Flash Flood Guidance
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Issued by NWS
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264 AWUS01 KWNH 180020 FFGMPD WIZ000-MNZ000-IAZ000-SDZ000-180600- Mesoscale Precipitation Discussion 0456 NWS Weather Prediction Center College Park MD 820 PM EDT Mon Jun 17 2024 Areas affected...Southern Minnesota, eastern South Dakota Concerning...Heavy rainfall...Flash flooding likely Valid 180019Z - 180600Z Summary...Showers and thunderstorms will increase in coverage this evening and begin to train along a surging warm front. Rainfall rates in excess of 2"/hr are likely at times, which could produce 2-4" of rain with locally higher amounts. Flash flooding is possible. Discussion...Showers and thunderstorms are expanding across portions of MN this evening as reflected by rapidly cooling cloud tops in the GOES-E IR imagery collocated with strengthening reflectivity via the regional radar mosaic. This activity is building in response to pronounced ascent due to the favorable positioning of the RRQ of an upper jet streak, a shortwave noted lifting out of SD, and increasing isentropic upglide atop a surface warm front analyzed by WPC from northern NE through western WI. Together these are producing strong lift into a favorable thermodynamic environment characterized by PWs of around 1.7-1.8 inches, and a sharp SBCAPE gradient to as high as 4000 J/kg over northern NE. Rainfall rates within ongoing convection are already estimated to be 1-1.5"/hr. The CAMS are struggling to initialize the current activity, as the latest reflectivity is much more widespread than the models indicate it should be. This reflects how potent the environment is for thunderstorm development, and the ingredients based approach indicates thunderstorms will continue to develop through the evening as the front begins to lift northward. The setup supports widespread development along and north of the warm front, with activity becoming more intense as the 850mb LLJ rises to 50-60 kts from the south, pegged above +6 sigma from the SREF, surging extreme moisture flux into the region. This will reinvigorate convection generally along the warm front where lift is most intense, while the additional synoptic lift results in widespread thunderstorms across the discussion area. Additionally, this surging LLJ will result in veering and collapsing Corfidi vectors in the next few hours, and signals are strong for back-building convection to the SW along the warm front, with cells then training N/NE at 15-25 kts. The lack of model support for the current activity indicates that the HREF probabilities are likely too low, but even they already suggest rain rates will eclipse 2"/hr at times, and where backbuilding/training occurs this has a 10-20% chance for as much as 5" of rainfall in the next 6 hours. Despite the uncertainty in the model output, the ingredients suggest instances of flash flooding are likely due to the heavy rates and training potential. Additionally, soils across this region are saturated from recent rainfall that has been 2-4" in the last 24 hours, and around 300% of normal the last 7 days. This has compromised FFG to 0.75-1.5"/3hrs, which will likely be exceeded, causing at least scattered instances of flash flooding. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ABR...FGF...FSD...MPX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 45939554 45909406 45469317 44849275 44259297 43769420 43379537 43169668 43239804 43759887 44179886 44539855 45119790 45669678