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A winter storm grounded hundreds of flights Wednesday as forecasters warned that most of the Northeast could be buried under as much as a foot and a half of snow Friday.

A day after meteorologists predicted that a storm system was unlikely to bring snow to New York City this week, the forecast was revised to warn that the city could expect 5 to 8 inches of snow through Thursday and Friday.

"It's going to be a pretty significant storm, which will cause major travel disruption for a lot of people early in the new year," said Dave Houtz, senior meteorologist for The Weather Channel. "Any untreated roads will be a real mess."

Almost 40 million Americans were under winter storm warnings, some of them in areas that could get hit much harder than New York.

The National Weather Service issued a blizzard warning for Nassau and Suffolk counties on Long Island in New York beginning at 6 p.m. ET Thursday well into Friday afternoon, predicting inch-an-hour snow with 45-mph winds during the worst of it Thursday night. Much of the rest of the Northeast was under a winter storm warning.

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In Boston, Mayor Tom Menino — in his last official act in office — pre-emptively declared a snow emergency for Thursday and closed the city's schools Friday as weather models pointed to up to 18 inches of new snow.

"What a New Year's gift, to receive one last snowstorm as mayor," Menino said Wednesday.

Buffalo was also predicted to get a 12- to 18-inch wallop, and accumulations of 8 to 12 inches were expected in areas of Maine and Vermont farther north.

What changed the forecast so drastically was the expected convergence of three separate low pressure systems from the south and east, hauling warm, wet air straight for the frigid Northeast, said Greg Postrel, a forecaster for The Weather Channel.

"That's setting up for a big snowstorm for New England," Postrel said — "big" as in near-zero visibility and howling winds with wind chills well below zero.

The storm system stretched from Kansas City, Mo., through Chicago and Detroit, which were already under several inches of snow Wednesday, with 5 to 8 inches more on the way.

Nearly 600 flights due in or out of Chicago's O'Hare International on Wednesday had been canceled by 6:30 p.m. ET, according to FlightAware.

At Mitchell International Airport in Milwaukee, where snow was already falling with as much as 9 inches expected, a Delta Air Lines flight arriving from Minneapolis skidded off the runway Wednesday, said a spokeswoman for the airport. No one was injured, but shaken passengers waited an hour for shuttle buses to arrive to take them inside to warmth.

The Winter Classic hockey game between the Detroit Red Wings and the Toronto Maple Leafs began with temperatures in the low teens Wednesday as snow fell steadily over the ice at Michigan Stadium in Ann Arbor.

The game-time temperature was announced at 13 degrees, with a wind chill of zero. Significant amounts of snow were shoveled off the ice during early stops in play.

The expected crowd of 107,000 would be a world-record attendance for a hockey game.