A deadly winter storm blanketed a huge swath of the United States Friday, grounding flights, turning highways into ice rinks and knocking out power to tens of thousands preparing for the Christmas holiday.

At least eight people in five states were killed by the dangerous road conditions since the storm formed near the Rocky Mountains and moved slowly eastward.

The powerful system dumped as much as two feet of snow (60 centimeters) in some areas and knocked down trees and power lines with winds gusting as high as 60 miles (100 kilometers) per hour.

Emergency shelters were opened to help those who lost power stay warm as utility crews struggled to reach downed lines on icy and snow-covered roads. Schools and government offices were closed, as were scores of businesses.

After pounding Wisconsin, Iowa, Nebraska, Illinois and Missouri on Thursday, the storm was centered over the Great Lakes and Appalachians on Friday morning and was expected to reach New England on Saturday.

"Strong winds will cause blowing and drifting, causing near blizzard conditions and very dangerous driving conditions," the National Weather service warned.

"Only travel in an emergency."

Smaller systems also snarled travel in California, Nevada and Oregon while high winds grounded and delayed flights in New York.

Thankfully, on Friday, the busiest travel day of the year, skies were clear in Chicago -- where an estimated 266,000 people will pass through the major aviation hub.

More than 600 flights were canceled Thursday at Chicago's O'Hare and Midway airports as the storm dumped freezing rain and a dusting of snow on runways and whipped up dangerously strong winds.

Hundreds more were canceled at smaller airports like Detroit, Minneapolis, and Kansas City, according to tracking service FlightAware.

Nearly 500 flights had been canceled by early afternoon Friday, primarily due to high winds in New York and San Francisco.

"This storm is not as large as some winter storms we've seen in the past that can result in thousands of cancelations in a day, but the impact is significant due to the number of holiday travelers," FlightAware chief Daniel Baker said.

More than 30,000 people remained without power in Wisconsin, where the governor declared a state of emergency and called up the National Guard to help rescue people stranded on snow-covered roads.

Two people were killed when their car slid into the path of a semi-trailer on a highway in Wisconsin's rural Rock County on Thursday, Channel 3000 news reported.

And an ambulance transporting a woman in labor got stuck on a Wisconsin highway at about 12:30 am Friday, the state's emergency management center said. A second ambulance sent to help also got stuck, so a snowplow was sent to drive in front of a third ambulance and get her safely to hospital.

In Iowa, two more people were killed and seven injured in a 25-vehicle pileup after conditions got so bad on a major highway Thursday that people couldn't see the cars and big trucks that had slowed down or stopped ahead of them, the State Patrol there said.

Dozens of people were also trapped in their cars, many for hours. One pileup was so bad emergency crews brought food and water to the stranded motorists while they waited for snow blowers and tow trucks to arrive.

Further west in Utah, one woman died of exposure after her car got stuck on an isolated road on Tuesday and she tried to walk out for help, KSL news reported. A man who was with her in the car was able to walk farther and reach shelter, but by the time rescue crews on snowmobiles found the woman she was dead.

And in Nebraska, two people were killed in separate crashes, KETV news reported. One man died after his car was struck by a big rig when blizzard conditions smothered a Kansas highway, the Dodge City Daily Globe reported.