Interactive: Hurricane Tracker (on this page)
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Hurricane Sandy lumbered toward the East Coast early Monday, leaving potentially tens of millions of residents with only hours to prepare for its onslaught of punishing wind, torrential rains and, at higher elevations, heavy snow.
A day before the storm was scheduled to make landfall, state and local officials issued mandatory evacuation orders for hundreds of thousands of residents in low-lying areas and began the slow, grinding process of shutting down New York City’s massive transit system.
Meanwhile, U.S. stock exchanges said they would not be trading on Monday and possibly Tuesday. In Washington, D.C., federal offices will remain closed Monday, and federal courts in affected areas sent out releases saying they, too, would be shuttered in anticipation of the historic storm.
Forecasters said Sandy has the ingredients to transform into a "super storm" as it merges with an Arctic jet stream, which could make the storm unlike anything seen over the eastern United States in decades. The storm is projected to make landfall late Monday .
"We're looking at impact of greater than 50 to 60 million people," said Louis Uccellini, head of environmental prediction for the National Oceanic and Atmospheric Administration (NOAA).
Sandy could be the largest storm ever to hit the United States, according to NOAA's website.
"The size of this alone, affecting a heavily populated area, is going to be history making," said Jeff Masters, a hurricane specialist who writes a blog posted on the Weather Underground.
Workers on Sunday night began shutting down New York City's subway, bus and commuter railroads , as ordered by New York Gov. Andrew Cuomo. In New Jersey, bus, rail and light rail services were gradually shut down starting Sunday afternoon.
In anticipation of widespread damage and vast power outages, states of emergency were declared in nine states from North Carolina to Maine, as well as in Washington, D.C., as Sandy made its way north after killing at least 65 people in the Caribbean.
“Sandy is a large hurricane and large systems pose multiple hazards for more people than … smaller systems of comparable intensity,” National Hurricane Center Director Rick Knabb said Sunday. “The large size of the system is why it is so capable of producing a life threatening storm surge, with that threat being applicable to many areas.”
A late Sunday advisory from the National Hurricane Center said Sandy was about 290 miles east of Cape Hatteras, N.C., and 470 miles south-southeast of New York City. It was moving northeast at 14 mph.
"The combination of an extremely dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters," the National Hurricane Center said in its advisory.
Forecasters said Sandy could make U.S. landfall anywhere between Maryland and southern New England. Some computer models show a likely landfall between Delaware and the New York/New Jersey area, where the storm surge could reach from 6 feet to 11 feet.
New York City Mayor Michael Bloomberg ordered mandatory evacuations (.PDF) of 375,000 people from low-lying areas in New York City, signifying officials' growing concern about the severity of the storm and its impact on the five boroughs. About 45,000 of those were people living in public housing, where the heat, hot water and elevator services will be turned off, Bloomberg said, according to NBCNewYork.com.
“This evacuation is mandatory. It is for your own safety,” Bloomberg said.
The city opened 65 shelters in public schools on Sunday morning and said more could be opened. Last year, the mayor called for an evacuation in anticipation of Tropical Storm Irene, which officials called unprecedented.
Three other states also were ordering mandatory evacuations.
Delaware Gov. Jack Markell ordered a mandatory evacuation of an estimated 50,000 people in coastal communities. In New Jersey, mandatory evacuations were ordered for Sandy Hook south to Cape May, an area that includes the casinos in Atlantic City and popular beach resorts.
Video: Bloomberg orders evacuation in NYC's low-lying areas (on this page)
Five shelters were open in Atlantic County for those five affected municipalities that are home to some 66,000 people, according to census data, said Linda Gilmore, a county spokeswoman. One of the shelters was already full.
“As conditions continue to deteriorate we’re seeing more people," with rain and increasing winds, she told NBC News.
Connecticut Gov. Dannel P. Malloy ordered mandatory evacuations on Sunday in vulnerable areas of Bridgeport and Fairfield.
Separately, several cities, including Stamford, Conn., and Islip, N.Y., also ordered residents out of flood-prone areas.
Not everyone was heeding the evacuation orders. Peter Mercatanti, 26, who lives on Deauville Beach in Mantoloking, N.J., said Sunday afternoon that he had kayaks ready and had packed wet suits, waterproof book bags, flares, glow sticks, flashlights, propane and food to bear out the storm.
He had stocked sandbags around his home and put up shelter blinds on all of the windows and the doors. But ocean water was hitting his fence and winds that he estimated to be about 25 to 30 miles an hour could be heard screaming on the beach.
“I just wanted to be here for it, see it, experience it, and try to take care of my house as much as possible,” Mercatanti, a chief of operations for a gym chain, told NBC News by telephone.
Though he is "extremely worried," he won't leave the island but will head to a friend's house nearby if need be.
"I’ve pretty much kind of prepared for every possible situation," he said. "I can be sustainable and floating."
