Dense Fog along has caused visibility to drop to near zero in many portions of the Chicago area south of Interstate-80 early this Sunday morning (see visibility observation map below). A Dense Fog Advisory was issued for that area. The fog has formed generally over counties hit by accumulating snow Friday – fog condensing/forming over colder snow surface. Conditions will improve later this morning – taking longer in areas that have the existing deeper snow cover.

If traveling over the affected area early this morning, prepare for a slow-go and anticipate a sudden drop to or extended period of very poor visibility.