Mesoscale Precipitation Discussion 0567 NWS Weather Prediction Center College Park MD 1114 PM EDT Sat Jul 23, 2022 Areas affected...Southeast MN...Southern WI...Northeast IA... Far Northern IL... Concerning...Heavy rainfall...Flash flooding likely Valid 240315Z - 240900Z SUMMARY...Increasingly more efficient heavy rainfall complex with embedded training cells pose possible flash flooding through early overnight. Rates up to 2"/hr and broad swath of 2-4", perhaps an isolated 5" amount is probable. DISCUSSION...GOES-WV suite depicts a weak shortwave ridge associated with a 3H speed max and weakly diffluent flow downstream crossing S MN into WI. Thunderstorms are already starting to blossom along and just north of an W-E oriented surface warm front generally parallel to the MN/IA border before crossing southern WI in the wake of the initial MCS. Strengthening but broad WSW LLJ is responding to the shortwave and upper-level UVV providing strong advection and eventual speed convergence through/along the front zone and downstream upglide. Strong convergence and axis of recharging/untapped 3000+ J/kg CAPE axis will support strong updrafts and moisture flux convergence of 1.9-2" TPW to support thunderstorms with capability of sub-hourly rates of .5-.75"/15 minutes; however, translation speeds will limit duration of any specific cell as zonal flow is strong at 35-40kts. As such, training/repeating cells will be critical for producing excessive rainfall. As mentioned, the orientation of the front/convergence axis is expected to remain fairly flat to the mean flow supporting a favorable axis for training and likely to support a few hours along the axis for totals of 2-4" to occur. Southward propagation due to cold pool generation and weak 500-1000 thickness divergence downstream into S WI/N IL remains the greatest uncertainty at this time. If southward propagation is stronger, surface based/most efficient cells should track south of due east and reduce duration for extreme totals... while best forcing/convergence axis will slide northward with best upglide over the front and present a training of elevated cells across central WI from LSE to OSH. As such, have broadened the eastern side of the area of concern to encompass this uncertainty. 00z Hi-Res CAMs denote this potential spread though there is some increase in agreement toward less southward propagation. Either way, the rates/totals are in the range of the lowered FFG values due to initial complex and as such scattered incidents of flash flooding are considered likely through early overnight hours. Boone-McHenry-Lake IL-De Kalb-Kane-DuPage-Kendall-Grundy-Kankakee- Northern Cook-Central Cook-Southern Cook-Northern Will-Southern Will- Eastern Will-Lake IN-Porter- Including the cities of La Grange, Carol Stream, Calumet City, Chicago, Portage, Kankakee, Crystal Lake, Wheaton, Woodstock, Palatine, Beecher, Schaumburg, McHenry, Downers Grove, DeKalb, Belvidere, Chesterton, Merrillville, Northbrook, Lombard, Oak Lawn, Gurnee, Manhattan, Aurora, Hammond, Waukegan, Bourbonnais, Lemont, Plano, Channahon, Buffalo Grove, Gary, Valparaiso, Bolingbrook, Sycamore, Wilmington, Minooka, Morris, Algonquin, Evanston, Peotone, Joliet, Oak Park, Cicero, Park Forest, Elgin, Des Plaines, Orland Park, Mokena, Oswego, Oak Forest, Plainfield, Yorkville, Bradley, Mundelein, Coal City, Crete, and Naperville 228 PM CDT Sat Jul 23, 2022 ...FLOOD WATCH IN EFFECT FROM 7 PM CDT THIS EVENING THROUGH SUNDAY MORNING... * WHAT...Flash flooding caused by excessive rainfall is possible. * WHERE...Portions of Illinois and northwest Indiana, including the following areas, in Illinois, Boone, Cook, De Kalb, DuPage, Grundy, Kane, Kankakee, Kendall, Lake, McHenry, and Will. In northwest Indiana, Lake and Porter. * WHEN...From 7 PM CDT this evening through Sunday morning. * IMPACTS...Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations. Creeks and streams may rise out of their banks. Flooding may occur in poor drainage and urban areas. * ADDITIONAL DETAILS... - Thunderstorms with high rainfall rates are expected this evening, with additional thunderstorms lingering across much of the watch area into Sunday morning. The high rainfall rates over saturated soils will lead to localized street flooding, runoff into already elevated streams, creeks and rivers, and renewed inundation of low-lying areas. PRECAUTIONARY/PREPAREDNESS ACTIONS... A Flood Watch for flash flooding means rapid-onset flooding is possible, but not yet certain, based upon the latest forecasts. Flash flooding is a dangerous situation. Persons with interests along area rivers, creeks, and other waterways should monitor the latest forecasts and be prepared to take action should flooding develop.