Flash Flood Guidance
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534 AWUS01 KWNH 160107 FFGMPD WIZ000-MNZ000-IAZ000-160700- Mesoscale Precipitation Discussion 0444 NWS Weather Prediction Center College Park MD 906 PM EDT Sat Jun 15 2024 Areas affected...southern Minnesota through western Iowa Concerning...Heavy rainfall...Flash flooding possible Valid 160105Z - 160700Z Summary...Showers and thunderstorms will gradually expand in coverage ahead of a warm front through early tonight. Rainfall rates of 1-2"/hr are likely, which through multiple rounds could produce 2-3" of rain with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic early this evening depicts increasing coverage of showers and isolated thunderstorms from western Iowa into much of southern Minnesota. These storms are firing in response to increasing ascent along a surface trough analyzed by WPC resulting in increased low-level convergence, and in an area of modest but sufficient upper level diffluence. This ascent is acting upon favorable thermodynamics, with the core of the most intense environment impinging into the surface trough thanks to advection on an 850mb LLJ that is arcing out of the S/SW at 30-40 kts according to regional VWPs. This is helping to draw northward PWs that are over 1.75 inches, collocated with MLCAPE above 2000 J/kg, resupplying the environment to enhance rainfall coverage and intensity. Recent radar-estimated rainfall rates are still modest at 0.5-1"/hr, but the coverage of these has steadily increased in the past 1-2 hours. As the evening progresses, the impressive LLJ is progged via the RAP to reach 40-50 kts, which when combined with a ribbon of PWs surging northward towards 2 inches, will result in 850-700mb moisture flux that could exceed +4 sigma into northern IA and southern MN. Wind speeds within this LLJ will be 1.5-2 times the mean 0-6km wind as well, suggesting even further enhancement of ascent, and where this collision occurs in the vicinity of the surface trough, repeated development of thunderstorms is likely as supported by simulated reflectivity from the recent CAMs. Not only will thunderstorms regenerate and repeatedly lift northeast, but the strong ascent into the enhanced PW/instability will drive rainfall rates which have a 10-20% chance of exceeding 2"/hr according to the HREF neighborhood probabilities. The greatest potential for the intense rates will be south of this surface front where instability is greater, but more modest rates may be more persistent to the north through regeneration. Although cells may move quickly to the northeast on the 25-35 kts 0-6km mean wind, the increasing and slowly veering LLJ will cause Corfidi vectors to veer and collapse to around 10 kts, further supporting the idea of regenerating and repeating cells. Where this occurs, more than 3 inches of rain is possible (20-30% chance) with locally higher amounts. FFG across the region is as low as 1.5"/3hrs in southern MN where 7-day rainfall has been more than 100% of normal in some areas. Otherwise, FFG is generally 2-2.5"/3hrs due to a relative lack of recent rainfall. This is reflected additionally by NASA SPoRT 0-40cm soil moisture that is near normal in IA, but around the 80th percentile into MN. Despite these differences, the HREF FFG exceedance probabilities are uniformly around 20-25%, suggesting instances of flash flooding are possible anywhere within the area into tonight. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ARX...DMX...FSD...MPX...OAX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 45669382 45629275 45049250 43889236 42599254 41659313 41019375 40699453 41239516 42379598 43389617 43989612 45049542