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