MESOSCALE PRECIPITATION DISCUSSION 0908
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
811 AM EDT MON OCT 23 2017

AREAS AFFECTED...SOUTHERN APPALACHIANS

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

VALID 231210Z - 231810Z

SUMMARY...MODERATE TO HEAVY RAIN IS EXPECTED ACROSS THE SOUTHERN
APPALACHIANS OVER THE NEXT SEVERAL HOURS.  HOURLY TOTALS UP TO 1"
WITH LOCAL AMOUNTS TO 4" ARE EXPECTED.

DISCUSSION...AN AREA OF RAIN WITH EMBEDDED THUNDERSTORMS IS MOVING
INTO THE PIEDMONT OF THE SOUTHERN APPALACHIANS AHEAD OF A
NEGATIVELY TILTED SHORTWAVE TROUGH.  SATELLITE IMAGERY HAS SHOWN
COOLING CLOUD TOPS IN THIS REGION AS OF LATE.  THIS IS ESSENTIALLY
AN ATMOSPHERIC RIVER PATTERN, ENHANCED COMPARED TO THE PACIFIC
NORTHWEST AS EMBEDDED PRECIPITABLE WATER VALUES OF 1.9-2" CLOSE TO
SEA LEVEL PER GPS DATA.  MU CAPE VALUES IN THE AREA ARE 100-500
J/KG PER SPC MESOANALYSES.  INFLOW AT 850 HPA IS 25-40 KTS PER VAD
WIND PROFILES, NEAR THE MAGNITUDE OF THE MEAN 850-400 HPA WIND. 
EFFECTIVE BULK SHEAR NEAR THE NORTHERN BORDER OF GA/SC IS 25-40
KTS.

THE RAP GUIDANCE SHOWS SOME SLIGHT REDUCTION IN THE MU CAPE ACROSS
THE REGION OVER THE NEXT SEVERAL HOURS AS LOW-LEVEL INFLOW
INCREASES, BUT SPC MESOANALYSES SHOW A SLIGHT UPWARD TREND AS OF
LATE, CASTING DOUBT ON THE RAP.  EVEN WITH 100-200 J/KG MU CAPE,
HOURLY RAIN TOTALS TO 1" ARE EXPECTED.  THE MESOSCALE GUIDANCE HAS
A GOOD SIGNAL FOR LOCAL AMOUNTS IN THE 4" RANGE THROUGH 18Z, WHICH
COULD OCCUR EVENLY UNTIL LATE IN THE PERIOD (AFTER 15Z) WHEN
INSTABILITY REDEVELOPS UPSTREAM IN SC AND CONVECTION COULD BECOME
MORE WIDESPREAD.  THIS COULD CAUSE LONGER DURATION HEAVY
RAIN-RELATED ISSUES IN URBAN AREAS AND WITHIN RUGGED TERRAIN.

ROTH

ATTN...WFO...FFC...GSP...MRX...RNK...

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

LAT...LON   36978101 36668024 35628095 34768195 34108308
            34968456 35888299