HURRICANE Sandy is heading north from the Caribbean - where it left nearly 60 dead - to threaten the eastern US with sheets of rain, high winds and heavy snow as officials warn millions in coastal areas to get out of its way.
Sandy is expected to affect up to 60 million people when it meets two other powerful winter storms.
Experts say it doesn't matter how strong the storm is when it hits land: the rare hybrid storm that follows will cause havoc over 1,300km from the east coast to the Great Lakes.
"This is not a coastal threat alone," said Craig Fugate, director of the Federal Emergency Management Agency (FEMA). "This is a very large area."
New Jersey was set to close its casinos this weekend. New York's governor was considering shutting down the subways to avoid flooding and half a dozen states warned residents to prepare for several days of lost power.
Sandy weakened briefly to a tropical storm before returning to Category 1 hurricane strength, packing 120kph winds about 539km southeast of Charleston, South Carolina, as of 5pm local time (0800 AEDT).
Experts said the storm was most likely to hit the southern New Jersey coastline by late Monday or early Tuesday.
Governors from North Carolina, where heavy rain was expected on Sunday, to Connecticut declared states of emergency. Delaware ordered mandatory evacuations for coastal communities by 8pm on Saturday.
New Jersey Governor Chris Christie broke off campaigning for Republican presidential nominee Mitt Romney in North Carolina on Friday to return home.
"I can be as cynical as anyone," the pugnacious chief executive said on Saturday. "But when the storm comes, if it's as bad as they're predicting, you're going to wish you weren't as cynical as you otherwise might have been."
The storm forced the presidential campaign to juggle schedules. Romney scrapped plans to campaign today in the swing state of Virginia and switched his schedule for the day to Ohio. First Lady Michelle Obama cancelled an appearance in New Hampshire for Tuesday, and President Barack Obama moved a planned Monday departure for Florida to later tonight to beat the storm.
In Ship Bottom, just north of Atlantic City, residents who were packing up just a few hundred metres from the ocean on Long Beach Island expressed fears that some might not take the warnings seriously.
"It's really frightening," one said. "But you know how many times they tell you, 'This is it, it's really coming and it's really the big one' and then it turns out not to be? I'm afraid people will tune it out because of all the false alarms before."
What makes the storm so dangerous and unusual is that it is coming at the tail end of hurricane season and the beginning of winter storm season, "so it's kind of taking something from both", said Jeff Masters, director of the private service Weather Underground.
Masters said the storm could be bigger than the worst east coast storm on record - the 1938 New England hurricane known as the Long Island Express, which killed nearly 800 people.
Experts said to expect high winds over 1,300km and up to 60cm of snow as far inland as West Virginia.
And the storm was so big, and the convergence of the three storms so rare, that "we just can't pinpoint who is going to get the worst of it", said Rick Knabb, director of the National Hurricane Center in Miami.
Officials are particularly worried about the possibility of subway flooding in New York City, said Uccellini.
New York Governor Andrew Cuomo told the Metropolitan Transportation Authority to prepare to shut the city's subways, buses and suburban trains by Sunday, but delayed making a final decision.
The city shut the subways down before last year's Hurricane Irene, and a Columbia University study predicted that an Irene surge just 30cm higher would have paralysed lower Manhattan.