With a warm humid rather unstable air mass in place (temperatures rising well into the 80s and dew points in the 70s) any kind of disturbance at the surface or aloft could trigger the development of showers/t-storms across the Chicago area in the next 24 to 48 hours. A large complex of storms moved out of Minnesota into Wisconsin this morning and will continue east, the southern portion of the complex likely introducing a new band of showers/t-storms across northern Illinois – strongest activity mid-late afternoon in the Chicago area. The Thunderstorm Outlook produced by the National Storm Prediction Center shows a 40 to 60 percent chance of thunderstorms between 3 and 7PM (blue-shaded area on the maps below. Some decrease in t-storms is expected this evening, then another storm complex is expected to intensify over Minnesota and move into Wisconsin overnight, the southern portion quite possibly reaching down into northern Illinois/the Chicago area well after midnight.
Strongest potentially severe storms are most likely in Iowa, Minnesota and southern/western Wisconsin during the next 24 hours – a portion of the Chicago area is in the Marginal Risk Category. Locally heavy, potentially flood-producing rainfall looks to be mainly confined to Minnesota and Wisconsin as well with northernmost sections of our area in the Marginal Risk area.