Flash Flood Guidance
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Issued by NWS
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094 AWUS01 KWNH 210133 FFGMPD IAZ000-NEZ000-SDZ000-COZ000-210730- Mesoscale Precipitation Discussion 0292 NWS Weather Prediction Center College Park MD 932 PM EDT Mon May 20 2024 Areas affected...far northeast CO...much of NE...far western IA...far southeastern SD Concerning...Heavy rainfall...Flash flooding likely Valid 210130Z - 210730Z Summary...Isolated to scattered instances of flash flooding are likely overnight from repeated rounds of heavy rainfall with rates generally 1-2"/hr. Localized totals of 4-7" are possible. Discussion...Convection is gradually increasing in coverage across portions of the Northern/Central Plains this evening, producing localized rainfall rates of 1-2"/hr. A warm and unstable air mass is in place across the region, with Tds rising through the 60s with SB CAPE of 2000-3000 J/kg. In addition, PWs range from 0.9-1.5 inches (above the 90th percentile, per LBF/OAX sounding climatology) and effective bulk shear of 30-60 kts. Continued convective proliferation and organization is anticipated overnight, likely evolving into an MCS that propagates rapidly east to northeast near/along the surface front. While the speed of the MCS should mitigate the extent and intensity of the flash flooding overall, the concern is that the ongoing storms prior to the arrival of the MCS will contribute substantially to the overall rainfall total (particularly across portions of southeast NE). Hi-res CAMs have done reasonably well with the depiction of storm mode and evolution thus far, and some of the more recent HRRR runs are particularly concerning with localized totals through 08z of up to 6". This signal has mainly been confined to southeast NE, where the 12z HREF indicates the highest chance for localized 3" exceedance (per the 40-km neighborhood probabilities of 20-40%). This is highly supported by the observational trends, as the surface front is located directly across this region with pre-MCS localized rainfall totals of 1-3" already (per MRMS estimates). Given that the MCS should track right through this area (bringing another round of rainfall with more widespread 2-3" totals), this should result in localized totals of 4-7". The corresponding 6-hr Flash Flood Guidance (FFG) generally ranges from 2.0-4.0", suggesting that the FFGs will almost certainly be eclipsed locally, likely resulting in isolated to scattered instances of flash flooding. Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...BOU...CYS...DMX...FSD...GID...GLD...LBF...OAX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 43389661 43119554 42079494 41169516 40629609 40189820 40000307 41440326 41870059 42949828