CHICAGO -- A winter storm moving across the Great Lakes that's forecast to drop about a foot of snow in some areas created treacherous driving conditions Friday, closed schools and forced the cancellation of hundreds of flights. The National Weather Service issued winter-weather warnings and advisories across the upper Midwest.
The snow that began falling late Thursday afternoon was expected to continue through Friday as the storm moves east.
Early Friday, the National Weather Service received reports of 4 inches of snow in northern Illinois, including near Chicago's Midway Airport.
Slide-offs and crashes were reported on the roads early Friday in the Chicago and Detroit areas.
About 550 flights were canceled at Chicago's O'Hare International Airport and about 230 flights were canceled at Midway, the Chicago Department of Aviation reported Friday morning. More than 100 flights were canceled at Detroit Metropolitan Airport in Romulus, Michigan, by early Friday.
American, United, Delta and Southwest airlines warned travelers to expect more flight cancellations to and from Minnesota, Michigan, Wisconsin, Illinois, Indiana and Ohio.
Meteorologist Heather Orow in Grand Rapids, Michigan, said Friday morning the storm is "generally going to be an issue for travel." People should stay off the roads if possible, but if they drive they should expect delays and hazardous conditions.
Some of the highest totals are forecast in northern Illinois, including the Chicago area, as well as northern Indiana and southern Michigan.
The expectation of up to 12 inches around Chicago prompted officials to close the city's public schools to about 390,000 students Friday. Classes were also canceled in the city's suburbs.
Chicago Department of Streets and Sanitation Commissioner John Tully said 300 salt-spreading plows hit the streets late Thursday and would continue their work through Friday.
Weather service meteorologist Charles Mott said the morning rush in Chicago "is gonna be trouble."
Court closings were reported in Chicago and Detroit, and driver's license offices were shut in the Chicago area.
CBS News correspondent Dean Reynolds reports Chicago Mayor Rahm Emanuel said some people in the city should be prepared to be snowed in by this storm.
Schools in Detroit shut for the day along with Wayne State University in Detroit and other schools across Michigan in the path of the storm. The Detroit area was in line for 6 to 9 inches.
Students in the Milwaukee Public School District -- Wisconsin's largest school district -- have Friday off, as well as those in the Baraboo, Kenosha, Janesville, Whitewater and hundreds of other schools.
AAA Michigan recommended that drivers stay off the road if possible.