Flash Flood Guidance
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Issued by NWS
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430 AWUS01 KWNH 221702 FFGMPD TNZ000-KYZ000-ILZ000-MSZ000-MOZ000-ARZ000-222300- Mesoscale Precipitation Discussion 0302 NWS Weather Prediction Center College Park MD 102 PM EDT Wed May 22 2024 Areas affected...The Ozarks eastward into the Tennessee Valley Concerning...Heavy rainfall...Flash flooding possible Valid 221700Z - 222300Z Summary...Showers and thunderstorms blossoming ahead of a cold front will expand and train into the evening. Rainfall rates as much as 2-3"/hr are likely, which could produce 2-3" of rain with local amounts as much as 5". Flash flooding is possible. Discussion...The GOES-E IR imagery this afternoon indicates rapidly expanding and intensifying convection noted by cooling cloud tops from eastern KY through northern AR. These storms are developing along/ahead of a southward advancing cold front analyzed by WPC draped from southern IN, through the Bootheel of MO, and into eastern OK. This front is moving into favorable thermodynamics for heavy rain rates, reflected by GPS measured PWs as high as 1.9 inches over AR, near the daily record at LZK, and SPC RAP analyzed SBCAPE as high as 4000 J/kg. Into these impressive thermodynamics, forcing for ascent is intensifying through low-level convergence aided by a convectively enhanced shortwave lifting out of OK, and an overlap of thickness and upper level diffluence downstream into the TN VLY. Within this robust environment, radar-estimated rain rates have already reached 1"/hr. As the aftn progresses, it is likely convection will become widespread ahead of this front as suggested by many of the available high-res CAMs. This is supported by the presence of the deep layer ascent acting upon an environment that will become increasingly unstable as a pool of higher SBCAPE gets drawn northeast ahead of the front. This will likely produce expansion and intensification of the thunderstorms, with HREF neighborhood probabilities for 2"/hr rain rates reaching 20-30%, and HRRR 15-min rainfall suggesting brief rates above 3"/hr, especially where storm organization can occur in the presence of 30-40 kts of bulk shear. Most concerning for the flash flood potential is that this convection should train on 850-300mb flow that is progressive at 20-25 kts, but aligned parallel to the front. With an intense pool of SBCAPE focused over AR, this flow should allow for regeneration and training from SW to NE, prolonging these rain rates in some areas. This is reflected by HREF probabilities for 3"/6hrs reaching 25% in some areas, with minimal, but non-zero, probabilities for 5" in the next 6 hours also present. FFG across the region is generally elevated at around 2.5"/3hrs, but pockets of 7-day rainfall which has been above 150% of normal according to AHPS has locally reduced FFG. Where the most intense rain rates occur, or where training is most prevalent, instances of flash flooding are possible. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...LMK...LZK...MEG...OHX...PAH... ATTN...RFC...ABRFC...LMRFC...OHRFC...NWC... LAT...LON 37378824 37338732 37018609 36418553 35958607 35488722 34828884 34569025 34559142 34579167 34599236 34779309 35109345 35559330 35959240 36609040 37178918