UNDATED (AP) - Tens of millions of Americans endured bone-chilling temperatures, blizzard conditions, power outages and canceled holiday gatherings Friday from a winter storm that forecasters said was nearly unprecedented in its scope, exposing about 60% of the U.S. population to some sort of winter weather advisory or warning.

More than 200 million people were under an advisory or warning on Friday, the National Weather Service said. The weather service's map "depicts one of the greatest extents of winter weather warnings and advisories ever," forecasters said.

More than 3,400 flights within, into or out of the U.S. were canceled Friday, according to the tracking site FlightAware, causing more mayhem as travelers try to make it home for the holidays. Some airports, including Seattle-Tacoma International Airport, closed runways.

More than 458,000 homes and businesses were without power Friday morning.

The huge storm stretched from border to border.

Forecasters said a bomb cyclone - when atmospheric pressure drops very quickly in a strong storm - had developed near the Great Lakes, stirring up blizzard conditions, including heavy winds and snow.

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The weather service is predicting the coldest Christmas in more than two decades in Philadelphia, where school officials shifted classes online Friday. Some surrounding districts canceled classes altogether.

In South Dakota, Gov. Kristi Noem late Thursday activated the state's National Guard to haul firewood from the Black Hills Forest Service to the Rosebud Sioux Tribe as some members were stranded in their homes with dwindling fuel.

Buffalo, New York, Mayor Byron Brown urged people to stay home as meteorologists warned that the city could see 2 to 4 feet (0.6 to 1.2 meters) of snow through the weekend.

In Maine, gusts approaching 70 mph (113 kph) were reported along the coast Friday morning. Atop New Hampshire's Mount Washington, the tallest peak in the Northeast, the wind topped 130 mph (210 kph).

It was so bad in Vermont that Amtrak canceled service for the day, and nonessential state offices were closing early.

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