By Tuesday afternoon, the airlines at O'Hare reported delays between 15 minutes and 45 minutes on both inbound and outbound flights. The city's aviation department is urging travelers to check their airlines' website for flight status.
Forecasters expect the storm to leave behind three to six inches of snow.
Roads throughout central and northern Illinois grew slick and some schools and government offices closed.
The National Weather Service says some heavy snow piled up early Tuesday in west-central Illinois. Both Galesburg and Macomb had more than 5 inches on the ground.