027  
AWUS01 KWNH 191825  
FFGMPD  
NCZ000-SCZ000-GAZ000-TNZ000-ALZ000-200000-  
 
MESOSCALE PRECIPITATION DISCUSSION 0960  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
225 PM EDT TUE AUG 19 2025  
 
AREAS AFFECTED...SOUTHERN APPALACHIANS  
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE  
 
VALID 191823Z - 200000Z  
 
SUMMARY...THUNDERSTORMS WITH 2-3"/HR RAIN RATES WILL EXPAND ACROSS  
THE SOUTHERN APPALACHIANS THIS AFTERNOON. THESE STORMS WILL BE  
VERY SLOW MOVING, LEADING TO 2-3" OF RAINFALL WITH LOCALLY AS MUCH  
AS 5" POSSIBLE. FLASH FLOODING MAY RESULT.  
 
DISCUSSION...THE GOES-E DAY-CLOUD PHASE MICROPHYSICS RGB THIS AFTN  
SHOWS A RAPID INTENSIFICATION OF UPDRAFTS NOW FEATURING CLOUD ICE  
AS CBS DEVELOP ALONG A LINE FROM WEST-CENTRAL NC INTO FAR NORTHERN  
GA. THIS ACTIVITY IS BLOSSOMING IN RESPONSE TO PERSISTENT UPSLOPE  
FLOW ON E/NE WINDS BEHIND A COLD FRONT THAT HAS SAGGED INTO THE  
GA/SC COASTAL PLAIN AS ANALYZED BY WPC. DESPITE BEING BEHIND THE  
FRONT, THIS LOW LEVEL FLOW IS POOLING MOISTURE ACROSS THE SOUTHERN  
APPALACHIANS AS REFLECTED BY PWS MEASURED BY GPS AS MUCH AS 1.5  
INCHES NEAR ASHEVILLE, NC AND 2.1 INCHES NEAR COLUMBIA, SC. AT THE  
SAME TIME, BREAKS IN THE CLOUDS ON THE WESTERN PERIPHERY OF THE  
COLD AIR DAMMING (CAD) IS ALLOWING FOR A RAPID INCREASE OF  
INSTABILITY WHICH IS NOW ANALYZED VIA THE SPC RAP TO BE 2000-3000  
J/KG. THE FRESH CONVECTION IN NORTHERN GA HAS ALREADY RESULTED IN  
MRMS HOURLY RAINFALL OF NEARLY 1.5 INCHES AND RESULTANT FLASH  
RESPONSE OF 300+ CFS/SMI UNIT STREAMFLOW.  
 
ALTHOUGH THE CAMS DIFFER IN THE COVERAGE OF CONVECTION THROUGH  
THIS AFTN, THE PERSISTENT UPSLOPE FLOW INTO THIS OVERLAP OF  
MOISTURE AND INSTABILITY SHOULD SUPPORT EXPANSION AND CONTINUATION  
WELL INTO THE EVENING. RAINFALL RATES AS PROGGED BY THE HREF  
NEIGHBORHOOD PROBABILITIES HAVE A 20-30% CHANCE OF EXCEEDING 2"/HR  
WITHIN ANY OF THIS CONVECTION, WHILE THE HRRR 15-MIN RAINFALL  
INDICATES UP TO 0.75" SUGGESTING BRIEF 3"/HR RAIN RATES. DESPITE  
THE UNCERTAINTY IN COVERAGE, THE HREF EAS PROBABILITIES INDICATE  
THE GREATEST POTENTIAL FOR HEAVY RAINFALL WILL BE ACROSS THE BLUE  
RIDGE AND SOUTHWARD INTO THE NORTHERN GA PIEDMONT, AND THIS IS  
SUPPORTED BY THE DIFFERENTIAL HEATING BOUNDARY CAUSED BY THE CLOUD  
BREAKS WITHIN THE CAD TO THE EAST. A RIDGE TO THE WEST WILL KEEP  
OVERALL FORCING GENERALLY LIGHT AND FLOW WEAK, INDICATING THAT  
STORMS WILL MOVE VERY SLOWLY ON 0-6KM MEAN WINDS OF JUST 5 KTS.  
THIS COULD BE PROBLEMATIC AS THE CONTINUED UPSLOPE FLOW WILL  
SUPPORT REGENERATING CELLS, AND AT TIMES STORMS MAY STALL WITH  
ZERO NET MOTION, ESPECIALLY DURING PERIODS OF BOUNDARY  
INTERACTIONS WITHIN THE OTHERWISE PULSE ENVIRONMENT.  
 
THE SLOW MOVEMENT AND REGENERATING BEHAVIOR OF THESE HEAVY RAIN  
RATES COULD PRODUCE 2-3" OF RAIN, WITH LOCALLY AS MUCH AS 5"  
POSSIBLE (10-15% CHANCE FROM THE HREF). THIS HEAVY RAIN WILL OCCUR  
ACROSS SOILS THAT ARE VULNERABLE BOTH DUE TO THE GENERAL  
SENSITIVITY OF THE COMPLEX TERRAIN, BUT ALSO DUE TO 0-10CM SOIL  
MOISTURE THAT IS ABOVE THE 95TH PERCENTILE ACCORDING TO NASA  
SPORT. ANY HEAVY SLOW MOVING HEAVY RAINFALL ACROSS THIS REGION  
COULD QUICKLY RESULT IN RUNOFF AND INSTANCES OF FLASH FLOODING.  
 
WEISS  
   
..PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT  
 
ATTN...WFO...BMX...FFC...GSP...HUN...MRX...RNK...  
 
ATTN...RFC...ALR...ORN...TIR...NWC...  
 
LAT...LON 36568158 36118170 35888189 35588221 34888283  
34168342 33818355 33658367 33518394 33568465  
33908534 34398560 34968542 35288499 35518446  
36268260 36438216  
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