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Flash Flood Guidance
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Issued by NWS
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705 AWUS01 KWNH 172327 FFGMPD TNZ000-KYZ000-MSZ000-LAZ000-MOZ000-ARZ000-180400- Mesoscale Precipitation Discussion 0455 NWS Weather Prediction Center College Park MD 727 PM EDT Mon Jun 17 2024 Areas affected...Portions of the Lower to Mid Mississippi Valley Concerning...Heavy rainfall...Flash flooding possible Valid 172326Z - 180400Z Summary...Showers and thunderstorms will continue to develop and lift northward into a surface trough through the evening. Rainfall rates within thunderstorms may peak above 2"/hr at times, creating rainfall of 1-3" with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this afternoon depicts widespread showers and thunderstorms within an exceptionally moist airmass across much of the lower to mid Mississippi Valley. These storms are developing in response to ample fuel in the form of PWs elevated to around 2.2 inches, above the daily record according to the SPC sounding climatology, and MLCAPE above 1500 J/kg. This warm/unstable airmass is further characterized by freezing levels rising to around 14,000 ft and mid-level lapse rates that are generally moist-adiabatic (6-7C/km) suggesting efficient warm rain processes are dominating. Synoptic ascent is modest on the western periphery of the building upper ridge, but weak shortwaves embedded within the flow, some subtle low-to-mid level convergence, and a surface trough analyzed from western TN into northern LA are providing sufficient lift into the robust environment to support the convection with heavy rainfall. Recent radar estimated rainfall rates have been as high as 1-1.5"/hr, and these are likely to continue through nightfall. Although storm organization is likely to be minimal due to a lack of bulk shear, the robust thermodynamics and sufficient ascent should result in continued development of convection through and even beyond sunset. This is reflected by most available CAMs, although many, including the HRRR, appear to be under-doing the convective coverage. Storms should continue to develop near the Gulf Coast/Lower Mississippi Valley and then lift northward on mean winds of 10-20 kts and within intensifying 850mb flow which may reach 25-30 kts early tonight. The 850mb wind exceeding the mean wind additionally suggests coverage will remain widespread through the evening, and although cells will be moving quickly, repeated rounds are possible in some areas. The greatest risk for repeating rounds or training will be along the surface trough where the most impressive moisture flux will focus, resulting in enhanced lift for regenerating or merging cells. With rain rates progged by the HREF to be 1-2"/hr, this could result in rainfall locally exceeding 3 inches. 7-day rainfall across this portion of the MS Valley has been minimal as noted via AHPS rainfall departures that are just around 10% of normal, which has resulted in 0-10cm RSM from NASA SPoRT that is just around 30%. However, heavy rainfall today has accumulated to 1-3" in some areas according to MRMS. While most of the soils should be able to handle progressive heavy rain rates, where training can occur, especially in the vicinity of the surface trough, or over urban areas or soils moistened by today`s rain, isolated instances of flash flooding are possible. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...JAN...LZK...MEG...MOB...OHX...PAH... ATTN...RFC...LMRFC...SERFC...NWC... LAT...LON 36818899 36568800 35788811 34528848 33638864 32718869 31758872 31628911 31668973 31869055 32309122 32419134 33399175 34459145 35839071 36628992