NOT ONLY
SUSTAIN THE LOW LEVEL MOISTURE TRANSPORT OUT OF THE GULF OF
MEXICO...BUT WILL ALSO ENHANCE THE UPSLOPE/OROGRAPHIC FORCING
ACROSS PORTIONS OF THE SOUTHERN APPALACHIANS. RAINFALL RATES HAVE
BEEN OBSERVED TO BE AT OR ABOVE 1 INCH/HR WITHIN THE CONVECTIVE
BANDS...BUT MAY BECOME ENHANCED FURTHER OVER THE NEXT COUPLE OF
HOURS GIVEN THE LOCALIZED UPSLOPE FLOW COMPONENT AND POTENTIAL FOR
SOME OVERSHOOTING TOPS WITHIN THE STRONGER CONVECTIVE CELLS.

SOME OF THESE AREAS HAVE ALREADY SEEN HEAVY RAINFALL EARLIER...AND
SOME CAM GUIDANCE...MAINLY THE 00Z NMMB AND HRRR ARE SUGGESTING
ADDITIONAL RAINFALL AMOUNTS OF 2 TO 4 INCHES WITH ISOLATED HEAVIER
AMOUNTS THROUGH MID MORNING. HOWEVER...GIVEN THE FAVORABLE SET-UP
FOR TRAINING CELLS...THE CAM GUIDANCE MAY TEND TO BE UNDER DOING
THE RAINFALL POTENTIAL. IN ANY CASE...THE ADDITIONAL HEAVY
RAINFALL WILL ELEVATE THE RISK OF FLASH FLOODING AND RUNOFF
PROBLEMS.

ORRISON

ATTN...WFO...BMX...FFC...GSP...HUN...MRX...RNK...

ATTN...RFC...LMRFC...OHRFC...SERFC...

LAT...LON   36618239 36518154 36038139 35288249 34608321 
            33918410 33158517 32408618 32558700 33028741 
            33628725 34348629 35118535 35648468 36308357