150 flights canceled at O'Hare by noon Thursday.
Southwest Airlines didn't take a wait-and-see approach to Thursday's expected snowfall, opting to cancel all flights after 4:30 p.m. out of Midway Airport.
Southwest officials warned travels to expect delays and to check southwest.com and airtran.com for updated information.
Passengers on other airlines were also urged to check departure times before leaving for Midway.
Thursday's rain and foggy conditions had already numerous delays and more than 200 cancellations at O'Hare International Airport by 2 p.m., and 150 at Midway.
By 3:30 p.m., delays at O'Hare were anywhere from 30 to 90 minutes for both inbound and outbound flights.
United Airlines waived change fees for travelers scheduled to fly out of O'Hare on Thursday who want to change their plans.