By Nicole Chavez, CNN
(CNN) -- More than 1,000 US flights have been canceled and drivers urged to stay off the roads as a winter storm slammed the Midwest on Friday with at least 1 inch of snow per hour.
Heavy snow and strong winds already are pounding Chicago and Detroit, where major public school systems opted not to open Friday, CNN meteorologist Haley Brink said.
"Make no mistake about it: This is a heavy snow, heavier than we have seen in a number of winters," Chicago Mayor Rahm Emanuel said Thursday at a news conference. "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."
More than 25 million Americans are under winter weather alerts as an arctic front pushes from Montana through the Midwest, CNN meteorologists said.
Chicago could get around a foot of snow -- 8 to 14 inches -- by late Friday, plus 4 more inches through the weekend, forecasters said.
The bulk of flight cancellations affect Chicago's O'Hare and Midway airports and Detroit Metropolitan Wayne County Airport, according to the tracking site FlightAware.com.
Four major US airlines -- Delta, United, Frontier, and Southwest Airlines -- have issued weather waivers for passengers, allowing travelers to change their reservations without incurring a fee.
A foot of snow is still a big deal for Chicagoans. Since 1884, the city has had only nine days when 12 or more inches of snow fell.
Authorities across Illinois prepared some 300 pieces of snow-fighting equipment and thousands of tons of salt for nearly 16,000 miles of highways and state roads.
Road conditions began deteriorating Thursday night as the system neared Chicago, the Illinois Department of Transportation tweeted.
"Our crews are out in full force clearing snow but please consider postponing any unnecessary travel!"
Chicagoans headed to grocery and hardware stores on Thursday to stock up on food and essentials before the storm.
"We have our shovels, we have our potassium for the sidewalks," Pam Smith told CNN affiliate WGN. "We have our food, our wine and our fireplace."
In 2011, residents weathered 20 inches of snow over two days. 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.
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