Flash Flood Guidance
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Issued by NWS
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098 AWUS01 KWNH 222355 FFGMPD WIZ000-ILZ000-MOZ000-IAZ000-230600- Mesoscale Precipitation Discussion 0491 NWS Weather Prediction Center College Park MD 753 PM EDT Sat Jun 22 2024 Areas affected...southern WI...eastern IA...north-central IL Concerning...Heavy rainfall...Flash flooding likely Valid 230000Z - 230600Z Summary...Hourly accumulations of 1-3" likely to lead to numerous instances of flash flooding, localized totals of 3-6". Some significant, life threatening flash flooding is possible, particularly in southern WI and surrounding portions of IA/IL (where the greatest coverage of flash flooding is expected). Discussion...A mesoscale convective system (MCS) is gaining organization and intensity over southwest WI and northeast IA late this afternoon. There has been a notable decrease in cloud top temperatures (via GOES-East longwave infrared imagery) over the past couple of hours, as overshooting tops have cooled an additional -10deg C (from -65deg C to -75deg C). In tandem, MRMS instantaneous rates have increased to the 4-6"/hr range, resulting in hourly estimated totals of 1-2" (as convection is quite progressive, reducing the residence time of these extreme instantaneous rates). The mesoscale environment ahead of the intensifying MCS is characterized by SBCAPE of 1000-3000 J/kg, preciptiable water values of 1.9-2.2" (at or above the max moving average and rivaling daily record values at DVN, per SPC sounding climatology), and effective bulk shear of 35-45 kts (aided by the right-entrance region of a 100 kt jet streak @ 250 mbs over Lake Michigan, providing large scale lift as well). This highly favorable environment for deep, organized convection should continue to support the MCS into the evening/overnight hours (further supported by MCS maintenance probabilities of 60-80% and derecho composite parameter values of 2-6). While the progressive nature of the MCS should largely limit the residence time of 1-3"/hr accumulations (as the 850-300 mb mean flow is 40+ kts), the concern is that a 2-4 hour period of training/repeating along the southwest flank of the MCS (where a strong southwesterly low-level jet is providing strong moisture transport/convergence for backbuilding) will result in localized totals of 3-6" (as indicated by both the HREF PMM/neighborhood exceedance probabilities and subsequent hourly runs of the HRRR). While there is relatively good agreement between the CAMs in the resultant QPF amounts, there is not as good agreement in the placement of those amounts (anywhere from southeastern WI through north-central IL and southwestern IA). Some recent HRRR runs (with the 21z run being the best example) depict the highest swath across southeastern WI, and this seems to be the most realistic scenario based on the latest observational trends (largley due easterly moiton with the deep layer wind). That said, the HRRR seems to be underdoing the overall intensity and coverage of the convection, and a stronger, more consolidated cold pool would likely allow for more significant upwind propagation (which would result in a southerly component in the storm motion, bringing a greater threat to northern IL). In addition, the upwind propagation vector favors a slower storm motion (near 20 kts), which combined with backbuilding could result in a more significant flash flood scenario (more likely to achieve the high-end 6" localized totals). Even without the backbuilding/training, Flash Flood Guidance (FFG) mostly ranges from 1-2" (over a 1-3 hour period), suggesting that numerous instances of flash flooding are likely (with some localized significant flash flooding possible). Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ARX...DMX...DVN...ILX...LOT...MKX... ATTN...RFC...NCRFC...NWC... LAT...LON 43768840 43388752 41388765 40588865 40228987 40469175 41029274 41979341 43089133 43458992