The first part of a narrow band of heavy snow arrived Thursday night at the United States' third-largest city by population. Schools will be closed as millions see accumulations of 8 to 14 inches by late Friday.
"Travel will be very difficult to impossible at times, including during the morning commute," the National Weather Service said in a winter storm warning.
The snow will linger into Saturday, and a lighter round is likely Sunday.
Detroit will get a snow dump of its own: The forecast is for 6 to 9 inches. It, too, has closed its schools.
Chicago officials say they are ready. Some 300 pieces of snow-fighting equipment and 220,000 tons of salt were in position, they said at a Thursday afternoon news conference.
"Make no mistake about it: This is a heavy snow, heavier than we have seen in a number of winters," said Mayor Rahm Emanuel. "Now, Chicago is used to this. On the other hand, over the last winters we haven't had a snow at this level. And it is not just one night, it is going to go through the weekend."
The Illinois Department of Transportation tweeted road conditions were deteriorating Thursday night across the top of the state as the system neared Chicago. "Our crews are out in full force clearing snow but please consider postponing any unnecessary travel!"
Chicagoans are a hardy bunch, but a foot of snow is still a big deal. Since 1884, Chicago has had only nine days with 12-plus inches of snow.
Residents weathered 20 inches over two days in February 2011. A storm in February 2015, logging in at 16.2 inches, was the last time Chicago had more than 10 inches of snow in a single storm.
Chicago and Detroit flight cancellations were on the way up, according to tracking website flightaware.com. More than 900 Friday flights involving the two cities were canceled.
To the east, snow emergencies have been declared in many metro Detroit communities, according to CNN affiliate WDIV
. Michigan State Police said "snowmageddon" was approaching.
"Please remember with all of the school closings that there will be lots of kids playing in the snow and they may not be watching! Drive carefully!" the agency tweeted.