Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 16 2025 22:00:56 AWUS01 KWNH 162200 FFGMPD INZ000-WIZ000-ILZ000-IAZ000-170300- Mesoscale Precipitation Discussion 0946 NWS Weather Prediction Center College Park MD 559 PM EDT Sat Aug 16 2025 Areas affected...southern WI and north-central IL, as well as surrounding portions of IA/IN Concerning...Heavy rainfall...Flash flooding likely Valid 162155Z - 170300Z Summary...Very heavy downpours with 1-3" short-term rainfall totals likely to result in isolated to scattered flash flooding. Discussion...A compact and potent mid-level shortwave trough is translating rapidly east-southeast over the Upper Midwest this afternoon, driving a classic series of mesoscale convective systems (MCSs) along the northern periphery of a blocked synoptic scale ridge (centered due south over the Midsouth region). Persistent convection has favored the ML CAPE gradient (500-4000 J/kg) over the past several hours, which has shifted southward coincident with prior MCS outflow and the associated tightening surface thetaE gradient. While some convection is still ongoing from the earlier morning MCS into northeast IL (rapidly approaching the Chicago metro with MRMS indicating very heavy downpours with 15-min rainfall totals nearing 1.0"), a trailing MCS (currently rapidly progressing through southern WI) is quickly becoming the more dominate expansive MCS. The mesoscale environment in the vicinity of theses MCSs is otherwise characterized by anomalously high tropospheric moisture content (PWs 1.6-2.0", at or above the 90th percentile per GRB/DVN sounding climatology), and effective bulk shear of 25-50 kts. While hi-res CAMs have certainly struggled with the precise convective evolution that has occured (rather typical in these highly dynamic, progressive MCS events), collectively analyzed through the HREF (and associated post-processing including time-lagged members) a valuable forward-looking signal for heavy rainfall still emerges. The 18z HREF local probability-match mean (LPMM) QPF suggests a period of localized 2-3"/3-hr associated with backbuilding along the west/southwest flank of the MCS (with new convective initiation occurring where low-level inflow and iscentropic lift coincide with the highest instability). While the HREF signal is likely too late/slow overall (as this is already beginning to occur into southwest WI), 40-km neighborhood exceedance probabilities for 2" exceedance range from 20-50% (confined to southern WI, though in reality lower-end probabilities for these amounts likely extend both upstream and downstream of the dominate MCS, due to underestimated forward propagation AND backbuilding by the CAMs overall). Although hourly runs of the HRRR have not proven as useful as the HREF signal (which also seems rather typical when the environment of the model initialization is mismatched with the observational assimilation), the experimental WoFS does suggest that similar 2"+ localized totals are possible (via an increasing trend in the 90th percentile QPF with a 27-km neighborhood 2" exceedance probs in the 20-40% range, notably displaced south and west of the HREF signal). While the greatest signal/risk for flash flooding is judged to be across southern WI and northern IL, the MPD also encompasses areas downstream (southeast into far northwest IN) and upstream (west into adjacent portions of IA) due to the aforementioned forward propagation and backbuilding threats. While these surrounding areas are less likely to realize localized 2"+ totals, the chances for 1"+ amounts (in as little as 15-30 min with very heavy downpours) are likely (per HREF/WoFS exceedance probs of 40%+) with associated 1-hr Flash Flood Guidance (FFGs) ranging from as low as 1.0-1.5" across a substantial portion of the region. As a result, isolated to scattered instances of flash flooding are considered likely (with conditional backbuilding of convection largely dictating the overall coverage and potential for an instance or two of more significant flash flooding). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!85RfJsxpSa7tuBurkDXC0aMjTJ4xX4BGbQjCHnUmnpeHIkFvJK0rzeISwXDJQFMobfZF= OoSfOBvy97Tpn6fDKz9qf-I$ for graphic product... ATTN...WFO...ARX...DVN...ILX...IND...IWX...LOT...MKX... ATTN...RFC...MSR...TIR...NWC... LAT...LON 43608890 43418789 42988769 42268726 41618674=20 40868651 40208687 39998789 40058882 40418974=20 40979028 41619055 42079078 42649143 42989143=20 43249113 43369075 43408979=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) .