The tropical cyclone formed south of Jamaica a week and a half ago and began to cruise north, paralleling the U.S. East Coast, dutifully following the track laid out by computer models. Hurricane Sandy was a thousand miles across, as measured by the extent of tropical-storm-force winds. The low pressure at its core was comparable to the infamous Long Island Express hurricane of 1938.

Normally such a storm would get caught in the jet stream and drift east, eventually blowing apart harmlessly in open ocean. This time, a ridge of high pressure blocked its path in the North Atlantic. The hurricane had nowhere to go but west, and late Monday it drove like a fist into the Eastern Seaboard.

The location was terrible, the timing equally so. Sixty million people live and work in the swath of America that Sandy targeted. It came ashore in New Jersey in the evening, close to a high tide enhanced by the full moon. Just to the north, New York City’s vulnerable harbor sat in the northeast quadrant of the hurricane, where the counterclockwise rotation of the storm shoved the ocean right up the Hudson and East rivers. Lower Manhattan was in a full-scale crisis Monday night, coping with flooded streets and power outages as the sea invaded densely populated neighborhoods.

The awesome scale of Sandy meant that an extraordinary number of people received something close to a direct hit. At least 16 deaths in New Jersey, New York, Maryland, Pennsylvania and Connecticut were blamed on Sandy, the Associated Press reported. Some of the victims were killed by falling trees.

As of early Tuesday, more than of 6 million residences and businesses had reportedly lost power. The storm put the presidential contest essentially on hold with Election Day just eight days away.

The storm came in slowly, delivering cold, deceptively gentle rain that gradually turned into something different, a pelting mess, with winds ratcheting up over the course of a long day that promised to become an even longer night.

“Part of the challenge for us has been to strike an appropriate balance between nonchalance and utter horror and fear,” Philadelphia Mayor Michael Nutter said at a briefing in West Philadelphia, where a high school was serving as an American Red Cross shelter. “All we can stress is, it’s going to get progressively worse.”

This was really not just a single storm, but two. From out of the northwest on Monday came an arctic cold front, and it collided with the hurricane. The two systems spent the latter half of Monday merging, with Sandy gradually losing its eye wall and cyclonic shape and turning into a bizarre hurricane/nor’easter hybrid.

The storm officially became “post-tropical,” but that did not mean it was weaker on the whole. The Weather Channel dubbed it “Superstorm Sandy.”

“It can be even stronger and not be a hurricane,” said Brian McNoldy, a storm expert at the University of Miami who blogs for The Washington Post’s Capital Weather Gang.

Sandy generated wind advisories as far west as Wisconsin. By Monday afternoon, more than 10,000 flights had been canceled. Atlantic City’s famed boardwalk, the priciest property in “Monopoly,” was obliterated as the ocean surged across the evacuated barrier island.

Garrett Tate, 56, who sells pinball machines to the arcades on the boardwalk, discovered that his New Jersey Transit bus home to Atlantic City was canceled. So he went to a Red Cross shelter in West Philadelphia, one of 140 people to do so. “It’s a very humble feeling here,” Tate said. “They want to help you here. With no agenda.”

The lights were on in some corner stores in the neighborhood of rowhouses, many of them boarded up, as customers ran in for last-minute provisions and lottery tickets. Some residents seemed resigned to losing power.

“If it happens, there’s nothing we can do about it,” said Angela Holt, 47, on her day off from her nursing-home job in the suburb of Bryn Mawr, Pa.

Wind gusts of up to 80 mph were expected in Washington and Baltimore as night approached. West Virginia braced for blizzard conditions and multiple feet of snow in the higher elevations of the Allegheny Mountains.

“This storm has something for everyone,” said Chris Vaccaro, spokesman for the National Weather Service.

New York City officials closed two tunnels leading into the city, multiple bridges and the subway system. Broadway went dark. So did Wall Street. So did the United Nations.

