A major winter storm battered the Northern Plains and Upper Midwest with high winds and heavy snow on Wednesday, forcing hundreds of schools to close, grounding air travel and making road travel difficult - if not impossible - across the US regions.

More than 50 million Americans were under winter weather advisories on Wednesday morning as the storm moved across a wide swath of the western and northern United States and into the East. Up to 2 feet (60 cm) of snow and winds of up to 60 miles (97 km) an hour were expected in some spots during the day and into Thursday, the National Weather Service said.

The storm also pounded California and brought a mix of snow and sleet to the East, including New England, where forecasters warned motorists to beware slick roads.

Snow-covered roads also will make travel treacherous in the Upper Midwest, and ice-covered power lines and falling trees could cause power outages late on Wednesday and into Thursday, said Frank Pereira, a forecaster with the weather service's Weather Prediction Center in College Park, Maryland.

"Travel will be near-impossible,” he said.

Experts say the growing frequency and intensity of such storms, interspersed with extreme heat and dry spells, are symptoms of climate change. While the American East has experienced a relatively mild winter, the Northern Plains has experienced an extreme winter in terms of snowfall and temperatures, according to the weather service.

The storm hit California on Tuesday and was expected to continue through the end of the week.

Winter storm warnings remained in effect on Wednesday in the state capital, Sacramento, and some areas in Northern California and the mountains, with heavy snow and wind gusts of up to 40 mph (64 kph). In Central California, the weather iced over roadways, the weather service reported.

More than 100,000 homes and businesses were without power, largely in Central California, according to the tracking site Poweroutage.us.

About 4 inches (10 cm) of snow were already on the ground in South Dakota, northern Nebraska and parts of Minnesota, but another foot was expected to fall later in the day and into Thursday.

Among the hardest-hit cities was Minneapolis, where some 20 inches (50 cm) of snow and 45-mph (72-kph) winds were expected to create whiteouts. If the forecast holds, it would rank as one of the heaviest snowstorms of all time for the Midwestern city.

Public school buildings closed, and Minneapolis' school system said it would hold classes remotely for more than 29,000 pupils for the rest of the week. Dozens of school districts canceled classes in the Dakotas, Colorado and Wyoming.

The storm wreaked havoc on morning air travel as hundreds of flights were delayed or canceled across the nation. More than 700 flights in and out of Minneapolis, Denver and Chicago were delayed or canceled over the past day, according to Flightware.com.

It also produced a band of freezing rain stretching from central Iowa through Chicago and into southern Michigan, coating roads, trees and power lines with up to a 1/4 inch (0.6 cm) of ice, the weather service's Pereira said.

"Heads up!” the Iowa Department of Transportation said in a Tweet early on Wednesday morning. “Watch for slippery travel in this area over the next few hours.”