Areas of fog have developed Tuesday evening thanks to both temperatures and dew point temperatures in the low to mid 30s. An incoming low pressure system will help keep winds at the surface from the south, holding temperatures in the 30s through a little after Midnight. This warmer air is leading to a little snow melt, which has caused some dense fog to develop.
Visibility in some locations already has fallen under a mile, and down to half a mile over northwest Illinois. Once the drier air comes in overnight, visibility should improve. However, temperatures falling below freezing and residual moisture/melting snow will lead to some black ice and slick conditions on the roads during the morning. Be careful when out for the morning commute.