A slow-moving autumn storm showed no signs of letting up in Colorado and the western Plains on Thursday, blanketing areas already buried with as much as 3 feet, closing schools and businesses and delaying flights.

Roads across Colorado and Wyoming were snow-packed and icy from the first big winter storm of the season in the West, and the snow's not likely to let up anytime soon. The storm spread a blanket of white from northern Utah's Wasatch Front to western Nebraska's northern border with South Dakota.

"There's definitely some adverse driving conditions right now, and it's expected to continue throughout a good portion of the day," said Bob Wilson, a Colorado Department of Transportation spokesman. Wilson said although some cars are sliding off roads, not many accidents had been reported.

In Colorado, U.S. Highway 6 is closed to Loveland Pass, while a 35-mile span of Interstate I-25 is closed from Wellington to Cheyenne. Wilson said the closure is to prevent traffic congestion going into Wyoming, where driving conditions are worse than in northern Colorado. A 40-mile stretch of Interstate 80 is closed from Cheyenne to Laramie.

Wyoming officials said they'd had reports of about 70 crashes, most of them on I-80, before deciding to close the road.

Laramie County District 1 schools have closed and some state offices are opening later in the day. High winds were causing drifting snow and reduced visibility, and two or three inches of snow were expected to fall Thursday, said meteorologist John Griffith with the National Weather Service in Cheyenne.

The storm that began Tuesday already added enough snow to break records for total snowfall in October for Wyoming. It was the biggest October snowmaker in the Denver area since 1997, said Byron Louis, a National Weather Service hydrologist in Boulder, Colo.

At Denver International Airport, all four runways were expected to open after snow crews worked through the night, said spokesman Chuck Cannon. The airport warned of more delays, saying it would see a foot of snow by Thursday afternoon.

United Airlines, the dominant carrier at the Denver airport with about 400 flights per day, canceled about half its flights, which is standard during such weather conditions, said spokesman Charlie Hobart. He said the move keeps delays and cancelations from spilling over into the next day.

"We're doing everything we can to accommodate the travelers and we're also asking them to check online for their flights," for cancelations, he said.

Denver-based Frontier Airlines canceled about 10 flights Thursday morning, said spokeswoman Lindsey Purves, and some flights have been delayed up to three hours.

The Denver metro area will be under a winter storm warning until 6 p.m. Thursday, with snow through the afternoon, blowing snow throughout the day and temperatures in the upper 20s, the National Weather Service said. As much as 7 inches could fall around parts of Denver before the storm ends.

"The plows are out, but the roads are kind of icy and snowpacked," said Ryan Drake, traffic operations specialist for the Colorado Department of Transportation. "Be patient and take your time."

Many schools in metro Denver remained closed Thursday, but the University of Colorado in Boulder and Colorado State University in Fort Collins, where 17.5 inches fell, decided to reopen, a day after sending students home early. At least three high school football playoff games set for Thursday in Nebraska were postponed.

Whiteout conditions were predicted Thursday for the plains areas of eastern Colorado and Wyoming and western Nebraska.

Winds were a concern farther west, too.

Winds gusting through Southern California forced a commuter train line to shut down and knocked a tree onto a car, but no serious injuries have been reported.

The National Weather Service warned of the possibility of further gusts up to 50 mph through Thursday morning in Los Angeles and Ventura counties. Fire danger warnings were up in some areas.