Snowstorm closes schools, grounds flights in U.S.

STORY: A major winter storm battered America's Northern Plains and Upper Midwest on Wednesday.

High winds and heavy snow forced hundreds of schools to close, grounded air travel, and made road travel difficult - if not impossible.

More than 50 million Americans have been placed under winter weather advisories.

The National Weather Service warned of winds of up to 60 miles an hour and expected up to 2 feet of snow to be dumped on towns and cities.

One of the hardest-hit areas in the Midwest was Minneapolis-St Paul where local officials declared emergencies.

Motorists were told to stay off the roads and some 3,500 flights were delayed or canceled across the nation.

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.

Experts say the growing frequency and intensity of such storms,

interspersed with extreme heat and dry spells, are symptoms of climate change.