MESOSCALE PRECIPITATION DISCUSSION 0047
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
540 AM EDT SUN MAY 19 2013
 
AREAS AFFECTED...NORTHERN GA AND NORTHWEST SC 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 190940Z - 191400Z
 
SUMMARY...AN AREA OF SLOW-MOVING THUNDERSTORMS IS EXPANDING IN
COVERAGE ACROSS NORTHERN GEORGIA, WHICH COULD LEAD TO FLASH
FLOODING.

DISCUSSION...THUNDERSTORMS HAVE INCREASED IN COVERAGE AND
INTENSITY OVER THE PAST FEW HOURS NEAR THE INTERSECTION OF WHERE
30-35 KNOT WESTERLY INFLOW INTERSECTS AN 850 HPA BOUNDARY ACROSS
NORTHERN GEORGIA.  THE WEAK 850 HPA WARM FRONT IS ALIGNED NNW-SSE
ACROSS THE STATE OF GA, AND DECENT DIVERGENCE ALOFT EXISTS TO THE
NORTHEAST OF A WEAK JET STREAK AT THE 250 HPA LEVEL.  PRECIPITABLE
WATER VALUES -- PWS -- ARE CLOSE TO 1.75 INCHES IN THIS REGION,
WHICH COULD HELP LEAD TO FLASH FLOODING WHERE CELLS TRAIN FOR TWO
HOURS OR MORE. FLASH FLOOD GUIDANCE VALUES -- FFGS -- ARE 3-4
INCHES WITHIN A FEW HOURS' TIME IN THIS REGION.  AS A RECENT
EXAMPLE OF THE SYSTEM'S POTENTIAL, FLOWERY BRANCH, GA PICKED UP
3.62" IN THREE HOURS, WITH 2.99" FALLING IN A SINGLE HOUR.  

UNTIL THE ADVENT OF THE 06Z HRRR, THE MESOSCALE GUIDANCE HAD BEEN
HANDLING THIS ACTIVITY POORLY, WITH GLOBAL MODELS SUCH AS THE 00Z
GFS UNDERESTIMATING THE 850 HPA INFLOW AVAILABLE IN THIS REGION. 
USING YESTERDAY'S CONVECTIVE TRENDS -- AND RECENT RAP RUNS -- AS A
GUIDE, EXPECT THE AREA TO SHIFT SOUTHWARD WITH TIME WITH CELLS
POSSIBLY MOVING INTO NORTHWEST SC -- FOLLOWING THE 850 HPA WIND
FLOW -- AND THROUGH CENTRAL GA -- DUE TO THE EXPECTED SOUTHWARD
PROPAGATION/CORFIDI VECTORS -- BEFORE FADING AROUND 14Z/10 AM EDT.
 

ROTH


ATTN...WFO...CAE...GSP...MRX...FFC...

LAT...LON   33428456 34578478 35238443 35278310 34368239 33208229
            32498299 32568397 33428456