The storm put the metropolitan Chicago area's more than 9 million people under a blizzard warning Sunday, and another 65 million people are under a winter storm warning across 18 states.
More than 2,000 U.S. flights were canceled Sunday, many of them in and out of Chicago, according to Flightaware.com, an online flight-tracking service. Already for Monday, more than 1,000 flights have been scratched.
Chicago could get as much as 15 inches of snow from the storm system, CNN meteorologist Michael Guy said.
"The streets will stay plowed and passable so people will be safe and secure," Chicago Mayor Rahm Emanuel promised at a news conference Sunday. More than 350 pieces of equipment, including plows and salt spreaders, fanned out across the city, officials said.
Still, Chicago Public Schools canceled classes for Monday, according to its Twitter page.
Up to a foot of snow was forecast in parts of Iowa, Illinois, Michigan and Wisconsin.
The storm will move east and affect major metropolitan areas such as New York City and Boston on Monday, Guy said.
"New York City could see a wintry mix of precipitation just in time for the morning commute with a possibility of sleet and ice," and Boston will "get another round of wintery weather with new snow accumulations from 10 to 12 inches," he said.
New York City Mayor Bill de Blasio issued a severe weather warning and hazardous travel advisory for Monday.
"The biggest threat in this case would be ice. We would expect a lot of icing on our roads and sidewalks -- up to a quarter inch of ice in some places, and a lot of that would happen in the early morning hours, so we'd want people to be very, very cautious tomorrow," he told reporters.
Slippery driving conditions could start as early as Sunday night.
"For anyone leaving Super Bowl parties, you're going to have to be ready to be careful at that point," said de Blasio. "I want to urge people to think ahead."