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Flash Flood Guidance
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Issued by NWS
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118 AWUS01 KWNH 212053 FFGMPD WIZ000-MNZ000-IAZ000-NEZ000-SDZ000-220300- Mesoscale Precipitation Discussion 0483 NWS Weather Prediction Center College Park MD 452 PM EDT Fri Jun 21 2024 Areas affected...southern SD...northern NE...southern MN...northern IA Concerning...Heavy rainfall...Flash flooding likely Valid 212100Z - 220300Z Summary...Numerous to widespread instances of flash flooding are likely with localized totals of 3-5" expected over soils that are already very saturated. Significant, life threatening flash flooding is also possible (particularly where the wettest antecedent conditions overlap with the strongest rainfall signal, indicated by where the pink and red areas overlap in the graphic). Discussion...Organized convection is set to proliferate once again this evening over portions of the Northern/Central Plains and into the Midwest, as a frontal zone remains draped across areas that have already been soaked by days of heavy rains. The wettest antecedent conditions (with corresponding Flash Flood Guidance of 1.5" or less) are across southeastern SD into northwestern IA and southern MN (denoted by the pink outlined area in the graphic), uncomfortably close to (and partially overlapping) a quasi-stationary surface front (analyzed by WPC). Meanwhile, a mid-level shortwave trough is evident (via GOES-East water vapor imagery) just upstream (over western NE/KS and northeastern CO), which should help to provide large scale lift for convection via differential positive vorticity advection (DPVA). In addition, the right-entrance region of a strong (100+ kt) jet streak should provide additional large scale lift via upper-level divergence. The parameter space is also quite impressive, as indicated by SPC RAP analysis with The mesoscale environment is characterized by ample instability (MUCAPE of 500-3000 J/kg), near record tropospheric moisture (precipitable water values of 1.7-1.9 inches, around the max moving average per SPC sounding climatology), and effective bulk shear of 35-55 kts. Hi-res model guidance is (overall) in remarkably good agreement, as the 12z HREF signal (probability matched mean QPF and 40-km neighborhood exceedance probabilities) lines up well with hourly HRRR runs (which in and of themselves have been quite consistent in depicting the highest QPF near/along the analyzed surface front). This swath of the highest potential QPF (denoted by the red outlined are in the graphic) is likely to see areal average QPF of 1-2" with localized totals of 3-5". Unfortunately there is a fair amount of overlap with this QPF guidance and the very wet antecedent conditions (including a limited area in the SD/MN/IA border region that has seen 6"+ of rainfall over the past 24 hours), raising concerns for numerous to widespread flash flooding impacts (including localized significant life threatening flash flooding). That said, the trend in the hi-res guidance (prior to the 12z cycle) has been to shift the highest QPF south, and if this axis verifies even farther south than currently depicted (which is not all that uncommon, as CAMs tend to have a northeast bias in QPF in association with MCSs) then this would limit the flash flood threat somewhat (both in terms of coverage and intensity). The latest HRRR runs give some indication of a potential southward shift in the highest QPF, depicting merging cells that become cold pool dominate and propagate southward. Even so, this propagation effect may be too little too late, as the strongest cells are expected to be capable of 2-3"/hr rainfall rates (which would only require a couple hours of residence time to lead to the expected localized 3-5" totals). Should cells NOT become cold pool dominate and result in longer-term training/repeating over sensitive areas (which is NOT currently indicated by the hi-res guidance), then widespread impacts would be a distinct possibility (with scattered instances of significant to catastrophic flash flooding). Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ABR...ARX...DMX...FSD...LBF...MPX...OAX...UNR... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 44619100 43879103 42799100 42709196 42569313 42329513 41729708 42099971 43250022 44239835 44599618 44459337 44609175