CHICAGO -- The first significant wintry storm of the season blanketed parts of the Midwest with a foot of snow and more was on the way Saturday, creating hazardous conditions as some travellers prepared to depart for the Thanksgiving holiday.

While winter has not officially begun, the shovels and snow blowers were out from South Dakota through southern Minnesota, Iowa and southern Wisconsin to northern Illinois and Indiana. The National Weather Service said the snow would continue in Illinois and Indiana on Saturday and move into Michigan. The front will head northeast to Canada late on Saturday and into Sunday.

"In those areas it is going to be a very tough travel day," said National Weather Service meteorologist Bruce Terry in College Park, Maryland.

Chicago's O'Hare International Airport, one of the busiest in the world, had recorded 10 centimetres of snow by early Saturday with more falling. Roads were slushy as freezing rain turned to snow in the Chicago area, which was forecast to get between 15 and 25 cm. Marengo, about 100 km northwest of Chicago, recorded 30 cm of snow, according to weather service meteorologist Bruce Sullivan.

Temperatures plunged behind the front with Sioux Falls, South Dakota recording -11 degrees Celsius early on Saturday and the town of Estherville in northern Iowa even colder, the weather service said.

The Illinois Tollway, which maintains interstate tollways in 11 counties of the state, said it had 185 snowplows ready to go and 84,000 tons of salt stockpiled for the winter.

Southeastern South Dakota got up to 45 cm of snow on Friday, Terry said. Tractor-trailers pulled off slow-moving interstates in South Dakota on Friday to park for the day, said Bret Brown, a cashier at Roadway Express truck stop in Sioux Falls.

In Iowa, Des Moines had recorded 15 cm by late Friday and amounts of a foot or more were common in northern Iowa. The highest snowfall in the state was 43 cm recorded in the far northwestern corner, the weather service said.

The Iowa Department of Transportation warned people in Des Moines and several other cities not to travel because of the hazardous conditions.

Iowa State Patrol Sgt. Nathan Ludwig said troopers in northern and western Iowa were seeing many cars in ditches, especially near Mason City and Council Bluffs, the Des Moines Register reported.

On Interstate 80, a semi-truck struck a snowplow, Ludwig said. No one was injured.

It is not unusual for snow to fall in the Midwest before Thanksgiving, Terry said.

"Some of those amounts are pretty impressive for this time of year," he added.

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Associated Press writers James Nord in Pierre, South Dakota, Carla K. Johnson in Chicago and Blake Nicholson in Bismarck, North Dakota, contributed to this report.