MESO: Nws Weather Prediction Center College Park Md

AWUS01 KWNH 070335
FFGMPD
NCZ000-VAZ000-WVZ000-SCZ000-TNZ000-GAZ000-070934-

MESOSCALE PRECIPITATION DISCUSSION 0272
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1135 PM EDT SUN OCT 06 2013

AREAS AFFECTED...SOUTHERN APPALACHIANS

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

VALID 070334Z - 070934Z

SUMMARY...A FEEDER BAND WITH ASSOCIATED RAINFALL RATES OF AN INCH
AN HOUR IS AIMED TOWARDS THE SOUTHERN APPALACHIANS. SINCE MOTION
OF INDIVIDUAL CELLS COULD ALLOW FOR A FEW HOURS OF TRAINING, FLASH
FLOODING IS CONSIDERED POSSIBLE ACROSS THE SOUTHERN APPALACHIANS.

DISCUSSION...THUNDERSTORM TOPS ARE COOLING IN ASSOCIATION WITH AN
INFLOW BAND CARRYING MOISTURE FROM POST-TROPICAL CYCLONE KAREN
INTO THE REGION, WITH ASSOCIATED RAINFALL RATES OF AN INCH AN
HOUR. THIS INFLOW BAND IS OCCURRING AHEAD OF A COLD FRONT MOVING
ACROSS EASTERN TN. RAINFALL EFFICIENCY IS BEING AIDED BY
PRECIPITABLE WATER VALUES -- PWS -- OF 1.5-2.1", WHICH IS NEAR THE
RECORD OR WITHIN THE 99TH PERCENTILE FOR NORTHERN GA FOR OCTOBER 7.

THE ATMOSPHERIC ENVIRONMENT IS SIMILAR TO THOSE ASSOCIATED WITH
WARM RAIN EVENTS WHICH HAVE CAUSED FLASH FLOODING OVER THE LAST
SEVERAL MONTHS ACROSS UPSLOPE PORTIONS OF CO AND THE WESTERN
CAROLINAS: PWS IN EXCESS OF THE 95TH PERCENTILE FOR THAT TIME OF
THE YEAR, LOW-LEVEL INFLOW WITH AN UPSLOPE COMPONENT OF AT LEAST
25 KT, CAPE VALUES OF 500-1000 J/KG PER RECENT ARW GUIDANCE AND A
MODIFIED 00Z FFC RAOB, AND BROAD DIVERGENCE ALOFT. DIVERGENCE
ALOFT IN THIS CASE IS GENERALLY CAUSED BY A DEEP CYCLONE MOVING
THROUGH IL. SAB MENTIONED A WEAK SHORTWAVE WITHIN THE SUBTROPICAL
STREAM ACROSS GA APPEARS TO BE ENHANCING DIVERGENCE AND RELATED
THUNDERSTORM ACTIVITY ACROSS NORTHEAST GA. SOME CLOUD TOP COOLING
WAS NOTED IN RECENT SATELLITE IMAGERY WITHIN A FEEDER BAND-LIKE
FEATURE DIRECTED INTO THE SOUTHERN APPALACHIANS. THE 850-400 HPA
MEAN LAYERED FLOW IS PARALLEL TO THE APPALACHIANS, IMPLYING CELL
TRAINING IS POSSIBLE. SHOULD CONVECTION BECOME ORGANIZED, CORFIDI
VECTORS ARE EXPECTED TO BE LIGHT IN THIS REGION FOR THE REMAINDER
OF THE NIGHT WHICH WOULD ALLOW FOR SLOW CELL MOVEMENT.

WITH TIME, THE LOW-LEVEL INFLOW IS EXPECTED TO INCREASE TOWARDS 40
KTS PER RECENT RAP GUIDANCE, WITH THE MAIN AXIS SHIFTING SLOWLY UP
THE EASTERN SIDE OF THE APPALACHIANS. SHOWERS AND THUNDERSTORMS
NEAR AND AHEAD OF THE COLD FRONT ITSELF COULD RENEW ACTIVITY
ACROSS NORTHERN GA OVERNIGHT. SAB MENTIONED RAINFALL RATES SHOULD
INCREASE SOME OVERNIGHT -- WHICH IMPLIES 1-1.5" RAIN RATES AN
HOUR. CAPES ARE EXPECTED TO REMAIN STEADY IN THE 500-1000 J/KG
RANGE INTO THE EARLY MORNING. THE MOST RECENT MESOSCALE GUIDANCE
IS FAIRLY UNANIMOUS IN PRODUCING A NARROW STRIPE OF 2"+ OF RAIN
THROUGH 09Z, WITH THE POSSIBILITY OF HEAVY RAINFALL CONTINUING
UNTIL 12Z. SINCE FLASH FLOOD GUIDANCE VALUES REMAIN FAIRLY HIGH
IN THIS AREA -- IN EXCESS OF THREE INCHES IN THREE HOURS -- AND
RAINFALL COULD TRAIN FOR 2-3 HOURS, REGIONAL FLASH FLOODING IS
CONSIDERED POSSIBLE ACROSS THE SOUTHERN APPALACHIANS OVER THE NEXT
SEVERAL HOURS.

ROTH

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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

LAT...LON 36518032 34828207 34128432 35018469 36158262 36438231
36738188 37168150 37488053 36518032

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