Heavy snow and high winds are beginning to pound the US East Coast along a front stretching from Maine in the north to North Carolina in the south, knocking out power, icing over roadways and closing hundreds of schools.
The storm dumped snow on Florida's capital Tallahassee for the first time in 30 years on Wednesday, and is expected to last throughout the day.
It is being dubbed the "bomb cyclone", and is the product of a rapid and rare drop in barometric pressure known as bombogenesis.
States of emergencies were in effect in Florida, Georgia, North Carolina and Virginia and there were blizzard warnings from the Canadian border as far south as Virginia.
Much of the eastern United States is in the grip of a sustained cold spell that has frozen part of Niagara Falls, played havoc with public works and impeded firefighting in places where temperatures barely broke 20F ( -6C).
Areas around Boston were forecast to see about one foot (30 cm) of snow on Thursday, and the National Weather Service predicted a similar amount and wind gusts of up to 55 mph (90 kmph) in New York City.
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Schools were ordered to close in both cities.
"This could bring some very dangerous conditions," New York Mayor Bill de Blasio said late on Wednesday.
"Both rush hours will be affected," Boston Mayor Marty Walsh earlier told a news conference. "Be patient. With the amount of snow we're getting here, we could be plowing your street and a half hour later it could look like we haven't been there."
