Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 12 2025 21:16:50 AWUS01 KWNH 122115 FFGMPD MIZ000-INZ000-ILZ000-130200- Mesoscale Precipitation Discussion 0919 NWS Weather Prediction Center College Park MD 514 PM EDT Tue Aug 12 2025 Areas affected...Greater Chicagoland and surrounding portions of IL/OH/MI Concerning...Heavy rainfall...Flash flooding possible Valid 122110Z - 130200Z Summary...Hourly rainfall totals of 1-2" may locally train/repeat over sensitive urban areas. Locally significant/life threatening instances of flash flooding are possible. Discussion...Convection has proliferated in the vicinity of a surface trough of low pressure this afternoon, extending from western MI through northern IL. While deep layer moisture flux convergence has spiked synoptically along the trough axis, an enhanced meso to microscale area of near surface convergence exists along the southern shore of Lake Michigan (and most pronounced along the IL shore). This is occurring as upper-level shear/divergence is only increasing, as the base of a potent shortwave trough progresses eastward across northern MN/WI. The idealized right-entrance region of an associated jet streak (~110 kts at 250 mb) may allow for this convection to persist for longer than anticipated, as CAMs have generally been much slower/weaker with convection than actual observational trends. With the mechanism for lift and convective longevity clearly established, plentiful instability (3000-3500 J/kg across northern IL) and anomalous tropopsheric moisture (~2.0" PWs above the 90th percentile, per ILX sounding climatology). MRMS estimates hourly rainfall amounts locally as high as 2", and even without training/repeating (thus far) the associated CREST unit streamflow values suggests minor to moderate flood impacts are already occurring (particularly where these 2" totals are co-located with more sensitive urbanized terrain). Going forward, CAMs suggest the potential for additional localized totals of 2-4" (HREF 40-km neighborhood exceedance probs for 3" threshold of 10-20% through 03z). Since most models were too slow to develop convection, much of this forecast rainfall may have simply occurred early. However, given the aforementioned favorable mesoscale environment (with little depletion of instability and increasingly favorable dynamics) it's quite possible that heavy rainfall will continue for several hours still, eventually coming to an end between 00-02z. Given the potential for training/repeating with little movement in the trough axis and baroclinic lake enhancement, a significant instance or two of flash flooding is possible (and may become life threatening in more sensitive, low-lying populated areas of Chicagoland). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-sAtEx_qMfvLfJ_Y_lQ-4btitGjJQjzNIHeugNXXgqFX5JYKzEWsIMiAeImX8FqLvam0= WiqkdDOBwKZk8uI-9NtV97I$ for graphic product... ATTN...WFO...ILX...IWX...LOT... ATTN...RFC...MSR...TIR...NWC... LAT...LON 42468708 42228647 41838622 41178670 40648751=20 40468857 40608902 41598821 42218789=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .