Hurricane Sandy headed 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 warned millions in coastal areas to get out of the way of the behemoth storm.
Sandy was expected to affect up to 60 million people when it meets two other powerful winter storms. Experts said it didn't matter how strong the storm was when it hit land: The rare hybrid that follows will cause havoc over 800 miles (1,300 kilometers) 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. "This is a very large area."
New Jersey governor Chris Christie declared a state of emergency Saturday as hundreds of coastal residents started moving inland and the state was set to close its casinos. 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 early Saturday but was soon back up to Category 1 hurricane strength, packing 75 mph (120 kph) winds about 335 miles (539 kilometers) southeast of Charleston, South Carolina, as of 5pm EDT (2100 GMT).
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 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 in a bit of understatement 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 on Sunday in the swing state of Virginia and switched his schedule for the day to Ohio.
First lady Michelle Obama canceled an appearance in New Hampshire for Tuesday, and President Barack Obama moved a planned Monday departure for Florida to Sunday night to beat the storm.
In Ship Bottom, just north of Atlantic City, Alice and Giovanni Stockton-Rossini spent Saturday packing clothing in the back yard of their home, a few hundred yards (meters) from the ocean on Long Beach Island. Their neighborhood was under a voluntary evacuation order, but they didn't need to be forced.
"It's really frightening," Alice Stockton-Rossi 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, and the one time you need to take it seriously, you won't. This one might be the one."
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 800 miles (1,300 kilometers) and up to 2 feet (60 centimeters) 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 one foot (30 centimeters) higher would have paralyzed lower Manhattan.
As the storm swirled away toward the US East Coast, officials in the Caribbean reported that the hurricane cost at least 58 lives in addition to destroying or badly damaging thousands of homes.
While Jamaica, Cuba and the Bahamas took direct hits from the storm, the majority of deaths and most extensive damage was in impoverished Haiti. The country's ramshackle housing and denuded hillsides are especially vulnerable to flooding when rains come.
Up and down the US East Coast and far inland, officials urged residents and businesses to prepare in big ways and little.
The Virginia National Guard was authorized to call up to 500 troops to active duty for debris removal and road-clearing, while homeowners stacked sandbags at their front doors in coastal towns.
Utility officials warned rains could saturate the ground, causing trees to topple into power lines, and told residents to prepare for several days at home without power. "We're facing a very real possibility of widespread, prolonged power outages," said Ruth Miller, spokeswoman for the Pennsylvania Emergency Management Agency.
Warren Ellis, who was on an annual fishing pilgrimage on North Carolina's Outer Banks, didn't act fast enough to get home.
Ellis' 73-year-old father, Steven, managed to get off uninhabited Portsmouth Island near Cape Hatteras by ferry Friday. But the son and his 10-foot (3-meter) camper got stranded when high winds and surf forced state officials to suspend service Saturday.
"We might not get off here until Tuesday or Wednesday, which doesn't hurt my feelings that much," said Ellis, 44, of Ammissville, Virginia "Because the fishing's going to be really good after this storm."
Last year, Hurricane Irene poked a new inlet through the island, cutting the only road off Hatteras Island for about 4,000.
In New Jersey, Christie's emergency declaration will force the shutdown of Atlantic City's 12 casinos for only the fourth time in the 34-year history of legalized gambling here. The approach of Hurricane Irene shut down the casinos for three days last August.
Atlantic City officials said they would begin evacuating the gambling hub's 30,000 residents at noon Sunday, busing them to mainland shelters and schools.
Tom Foley, Atlantic City's emergency management director, recalled the March 1962 storm when the ocean and the bay met in the center of the city.
"This is predicted to get that bad," he said.