Frost Advisories Covering Wisconsin, Michigan, Indiana for tonight
Portions of Indiana, Michigan, Wisconsin and a sliver of Ohio are covered in a Frost Advisory, meaning patches of frost may/likely will be present in the areas under the advisory. The reason for this is the clear skies tonight. In the winter, clouds act as a blanket, covering up cities and thus holding down the sun's heat to make for a warmer night. However, with a clear night, that heat built up from the day bounces off into space and makes for a much colder night. Here's a real time image of infrared satellite imagery for the area in question.