Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 27 2024 18:04:17 AWUS01 KWNH 271804 FFGMPD VAZ000-NCZ000-WVZ000-OHZ000-KYZ000-INZ000-272300- Mesoscale Precipitation Discussion 1078 NWS Weather Prediction Center College Park MD 203 PM EDT Fri Sep 27 2024 Areas affected...Lower Ohio Valley into the Appalachians of WV/VA/NC Concerning...Heavy rainfall...Flash flooding possible Valid 271802Z - 272300Z Summary...Showers and thunderstorms developing around Tropical Storm Helene will train to the NW through the afternoon. Rainfall rates as high as 1.5"/hr are likely, which through this training could produce 1-3" of rain. Additional flash flooding is possible. Discussion...The regional radar mosaic this afternoon shows an expansive area of rainfall from the MS VLY eastward into the coastal Mid-Atlantic states. This precipitation shield is associated with the interaction of Tropical Storm Helene and an upper low centered near western TN. Drier air noted in WV imagery being ingested into Helene is impinging into an axis of higher boundary layer theta-e air over NC, driving local ascent to help to reinvigorate convection within this moist plume. PWs measured by GPS are generally 1.9 to 2.4 inches, nearing daily records, with U/A soundings featuring deep moist-adiabatic lapse rates to support efficient warm-rain processes. Instability is severely limited west of the Appalachians as noted via the SPC RAP analysis, but a ribbon of MLCAPE approaching 1000 J/kg over NC is slowly advecting westward on impressive 850mb SE inflow of 40-50 kts. The evolution the next few hours is somewhat uncertain across this region due to CAM disagreement, but the models appear to be under-analyzing the current convective regeneration along the theta-e gradient across NC. This suggests that the overlap of ascent and thermodynamics is still robust, and sufficient to drive new convection to the NW through the aftn. The pronounced 850mb inflow should help advect the higher instability NW as well, which will support an expansion of rain rates which the HREF neighborhood probabilities suggest have a 20-40% chance of exceeding 1"hr at times. Mean storm motions will remain progressive on 0-6km winds of around 40 kts, but the accompanying 850mb wind evolution will also result in collapsing and veering Corfidi vectors to become more anti-parallel to the flow indicating an increased potential for backbuilding and training. While the CAMs disagree on the greatest risk area for heavy rainfall this aftn, there is a 90% chance for 1"+ and 10-30% chance for 3+" of rain in both the HREF and REFS ensembles where the most pronounced training occurs. Much of this area has seen heavy rainfall the past 24-hrs reaching 1-2" in the lower OH VLY and as much as 4-6" in SW VA. This suggests that the most vulnerable soils will remain in the terrain of the Appalachians of WV/VA and far eastern KY, but even into the OH VLY 3-hr FFG has fallen to around 1.5", and HREF FFG probabilities indicate a 20-50% chance of exceeding these values. While the greatest risk for flash flood impacts will be in the Appalachians atop the most saturated soils, any training could cause flash flooding into the evening. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4omrvPxFtolww6G2BzTDPQAYMxMUGNsW76EE3xIbu6Wu_zyaBoWT_l1x4nLoC3RZAmDM= TUoM0yEMkXX8UP1QWgWlX4c$ for graphic product... ATTN...WFO...GSP...ILN...JKL...LMK...MRX...RAH...RLX...RNK... ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...NWC... LAT...LON 39508400 39448269 39058161 38338063 37347964=20 36577943 36027944 35767998 35838043 36218101=20 36748170 37198252 37818367 38168437 38278454=20 38588491 39098472=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .