A strong area of low pressure will move across the middle of the country bringing with it rain and thunderstorms. Storms will begin out west over the Plains Tuesday evening and slide eastward into Iowa, Missouri and Oklahoma by Wednesday afternoon and evening.
Scattered showers are likely during the day in Northern Illinois and Southern Wisconsin, but the more steady rainfall will arrive closer to supper time and overnight. While a few thunderstorms can't be ruled out, it looks like the higher chance for those will arrive late in the evening. This will be as a cold front and area of low pressure move closer.
With still a little instability lingering into the morning and early afternoon Thursday we can't completely rule out a few more thunderstorms developing over Northern Illinois, but it looks like the higher threat for storms will be over Northeast and East-Central Illinois.
Scattered showers are possible again Friday as colder air moves into the Great Lakes. We could even get a little thunder with that, as well.