MESOSCALE PRECIPITATION DISCUSSION 0946
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
949 AM EST WED DEC 20 2017

AREAS AFFECTED...ERN CENTRAL AL...NORTHERN GA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 201450Z - 201900Z

SUMMARY...TRAINING CONVECTIVE ELEMENTS POSE CONTINUED FF THREAT
THOUGH AREAL EXPANSION OF RAIN SHIELD AND DRIER SOIL CONDITIONS
SHOULD LIMIT THREAT WITH TIME TOWARD AFTERNOON.

DISCUSSION...CURRENT WSR-88D BASE REFLECTIVITY MOSAIC AND SURFACE
OBSERVATIONS DEPICT FLAT EAST-WEST ORIENTED STATIONARY FRONT HAS
BEGUN TO SAG SOUTHWARD ON SOME WEAK COLD POOL DEVELOPMENT ACROSS
NE AL/N GA. ISENTROPIC ASCENT ACROSS THIS BOUNDARY IS REDUCING IN
BEST EFFECTIVE ANGLE WITH THE CONTINUING VEERING LLJ; YET VWP
SHOWS 50-55KT 850MB FLOW COINCIDENT WITH NARROWING MOISTURE AXIS
WHERE VERTICAL STACKING OF MOISTURE STREAMS SHOWS NEARLY SATURATED
SFC-6H PROFILE CONCENTRATING TPWS IN EXCESS OF 1.7" MAINLY BELOW
5H ACCENTUATING RAINFALL EFFICIENCY. THIS EFFICIENCY HAS PRODUCED
1.25-1.75"/HR RATES VERIFIED THROUGH GAUGE REPORTS AS RADAR
APPEARS TO UNDERESTIMATING BY .5-.75"/HR AT TIME...ESPECIALLY
NOTED OVER N AL AND N GA. GIVEN THE NARROWING/VEERING LOW LEVEL
FLOW INSTABILITY IS ALSO REDUCING AND SUPPORTING THE SOUTHWARD
PROPAGATION OF THE BOUNDARY AND THE ENHANCED CONVECTIVE ELEMENTS
ALONG IT.  STILL GIVEN THE OVERALL EASTWARD PROGRESSION AND LENGTH
OF THESE ELEMENTS...THERE SHOULD REMAIN A SHORT-TERM THREAT FOR
TRAINING AND TOTALS OF 2-3" IN 2-3HRS ACROSS EASTERN CENTRAL AL
INTO WESTERN CENTRAL GA.  

FURTHER DOWNSTREAM...VEERING IN THE LOW LEVELS ACROSS AL/GA ALLOWS
FOR INCREASED UNIDIRECTIONAL FLOW ABOVE THE BOUNDARY ALLOWING FOR
MODERATE SHIELD PRECIPITATION TO CONTINUE ACROSS N GA AND
FOOTHILLS OF S APPALACHIANS.  WHILE NOT LIKELY TO BE FLASHY IN
NATURE DUE TO THE RATES...THE COMPLEX TERRAIN AND CHANNELING IN
THE FOOTHILLS FROM MODEST 1-1.5" OVERNIGHT/EARLY MORNING COMBINED
WITH AN ADDITIONAL 1-2" COULD LEAD TO FLOODING CONCERNS THROUGH
THE EARLY AFTERNOON HOURS.

TOWARD LATE MORNING/EARLY AFTERNOON... CONVECTION CONTINUES TO SAG
SOUTH WHILE INSTABILITY REDUCES COINCIDENT WITH HIGHER FFG VALUES
ACROSS CENTRAL GA AND WITHOUT PRECURSORY MODERATE RAIN SHOWERS
OVERNIGHT LIKE LOCALES NORTH...FLOODING THREAT SHOULD BE ENDING. 

GALLINA

ATTN...WFO...BMX...FFC...GSP...

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

LAT...LON   34848373 34178286 33798272 33378298 32928419
            32548593 32598736 33518650 34698490