MESOSCALE PRECIPITATION DISCUSSION 0514
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
305 PM EDT THU SEP 24 2015
 
AREAS AFFECTED...MUCH OF SC & SOUTHEASTERNMOST NC 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 241905Z - 250105Z
 
SUMMARY...EFFICIENT RAINFALL HAS BEGUN TO FALL ACROSS SC.  RAIN
RATES COULD INCREASE OVER THE NEXT SEVERAL HOURS CLOSER TO THE
NC/SC COAST AS WELL AS ACROSS THE PIEDMONT.  FLASH FLOODING IS
POSSIBLE.

DISCUSSION...A PAIR OF SURFACE LOWS SHOW UP IN SURFACE
OBSERVATIONS WITHIN WHAT APPEARS TO BE AN INFLOW BAND ON SATELLITE
IMAGERY OFFSHORE THE CENTRAL SC COAST WITH WHAT HAS BEEN THE MAIN
CIRCULATION EAST OF SAVANNAH GA.  INFLOW INTO THE REGION IS
ROUGHLY OUT OF THE EAST-SOUTHEAST AT 25 KTS PER VAD WIND PROFILES.
 SOME CIN STILL EXISTS ACROSS THE REGION AS IT HAS BEEN CLOUDY ALL
DAY.  MUCAPES ARE IN THE 250+ J/KG RANGE ACROSS PORTIONS OF
CENTRAL SC, WITH HIGHER VALUES CREEPING INTO THE UPPER SC COAST. 
AS SAB POINTED OUT IN THEIR SPENES MESSAGE AT 1733Z, CONVERGENCE
IN THE LOW-LEVELS IS SHOWING UP IN MODEL MASS FIELDS AS WELL AS
SATELLITE AND RADAR IMAGERY ACROSS THE NORTHERNMOST COAST OF SC. 
OBSERVATIONS FROM THE COLUMBIA SC AREA INDICATE RAINFALL COULD BE
EXCEEDING RAINFALL ESTIMATES, WITH 1.26" FALLING IN ONE HOUR AT
CAE.  PRECIPITABLE WATER VALUES ARE IN THE 2-2.2" RANGE PER GPS
VALUES, 1.5-2 SIGMAS ABOVE THE MEAN FOR LATE SEPTEMBER.

THE GUIDANCE SHOWS INSTABILITY INCREASING THROUGH THE AFTERNOON
INTO THE EVENING HOURS NEAR THE SC/NC BORDER, WHILE VALUES ACROSS
SC REMAIN UNDER 500 J/KG.  MEANWHILE, 850 HPA INFLOW RAMPS UP TO
30-40 KTS ACROSS SC, WHICH WOULD BE AT LEAST DOUBLE THE 850-400
HPA MEAN WIND IN THE AREA.  RAP FORECASTS INDICATE PWS MAY BREACH
2.25" OVER THE NEXT SEVERAL HOURS.  WHAT HAS BEEN A GENERALLY LONG
DURATION RAIN EVENT MAY BE TRANSITIONING INTO A FLASH FLOOD
SCENARIO. THE CAM GUIDANCE INDICATES LOCAL AMOUNTS OF ~3" OVER THE
NEXT SEVERAL HOURS.  UPSLOPE FLOW INTO THE TOPOGRAPHY OF THE
CAROLINAS CAN BE TRICKY, AND THE GUIDANCE (AS WELL AS RADAR
ESTIMATES) CAN UNDERESTIMATE THE QPF MAGNITUDE WITHIN THIS SORT OF
REGIME.  DUE TO THIS, THOUGHT IT PRUDENT TO ISSUE AN MPD FOR
POSSIBLE FLASH FLOODING.  HOURLY RAIN RATES NEAR THE NC/SC BORDER
COULD REACH 2.5" IN THIS MOIST AND UNSTABLE AIR MASS, WHILE HOURLY
RATES UP TO 1.5" ARE EXPECTED INLAND.  OVER A FEW HOURS, THIS SORT
OF RAINFALL COULD EXCEED THE HIGH FLASH FLOOD GUIDANCE VALUES IN
THIS AREA.   

ROTH

ATTN...WFO...CAE...CHS...GSP...ILM...RAH...

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

LAT...LON   34898138 34847936 34377771 33937791 33817798 
            33747861 33357901 32967931 32638009 33298110 
            34028254 34638262