Story highlights Some 25 million Americans are under winter weather alerts Friday

Chicago could see 14 inches of snow Friday, then 4 more inches this weekend

(CNN) More than 1,500 US flights have been canceled Friday as a winter storm slammed the Midwest with at least 1 inch of snow per hour.

Heavy snow and strong winds already pounded Chicago and Detroit, where public school systems opted not to open Friday, CNN meteorologist Haley Brink said.

"We are getting more snow today, tomorrow and Sunday than we have normally gotten in the last two winters in any one period of time," Chicago Mayor Rahm Emanuel said in a Friday morning news conference, adding that the city's top priority is the safety of residents.

"Whether that's young kids, homeless, elderly, sick, commuters -- in every capacity we are responding and ensuring that people are able to handle this weather."

About 23 million Americans are under winter weather alerts as an arctic front pushes from Montana through the Midwest, CNN meteorologists said.