CHICAGO (AP) -- A winter storm moving across the Great Lakes that's forecast to drop about a foot (1/3 meter) 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 (10 centimeters) of snow in northern Illinois.
Slide-offs and crashes were reported on the roads early Friday in the Chicago and Detroit areas. American, United 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 (30 centimeters) 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."
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 (15 to 23 centimeters)
AAA Michigan recommended that drivers stay off the road if possible.