Flash Flood Guidance
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Issued by NWS
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035 AWUS01 KWNH 252015 FFGMPD NCZ000-SCZ000-GAZ000-TNZ000-260200- Mesoscale Precipitation Discussion 1057 NWS Weather Prediction Center College Park MD 414 PM EDT Wed Sep 25 2024 Areas affected...North Georgia and the Southern Appalachians Concerning...Heavy rainfall...Flash flooding likely Valid 252013Z - 260200Z SUMMARY...Training thunderstorms will continue to lift over the Atlanta Metro area rest of this afternoon and over the eastern side of the southern Appalachians through tonight. Flash flooding is likely, particularly in Appalachian terrain, through this evening before becoming considerable to catastrophic overnight. DISCUSSION...Convergence of the tropical plume of moisture ahead of Helene and a cold front over southern Alabama has allowed a particularly heavy line of slow-moving and training thunderstorms to develop over southeast AL into western GA. This northward shift in moisture is driven by an upper low centered over far western KY. Recent rainfall estimates of 2 to 3" per hour are from KFFC and KMXX over eastern AL/western GA. The heaviest rain thus far has stayed south of the Atlanta metro and outflow is seen on KFFC which indicates the main threat for now to be avoiding that particular area. However, the tropical moisture plume with PW of 2 to 2.25" will shift up the AL/GA border rest of the afternoon, pushing moisture and instability back over the Atlanta metro and then up the eastern side of the southern Appalachians through this evening. Terrain enhanced rainfall there will quickly fill in a gap of rainfall over the past day that in western NC (surrounding Asheville). Rates of 2"/hr is likely by this evening which would exceed the FFG of 1.5-2"/hr. Flash flooding is likely through 02Z. The 18Z HRRR depiction of 2-4" in terrain over western NC and northeast GA is reasonable, but around 1" additional seems low for north-central GA and the Atlanta metro with a few additional inches possible. The much greater threat for flooding area-wide is expected to be later tonight as even stronger southeasterly flow associated with the approach of Helene pushes ever greater moisture into the same eastern slopes of the southern Appalachians with potentially catastrophic flash flooding overnight. Jackson ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...FFC...GSP...MRX...RNK... ATTN...RFC...LMRFC...OHRFC...SERFC...NWC... LAT...LON 36478281 36268158 34908229 33878304 33158365 33158508 33548517 34468443 35388418