The Coast Guard also rescued a surfer near Sea Bright, N.J., where authorities have issued mandatory evacuations.
"We can't stress enough to stay clear of beaches," said Chief Petty Officer Rodger Krass, the command duty officer in the New York sector, said in a statement. "Now is not the time to head out onto the water to go surfing as sea conditions are extremely dangerous and are continuing to worsen as Hurricane Sandy approaches shore."
President Barack Obama on Sunday visited the National Response Coordination Center at FEMA Headquarters, where the deployment of federal resources and teams to states up and down the East Coast will be coordinated, the White House said.
Calling it a "very serious storm event," he said: "This is going to affect a big swath of the country and because it's slow moving, it could be a number of days until people get power back."Video: N.J. coast residents ordered to evacuate
FEMA Administrator Craig Fugate said earlier Sunday that the "time for preparing and talking is about over."
"People need to be acting now particularly as this storm … is becoming a rather substantial risk to storm surge," he added. The hurricane is "going to produce very high potential life threatening storm surge … that may require additional evacuations today."
Coastal flooding posed a major threat, particularly in low-lying areas like New York City, the global financial nerve center, and Alexandria, Va., across the Potomac River from Washington, D.C. Flooding also could occur in parts of North Carolina, Virginia, Maryland, Connecticut, Rhode Island and New York, the National Hurricane Center said.
But Fugate warned that "this is not a coastal threat alone" and that the impact will be felt "well inland."
Some areas could experience up to a foot of rain, while others, such as West Virginia, Virginia and the area around the North Carolina-Tennessee border, could get from 1 to 3 feet of snow.
Sandy's winds were not overwhelming for a hurricane -- sustained near 75 mph Sunday afternoon with higher gusts. But its width is exceptional. Hurricane force winds extended up to 175 miles from its center while its lesser tropical storm-force winds reached across 520 miles.Slideshow: Recovering after Sandy (on this page)
Officials urged residents to stock up on emergency supplies, but items like lanterns, generators, propane, rain boots and batteries were hard to find at stores throughout the region. Grocery stores also reported runs on bread, bottled water and canned foods.
Long lines also were reported at gas stations that remained open.
Sandy also was disrupting travel in the region. More than 6,800 flights have been canceled so far, according to FlightAware.com, a flight tracking website. Rail traffic also was impacted, with Amtrak canceling all of its northeast corridor service in addition to some other lines it operates in the region.
Some local governments, including New York City and Washington, D.C., announced schools would be closed on Monday and Tuesday. Federal government offices in Washington, D.C., will also be closed Monday and non-emergency employees there as well as in Connecticut and Massachusetts have been told not to come to work.
The Broadway League said it was calling off all Broadway performances on Sunday and Monday evenings.
Workers also halted construction on Saturday at the World Trade Center, where efforts to secure the site were ongoing, Cuomo said in a statement. All construction materials have been tied down and nailed to concrete floors, or secured with heavy chains, and more than 1,000 sandbags had been placed around low-lying areas next to Battery Park and the Hudson River.
"I've directed the Port Authority to ensure that the World Trade Center site and the Memorial are protected from flooding and high winds due to Hurricane Sandy," Cuomo said. "The World Trade Center is being built well above and beyond normal flood protection standards; however we must take every precaution to ensure the site is protected from a potentially historic storm."
Coming in the hectic run-up to the U.S. presidential election on Nov. 6, the storm presented a challenge to the campaigns of Obama and his Republican challenger, Mitt Romney.
Obama canceled campaign stops in Virginia and Colorado early next week because of the hurricane and was planning to remain at the White House, his spokesman said Saturday.
Romney rescheduled his campaign events for Virginia on Sunday and headed to Ohio instead.
Obama said officials did not expect the storm to impact voting, "but we're obviously going to have a look." Meantime, Maryland’s governor also announced that early voting in the state would be canceled Monday, so as not to jeopardize the safety of voters, NBC Washington reported.
Power outages caused by the storm, expected to affect millions or residents and businesses, could continue through the election, NBC meteorologist Bill Karins warned. Some 3,000 people were without power in Virginia, where it was raining Sunday and up to 200 people were going to be evacuated, said Gov. Robert McDonnell.
"After the storm hits, expect the cleanup and power outage restoration to continue right up through Election Day," Karins said. "Serious and life-threatening weather conditions are expected from Outer Banks to New England."
Although Sandy is not forecast to be as strong as other recent storms to hit the Northeast -- such as Hurricane Irene in August 2011, which left $4 billion in damage -- it holds the potential to cause significant damage because it will be moving slowly.
Some experts predict at least $1 billion in damage in the United States.
NBC News' Isolde Raftery, Ian Johnston, Miranda Leitsinger, Miguel Llanos, Andrew Mach and Jim Gold, The Associated Press and Reuters contributed to this report.