An “Alberta clipper” low pressure system, so named because it originated over the Great Plains of Canada’s Alberta Province, has Illinois in its sights. Rain and snow associated with the weather system spreads across Illinois Tuesday, but the bulk of precipitation falls from Chicago southward. The center of low pressure is expected to cross central Illinois Tuesday evening.
Wind circulating in counterclockwise fashion around the low suggests Chicago’s winds will be easterly — off the relatively mild waters of Lake Michigan. Chicago’s temperatures, moderated by the lake and the warmth of the city itself, should be high enough that rain is likely in the city, but, in an unusual reversal of normal precipitation patterns, wet snow (maybe an inch) is the best bet across southern portions of the metropolitan area that are less affected by lake and city warmth.