At least 17 people have died as dangerously cold temperatures grip wide swaths of the US from Texas to New England.
A brutal winter storm hit the coastal southeast with a rare blast of snow and ice on Wednesday, hitting parts of Florida, Georgia and South Carolina with their heaviest snowfall in nearly three decades.
Forecasters are warning the same system could soon strengthen into a "bomb cyclone" as it rolls up the East Coast, bringing hurricane-force winds, coastal flooding and up to a foot of snow.
A winter storm warning extends from the Gulf Coast of Florida's "Big Bend" region all the way up the Atlantic coast, with forecasters warning that hurricane-force winds blowing offshore on Thursday could generate seven metre seas.
Schools in the southeast called off classes just months after being shut down because of hurricane threats, and police urged drivers to stay off the roads in a region little accustomed to the kind of winter woes common to the northeast.
Airports shut down in Savannah, Charleston and elsewhere as airlines cancelled 500 flights on Wednesday, and at least 1,700 more were cancelled on Thursday.
The weather service says the winter storm will probably intensify into a "bomb cyclone" that could dump more than 20 centimetres of snow on the Boston area on Thursday and at least 15 centimetres in the New York City region.
A bombogensis happens when it drops 24 millibars of pressure in 24 hours and the National Weather Service's Weather Prediction Centre says it appears likely the storm will intensify at twice that rate.