MESOSCALE PRECIPITATION DISCUSSION 0517
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
754 AM EDT FRI SEP 25 2015
 
AREAS AFFECTED...CAROLINA PIEDMONT/APPALACHIANS 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 251154Z - 251754Z
 
SUMMARY...A LONGER DURATION RAIN EVENT COULD OCCASIONALLY
TRANSITION TO A FLASH FLOOD EVENT OVER THE NEXT SEVERAL HOURS
WITHIN A MOIST ATMOSPHERE WITH INCREASING UPSLOPE FLOW.

DISCUSSION...SATELLITE IMAGERY REVEALS A MID TO UPPER LEVEL LOW
NORTH OF COLUMBUS GA AND A WARM CONVEYOR BELT EXTENDING FROM FL'S
GOLD COAST INTO THE CAROLINAS.  CONVERGENCE IN THE LOW-LEVELS IS
SIGNIFICANT BETWEEN A BLOCKING RIDGE AND A DEEP CYCLONE IN SC PER
SURFACE OBSERVATIONS, VAD WIND PROFILES, AND LOW-LEVEL CLOUD
MOTIONS ON INFRARED SATELLITE IMAGERY.  GPS-DERIVED PRECIPITABLE
WATER VALUES ARE 1.75-2" IN THIS REGION, 1.5-2 SIGMAS ABOVE THE
MEAN FOR LATE SEPTEMBER.  INFLOW AT 850 HPA IS 25-30 KTS, NEAR THE
MAGNITUDE OF THE MEAN 850-400 HPA WIND.  MUCAPES IN THE REGION ARE
ROUGHLY 250-500 J/KG PER A FAIRLY RECENT SPC MESOANALYSIS.

RADAR IMAGERY REVEALS NEW THUNDERSTORM ACTIVITY ACROSS WESTERN SC,
INDICATIVE OF A POCKET OF INSTABILITY LURKING UPSTREAM FROM THE
GA/SC BORDER INTO CENTRAL SC.  CONSIDERING THE HIGH PWS AVAILABLE
AND RELATED HIGH WET BULB ZEROS IN THE REGION (13,000 FT), LITTLE
INSTABILITY WOULD BE NECESSARY TO FLARE NEW CONVECTION AND ENHANCE
RAIN RATES.  SATELLITE AND RADAR IMAGERY TENDS TO SHOW MUTED
SIGNALS IN SUCH A REGIME, BEAM BLOCKING ASIDE.  THE GSP/RNK
OFFICES SUGGEST THAT RADAR ESTIMATES ARE ROUGHLY 30-35% TOO LOW IN
THEIR AREA, IMPLYING 1.5-2" HAS FALLEN SO FAR.  WITHIN THIS
ENVIRONMENT, TRAINING CELLS COULD LEAD TO HOURLY RAIN RATES UP TO
1", WHICH OVER A FEW HOURS COULD LEAD TO FLASH FLOOD ISSUES --
RANDOM POCKETS OF INSTABILITY COULD ENHANCE HOURLY RAIN RATES
FURTHER.  RECENT RAP RUNS SUGGEST 850 HPA INFLOW SHOULD RAMP UP
INTO THE 40 KT RANGE BY 18Z, NEARLY DOUBLE THE 850-400 HPA MEAN
WIND.  THE CAM GUIDANCE INDICATES LOCAL AMOUNTS OF 2" ARE POSSIBLE
OVER THE NEXT SEVERAL HOURS, WHICH COULD FALL QUICKLY SHOULD CELL
TRAINING OCCUR.  THE LONG DURATION RAINFALL HAS DROPPED AREA FLASH
FLOOD GUIDANCE VALUES AS SOILS SATURATE.  THE TRANSITION FROM
LONGER DURATION FLOOD TO FLASH FLOODING IS HARD TO ANTICIPATE IN
THIS SORT OF REGIME, AND IS CONSIDERED POSSIBLE OVER THE NEXT
SEVERAL HOURS.

ROTH

ATTN...WFO...CAE...FFC...GSP...MRX...RAH...RNK...

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

LAT...LON   36968089 36638038 35917921 34698065 33868237 
            34418311 34728354 34598409 35278343 36228225 
            36918134