Flash Flood Guidance
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Issued by NWS
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967 AWUS01 KWNH 020706 FFGMPD MOZ000-IAZ000-NEZ000-021200- Mesoscale Precipitation Discussion 0538 NWS Weather Prediction Center College Park MD 305 AM EDT Tue Jul 02 2024 Areas affected...southwest IA...southeast NE...adjacent portions of far northwest MO Concerning...Heavy rainfall...Flash flooding likely Valid 020705Z - 021200Z Summary...Rainfall rates of 1-2"/hr will persist with elements of backbuilding/training supporting additional localized totals of 2-5". Numrous instances of flash flooding are likely. Discussion...A large, disjointed mesoscale convective system (MCS) continues to bring heavy rainfall to portions of the Upper Midwest and Central Plains late tonight. While the primary area of thunderstorms (and resulting significant flash flooding) has occurred along the I-80 corridor in southeast NE, a secondary area of storms has also manifested farther northeast (over northeast IA, southeast MN, and adjacent portions of WI). All of this activity (with a large area of stratiform precipitation in between) is being driven by a strong low-level jet (LLJ) ushering in warm air advection/moisture transport and subsequent destabilization (with 3-hr MU CAPE increases of 400-1000 J/kg) across a broad area from the Southern/Central Plains to the Upper Midwest. The strongest convection (being rooted closer to the surface) continues to trail behind in southeast NE and southwest IA, and recent backbuilding to the west (towards the most Theta E rich air at the surface) has resulted in repeating of 1-2"/hr rainfall rates (with MRMS estimating localized totals of 2-4" over a 2-3 hour period in the vicinity of the Lincoln and Omaha metro areas). This is in stark contrast to the aforementioned secondary area of heavy rainfall farther northeast, which remains quite progressive with hourly rates capped closer to 1.5" (and importantly without training/backbuilding elements). Going forward, hi-res guidance is actually in better spatial agreement (per HREF ensemble agreement scale, EAS, neighborhood probability fields) with the secondary area of heavy rainfall, suggesting higher confidence in 1-2" accumulations in this area. That said, when focusing on higher-end amounts (additional accumulations of 3"+), the 40-km neighborhood exceedance probabilities are much higher (20-40%) along and downstream of the hard hit Lincoln/Omaha metro areas. The 00z HREF probability matched mean QPF depicts 2-5" totals in this region (primarily in southwest IA going foward), and this is driven by the ARW, ARW2/NSSL, and HRRR members (with subsequent runs of the HRRR remaining fairly consistent with 2-5" depictions through 12z, though notably too far north). Given the observational trends, consistent model guidance, and impressive parameter space (MU CAPE 1000-2000 J/kg, PWATs 1.8-2.2", and effective bulk shear of 30-50 kts) in this region, numerous instances of flash flooding are likely (and will likely continue in already hard-hit areas). Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...DMX...EAX...GID...OAX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 41959347 40939349 40279591 40769746 41679621