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Flash Flood Guidance
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Issued by NWS
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373 AWUS01 KWNH 210100 FFGMPD MNZ000-IAZ000-NEZ000-SDZ000-210700- Mesoscale Precipitation Discussion 0474 NWS Weather Prediction Center College Park MD 859 PM EDT Thu Jun 20 2024 Areas affected...northern NE...southern SD...surrounding portions of MN/IA Concerning...Heavy rainfall...Flash flooding likely Valid 210100Z - 210700Z Summary...Additional rainfall of 1-3" expected with localized totals of 4-6" possible. Given the sensitive conditions over southeastern SD (and surroundings), scattered to numerous instances of flash flooding are likely (with localized significant flash flooding possible). Discussion...A complicated evolution of convection is ongoing across portions of the Northern/Central Plains this evening, primarily in the vicinity of a warm front slowly lifting north from NE into SD. The most organized convection is located over the Sand Hills of NE, where earlier discrete cells have rapidly grown upscale and organized into an MCS (mesoscale convective system) with a distinct bow echo. Farther downstream, a prolific supercell has been anchored near the warm front (nearby Ainsworth, NE), and both surface-based (and elevated) convection is firing around (and north) of this storm. Continued upscale growth of convection is expected through the evening hours, as the mesoscale environment is characterized by ML CAPE of 1000-2000 J/kg (increasing/advecting towards the northeast), precipitable water values of 1.4-1.8 inches (between the 90th percentile and max moving average, per SPC sounding climatology), and effective bulk shear 35-55 kts. While the primary flash flood threat as of late has come from the aforementioned supercell (which produced very impressive rainfall rates of 2-4"/hr at its peak, per MRMS estimates), the main threat going forward will come from training/repeating of relatively high rainfall rates (1-2"/hr) along and north of the warm front. This is quite problematic, as the most concerning antecedent conditions exist across southeastern SD (and some of the immediate surroundings of MN, IA, and NE). Much of this region has had 1-3" (and locally 4-5") of rainfall over just the past 6-12 hours (and anywhere from 200-600% of normal over the past 7 days). Additional totals of 1-3" are expected (and will happen relatively quickly) in association with the expanding MCS/bow echo, and localized totals may reach 4-6" along the northern periphery of the MCS where training/repeating of cells occur. Given the sensitive conditions (with a large area of Flash Flood Guidance values of 1.5" or less), scattered to numerous instances of flash flooding are likely (with localized significant flash flooding possible). Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ABR...FSD...LBF...OAX...UNR... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 44779814 44329632 43249601 41699649 41929938 41150136 41050193 41220245 42090261 43020243 44100061