512  
AWUS01 KWNH 201948  
FFGMPD  
GAZ000-TNZ000-ALZ000-MSZ000-210100-  
 
MESOSCALE PRECIPITATION DISCUSSION 0963  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
347 PM EDT WED AUG 20 2025  
 
AREAS AFFECTED...PORTIONS OF TENNESSEE, ALABAMA, AND MISSISSIPPI  
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE  
 
VALID 201947Z - 210100Z  
 
SUMMARY...SLOW MOVING SHOWERS AND THUNDERSTORMS WILL EXPAND  
THROUGH THE AFTERNOON. RAINFALL RATES WILL LIKELY EXCEED 2"/HR AT  
TIMES, LEADING TO POCKETS OF 2-4" OF RAINFALL. THIS MAY LEAD TO  
INSTANCES OF FLASH FLOODING, ESPECIALLY OVER URBAN AREAS.  
 
DISCUSSION...THE REGIONAL RADAR MOSAIC THIS AFTN INDICATES RAPID  
DEVELOPMENT OF SHOWERS AND THUNDERSTORMS FROM SOUTH-CENTRAL TN  
THROUGH CENTRAL MS. THIS CONVECTION IS DEVELOPING AHEAD OF AN  
APPROACHING COLD FRONT AND ALONG A LOW-LEVEL CONVERGENCE BOUNDARY  
FORMED BY THE INTERACTION OF CONVECTIVE OUTFLOWS AND A SURFACE  
TROUGH. ASCENT ACROSS THIS REGION IS BEING ADDITIONALLY ENHANCED  
BY A MODEST SHORTWAVE TRACKING E/SE NEAR THE MS/TN BORDER.  
 
THIS BROAD ASCENT IS ACTING UPON A FAVORABLE THERMODYNAMIC  
ENVIRONMENT TO SUPPORT HEAVY RAINFALL. PWS AS MEASURED BY GPS ARE  
BETWEEN 1.8 AND 2.1 INCHES, AROUND THE 90TH PERCENTILE FOR THE  
DATE, OVERLAPPING MUCAPE THAT IS BETWEEN 3000-4000 J/KG SOUTH OF  
THE COLD FRONT. THIS IS DRIVING THE RAPID EXPANSION OF CONVECTION  
NOTED IN THE REGIONAL RADAR, WITH ADDITIONAL STORMS FORMING DUE TO  
OUTFLOW COLLISIONS AND STORM MERGERS IN THE GENERALLY PULSE-TYPE  
ENVIRONMENT.  
 
RAINFALL RATES HAVE BEEN ESTIMATED VIA LOCAL RADARS TO BE AS HIGH  
AS 2-2.5"/HR FROM KHTX AND KOHX, LEADING TO MRMS HOURLY RAINFALL  
AS MUCH AS 2.5 INCHES. AS THE AFTN PROGRESSES, THE CONTINUED  
ASCENT DRIFTING SOUTHWARD INTO A RIBBON OF ENHANCED MOISTURE  
CONFLUENCE COULD SUPPORT MORE FREQUENT RAINFALL RATES ABOVE 2"/HR  
AS SUPPORTED BY HREF NEIGHBORHOOD PROBABILITIES, AND THE HRRR  
15-MIN PRECIPITATION ACCUMULATION SUGGESTS BRIEF RATES IN EXCESS  
OF 3"/HR ARE POSSIBLE. WHILE STORMS SHOULD REMAIN GENERALLY PULSE  
DUE TO MINIMAL BULK SHEAR, AND MEAN WINDS SUPPORT A SLOW SOUTHWARD  
ADVANCE OF CELLS AT 5-10 KTS, FREQUENT MERGERS AND COLLISIONS WILL  
LEAD TO SLOWER AND AT TIMES CHAOTIC MOTION WHICH COULD ENHANCE THE  
DURATION OF RAINFALL. WHERE THIS OCCURS, TOTAL RAINFALL THROUGH  
THIS EVENING COULD REACH 2-4" AS REFLECTED BY HREF 6-HR RAIN  
PROBABILITIES EXCEEDING 40% FOR 3 INCHES.  
 
7-DAY RAINFALL ACROSS THE AREA HAS BEEN SCATTERED, BUT IN SOME  
PARTS OF THE DISCUSSION AREA, ESPECIALLY IN AL/TN, HAS BEEN AS  
MUCH AS 300% OF NORMAL, LEADING TO 0-10CM SOIL MOISTURE FROM NASA  
SPORT THAT IS ABOVE THE 95TH PERCENTILE. WHILE FFG REMAINS  
ELEVATED, AND FFG EXCEEDANCE PROBABILITIES ARE MODEST IN RESPONSE,  
IT IS POSSIBLE THAT ANY SLOW MOVING/LINGERING HEAVY RAINFALL COULD  
OVERWHELM SOILS TO PRODUCE RAPID RUNOFF. HOWEVER, THIS WILL BE  
MOST LIKELY ACROSS ANY URBAN AREAS WHICH HAVE THE GREATEST  
POTENTIAL FOR ISOLATED FLASH FLOOD IMPACTS INTO THIS EVENING.  
 
WEISS  
   
..PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT  
 
ATTN...WFO...BMX...FFC...HUN...JAN...MEG...MRX...OHX...  
 
ATTN...RFC...ALR...ORN...TIR...NWC...  
 
LAT...LON 35948609 35908485 35648452 35018456 34358490  
33818568 33458724 33288747 32828818 32448880  
32188955 32178993 32489015 33079011 33699002  
34638895 35448751  
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