Here are two GFS forecast soundings valid at the same time. One is from Waukegan and one is from Rockford.
First up, Waukegan...
Here we see a deep warm layer above 0C from just under 850 mb all the way up to 700 mb. There is a respectable cold layer colder than 0C underneath this, but its depth and magnitude is not quite like what the warm layer is. There is also a bit of mid-level dry air/lack of saturation. These factors point toward this being more of a freezing rain sounding instead of sleet. If you notice toward the bottom of the graph, the solid red line is essentially touching the light blue dashed line, which represents sfc temps right around freezing.
Now going over to Rockford...
Here we see a deeper warm layer aloft that goes from about 900 mb to 700 mb, and it's also warmer. There's also less of a cold layer underneath than there is at Waukegan at that same time. Interestingly, despite this larger/warmer warm layer and less pronounced cold layer, you see similar sfc temps as Waukegan, which I have to believe is the model trying to account for marine influence vs no marine influence. Even with sfc temps around freezing at Rockford on that forecast sounding, I would argue that it would be difficult to keep sustaining freezing rain given the absolutely massive warm layer aloft and marginal sfc temps, assuming this depiction is correct of course.
All in all, will be interesting to see how this turns out. You'd put the icing threat north of Chicago at this point... especially the core of Chicago. But I don't think it's as simple as to say that there won't be icing near the western shores of Lake Michigan. It will likely depend on how far north you are.