A mass of cold, cold air from the polar region is pushing south into the central United States. Below is a map from MesoWest showing the Real-Time Mesoscale Analysis of the temperatures over the United States at about 21:30 GMT (2:30 PM Mountain Time). You can see the cold air extending into Illinois. Also, notice the temperatures over the Great Lakes, which haven't cooled off as fast. This is because water has a high specific heat.
The National Weather Service stations in the area are reporting temperatures as cold as -20 F (at 2:30 PM Mountain Time). That is bitter cold and very dangerous. I suppose I won't complain about my comfy 32 degrees.