A cold front will stretch down through the Midwest associated with a low pressure system in Canada. That front will ignite some showers going into the 7 pm hour this evening, with the heaviest precipitation from about 8pm-12am tonight. Here's this evening's forecast 500 mb chart, where we have outlined the cold front reaching down from Canada.
Since it is Fall, temperatures across the nation have gone to a point where the violent spring and summer thunderstorms are no longer something to worry about as much. They can be a threat, but not nearly as much as in the spring or summer. Thus, this evening, there will only be some showers and embedded thunderstorms. If you're really curious about where the convection/thunderstorms will occur, check the image below to see where convective precipitation will occur at around 1 am this evening.
The image is in millimeters, so it won't be as bad as you may initially think by looking at the map only without a legend.
Anyhow, below we have the overall set-up for this evening around 1 am below.
The image is from the GFS model, which gives the set-up. But the cool thing is that we have a model called the WRF that can show what the radar may look like in the future. That said, here is the forecast radar reflectivity for 1 am.
All in all, there will be several disorganized fragments of showers in the Illinois region, some more thunderstorms in the Arkansas region, and maybe some embedded thunderstorm pieces in the Wisconsin area into Michigan.
Stay with The Weather Centre for the latest information on this storm system.