Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 09 2023 18:31:17 AWUS01 KWNH 091831 FFGMPD VAZ000-NCZ000-SCZ000-GAZ000-TNZ000-KYZ000-ALZ000-100030- Mesoscale Precipitation Discussion 1045 NWS Weather Prediction Center College Park MD 230 PM EDT Sat Sep 09 2023 Areas affected...Southern Appalachians Concerning...Heavy rainfall...Flash flooding possible Valid 091830Z - 100030Z Summary...Slow moving thunderstorms will increase in coverage along the Southern Appalachians through this evening. Rainfall rates may exceed 2"/hr at times, producing locally more than 3" of rain. Flash flooding is possible. Discussion...The center of an amplified mid-level trough is pivoting across Tennessee this afternoon, with spokes of vorticity rotating through it to the east. This trough will continue to shift east, with height falls and PVA combining with modest jet level ascent to drive deep layer lift today. At the surface, a warm front was analyzed by WPC to be draped from near Atlanta, GA northeast towards Roanoke, VA, with low-level southeast flow advecting PWs of more than 1.5 inches and elevated SBCAPE above 2000 J/kg northwestward. This is not only enhancing the thermodynamics across the area, but these parcels are also being lifted isentropically atop the warm front to enhance ascent. Together, this overlap of forcing and moisture/instability is yielding rapid expansion of convection noted on the regional radar mosaic, with radar-estimated rain rates of 1-1.5"/hr. During the next several hours, the continued movement of the anomalous trough overhead and the persistence of the robust thermodynamics will result in increased coverage and intensity of thunderstorms. This is reflected by most available high-res models, with HREF hourly rainfall rate probabilities increasing for 2"/hr, and the HRRR 15-min rainfall product indicating locally up to 0.75" in some areas. While effective shear is likely to remain weak suggesting generally pulse convection, cells containing these rates will move very slowly, reflected by 0-6km mean winds of just 5-10 kts and similar Corfidi vectors. This additionally suggests that as convective coverage increases, storm mergers and boundary collisions will result, driving briefly even more intense rain rates and chaotic motions. Both the HREF and RRFS TL indicate 20-25% probabilities for more than 3 inches of rain, with locally higher amounts likely. There may be a local axis of heaviest rainfall along the Cumberland Plateau where upslope flow beneath the core of the upper trough may drive the most persistent and regenerative convection. 14-day rainfall according to AHPS has been generally slightly above normal, but this has still been sufficient to produce USGS streamflow anomalies that are running slightly above to well above normal. Additionally, FFG is compromised in some areas to as low as 1.5-2.5"/3hrs, with sensitive terrain features likely requiring even less for significant runoff. The slow motion of these intense rates will likely be sufficient to overwhelm these soils in some areas, which could produce instances of flash flooding through the aftn and evening. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8IB191M9FT_FLf_LmohQTB9ZqHi-GnXWCBb4OrqyjoZXODk0bo3wjAc9hd_sz7A43dcl= pvgcxifgDEt5UrPwEU3vrnY$ for graphic product... ATTN...WFO...BMX...CAE...FFC...GSP...HUN...JKL...LMK...MRX... OHX...RLX...RNK... ATTN...RFC...LMRFC...OHRFC...SERFC...NWC... LAT...LON 37208278 36348094 35938045 35348077 34488182=20 33348316 33028420 33168511 33628589 34328663=20 35208642 36308568 37008445=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .