Flash Flood Guidance
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Issued by NWS
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191 AWUS01 KWNH 090128 FFGMPD NCZ000-SCZ000-GAZ000-TNZ000-KYZ000-ALZ000-ILZ000-MOZ000-090700- Mesoscale Precipitation Discussion 0244 NWS Weather Prediction Center College Park MD 928 PM EDT Wed May 08 2024 Areas affected...Tennessee River Valley into Southern Appalachians... Concerning...Heavy rainfall...Flash flooding likely Valid 090130Z - 090700Z SUMMARY...Significant life-threatening flash flooding is currently ongoing across northern Middle TN; with additional thunderstorms developing upstream and a narrowing corridor for training downstream suggesting further expansion of considerable flooding through the rest of Middle and southeastern TN. DISCUSSION...01z surface analysis depicts a surface low over south central MO between TBN and VIH with warm front draped southeast across W KY into northern TN. Here, strong convection is reinforcing the frontal zone across northern TN; steepening the isentropes in the area. A secondary convergence zone likely from this morning`s MCS reinforced by differential heating and weak outflow can be analyzed from NW TN south-southeast to near HSV to near RMG. Weak TCU bubbles along this boundary to help denote it. VWP along with RAP analysis and short-term forecast show broad southwesterly 35-40kt LLJ across the MS River Valley intersecting the boundaries fairly orthoganally to induce stronger vertical ascent, given the strength and forcing pressing in from the west; there is limited expected southward sag with time allowing for additional development in W KY and eventually further upstream out of MO to once again align with already saturated areas with falsh flood emergencies across that have seen 3-6". Very moist/unstable enivornment with 3500-4000 J/kg of MLCAPE and TPW to 1.75" should support 2-2.5"/hr rates. So, an additional 3-5" along this swath is possible, broadening the area of significant life-threatening flash flooding along and likely south of the axis from Marshall/Trigg county, KY to Sumner to Cumberland county. Elsewhere, the eastern gradient of the LLJ and enhanced moisture of 1.75" is aiding moisture convergence ascending over the outflow boundary across N AL. Supercells continue to develop and track a bit south of southeast with flanking line/RFD trying to reinforce the outflow boundary. Given upstream LLJ will continue to strengthen, isentropic ascent should support back-building/regeneration upstream and allow for additional training with 2"/hr rates likely to be the norm. As such, this should be another axis of 4-6"+ probably a bit south of this morning`s heavy rainfall. The HRRR has been persistent with this evolution and provides some confidence, though trends suggest it may be a bit too far south, but storm scale interaction/cold pool generation is likely to be the main driver of ideal training axis which is very difficult to lock down this far out. Still the risk for However, this is across more complex orography in the upslope of the Cumberland Plateau, and an axis of considerable flash flooding is likely to evolve. Eventually, leading edge convection should be fast moving but will move through higher terrain of the southern Appalachians and so portions of W NC, N GA are included in this MPD. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...BMX...FFC...GSP...HUN...LMK...MEG...MRX...OHX... PAH... ATTN...RFC...LMRFC...NCRFC...OHRFC...SERFC...NWC... LAT...LON 37788917 37738772 36828547 36108396 35418316 34638334 34058404 34058525 34598660 35618820 36688928