After shutting off power in parts of the city to preserve its equipment Con Edison reported later Monday night that flooding in company substations had zapped power in a large section of Manhattan from East 39th Street to the lower tip of Manhattan. The company said approximately 250,000 customers in Manhattan were without power. The company also cut power to parts of southern Brooklyn, including Coney Island.

Like the opening of a big Broadway production, Sandy’s arrival in New York City came with a lot of buildup and buzz. New Yorkers began the day by throwing on jogging sweats, hopping on bicycles and heading straight to Battery Park City to watch the rising surf that threatened to engulf Lower Manhattan.

“New Yorkers are always going to gawk,” said Eric Bates, an editor at Rolling Stone magazine who showed up with his family.

In Park Slope, a neighborhood in northern Brooklyn lined with oak trees and three-story brick homes, just a few souls darted through the rain to stock up on fresh provisions Monday afternoon. Hannah Rad and Vida Ventura, two DJs, made a final run for toilet paper, Starbucks drinks and adult beverages. “The tequila and champagne stock was lessened,” Rad said. Referring to her new supply of cookies, chips and Twizzlers, she said, “And our snack game is super-tight now.”

The cavalier attitude toward the storm contrasted with the sense of urgency conveyed by the region’s political leaders, many of them possible presidential candidates down the road.

Republican Gov. Chris Christie of New Jersey warned people not to jury-rig last-minute energy sources: “If it looks stupid, it is stupid. You’re going to wind up hurting yourself.” Newark Mayor Cory Booker worried that residents were not taking the storm seriously enough. Andrew Cuomo, the Democratic governor of New York, issued a warning to residents along the waterfront: “Do not underestimate this storm. The forecasts for the surge are really extraordinary. They are talking about surges that we have not seen before.

But many New Yorkers, including those along the flood zones that had been ordered evacuated, shrugged off the warnings. Alissa Quinones, a 35-year-old nursery school teacher who lives in a high-rise at the center of the flood zone at Battery Park City, just on the edge of the Hudson River, said her husband and two children evacuated during Hurricane Irene last year but decided to stick it out this time around.

“We are on the 33rd floor, so we feel like we are not in danger of flooding up so high,” she said. Then she added, “But you kind of wonder if you made the right decision.”

Phil Everson and Kathleen Corrigon, a couple who live on the ground floor of a renovated pre-war brick building on Manhattan’s Lower East Side, also ignored the evacuation appeal in their neighborhood. “We’ve been through this before. We’ve been through 9/11,” Corrigon said. She added that her greatest worry is rats. The rodents infested the area after Irene hit last year.

Across the city, shopkeepers and business owners in the ­lowest-lying areas had stacked sandbags in front of their buildings and installed water pumps to battle the surge. Some residents were unhappy with Mayor Michael R. Bloomberg’s decision to evacuate low-lying areas.

Jack Lupin, a retired New York City police boat captain who lives on Coney Island, declined to evacuate — he has two small dogs that, he says, “fly away” in a stiff breeze. But he was stunned to discover that the city had shut down the elevators in his high-rise building and sent the security guards home. He and his girlfriend live on the 23d floor. He thinks Bloomberg overreacted. “It is incredible because I had to walk the dog, and I’m going down 23 flights of stairs,” Lupin said. “There’s really no reason for it. It’s uncalled for.’’

The U.S. Postal Service urged any voters with absentee ballots to mail them as soon as possible. Post offices along the North Carolina, New Jersey and New York coastlines closed or suspended deliveries Monday as the storm approached. FedEx said it was suspending or curtailing flights to certain East Coast cities, including New York, Boston, Philadelphia, Baltimore and Washington.

The center of the storm was expected to migrate through Pennsylvania and then north to Canada as it gradually weakens. Along the battered East Coast, conditions will remain harsh and hazardous through Tuesday at least, with winds not abating until Thursday.

“A long-duration event” was the ominous phrase used by Rick Knabb, director of the National Hurricane Center, at a news conference Monday.

Lynch reported from New York. Suzanne Sataline and Katie Van Syckle in New York, Lisa Rein in Philadelphia and Jason Samenow and Ed O’Keefe in Washington contributed to this report.