Dreaming of a white Christmas? Be careful what you wish for.

A "significant storm system" is expected to move through the northern Plains on Christmas Day, bringing whiteout conditions and driving winds that could make travel "very difficult to impossible," the National Weather Service said Wednesday.

Heavy snow is possible beginning Friday over most of Wyoming before moving east to the Dakotas and northern Minnesota over the weekend, the National Weather Service said.

Storm system moving in Friday could bring blizzard conditions to northern Plains on Christmas Day. pic.twitter.com/js9DUSTIhS — Alex Johnson (@MAlexJohnson) December 21, 2016

"It's going to be a pretty significant storm," said Ari Sarsalari, a meteorologist for The Weather Channel.

"A straight-up blizzard is likely to occur on Christmas Day" in the Dakotas, Sarsalari said, but "the more important part of this storm is going to be the wind and how much it's going to blow the snow around and make it hard to see."

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The National Weather Service — which issued a Christmas Day blizzard watch for western South Dakota on Wednesday, four days ahead of time — said sustained winds as high as 60 mph could cause whiteout conditions and pile up snowdrifts measured in feet.

Winter storm watches for the weekend, which were already in effect for most of Wyoming and eastern Idaho, were extended south into Nevada late Wednesday.

While it's too early to say just how much of the country will get dumped on, the weather service said a strong low-pressure system sweeping in from the Pacific Northwest is expected to intensify, with the potential to become a "widespread and significant winter storm" across the entire Northern Plains by Sunday.

A man shovels snow from his driveway last week in Sioux Falls, South Dakota. Parts of the state are already under a blizzard watch for Christmas Day. AP

South of the snow area, ice storms could slick southern Minnesota and central Michigan south into Iowa, with severe thunderstorms stretching all the way south through Texas, forecasters said. A few Christmas tornadoes could even be possible in southern areas.

Farther east, a messy mix of snow, rain and ice is likely to develop beginning Friday from northern New England on south. Forecasters said significant icing is possible over a large area from northern Iowa through the upper Midwest, including Minneapolis and Detroit, into Sunday.

The ice could arrive in upstate New York and northern New England on Monday, where "you may have some travel concerns," said Domenica Davis, a meteorologist for The Weather Channel.

AAA said that with Christmas falling on a Sunday, the holiday travel period is a day shorter than usual. Even so, it expects 103 million Americans, the most on record, to travel for the holidays this year.

Nine out of 10 of them will do so by road, said AAA — which predicted that it will have to rescue almost 1 million of them.