Looking at the zoomed in maps for the Chicago area, it's actually more like a 10 degree difference in the highs between Chicago and the areas well inland. Not making a call on high temps yet, but I am confident there won't be that large of a spread in highs. No way.
Something seems off with those maps, and it's the large discrepancy between Cook county and the rural/well inland areas of northern IL. You'd expect it to be warmer in the urban area, but a 6-8 degree difference during the afternoon seems a little egregious. So I think it will be warmer inland than shown, cooler around Chicago than shown, or some combination of that.