Interactive: Hurricane Tracker (on this page)
Hurricane Sandy closed in on the United States early Sunday as coastal communities along the East Coast scrambled to prepare for torrential rains, high winds, major flooding, power outages and heavy snow a week before the presidential election.
Forecasters said Sandy has all 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.
"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.
States of emergency were declared in eight states from North Carolina to Massachusetts, as well as Washington, D.C., as Sandy made its way north after killing 41 people in the Caribbean. Governors of states in Sandy's path ordered mandatory evacuations of vulnerable areas. New York City officials also discussed whether to shut the subway system on Sunday evening in advance of the storm.
On its current projected track, Sandy could make U.S. landfall on Monday night or Tuesday morning anywhere between Maryland and southern New England, forecasters said. Some computer models show a likely landfall between Delaware and the New York/New Jersey area.
While Sandy's winds were not overwhelming for a hurricane, its width was what made it exceptional. Hurricane force winds extended 105 miles from its center while its lesser tropical storm-force winds reached across 700 miles.
Sandy could be the largest storm to hit the United States, according to the National Oceanic and Atmospheric Administration's website.Slideshow: Recovering after Sandy (on this page)
"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.
Officials urged residents to stock up on food, water and batteries. Worried residents packed stores, buying groceries, generators, candles, and other supplies in anticipation of power outages. Some local governments announced schools would be closed on Monday and Tuesday.
"They're freaking out," said Joe Dautel, a clerk at a hardware store in Glenside, Penn. "I'm selling people four, five, six packs of batteries -- when I had them."
As the threat of a monster storm began to sink in, shoppers crowded hardware stores and supermarkets looking to stock up on emergency supplies. At a Trader Joe's store in Millburn, New Jersey, shelves were stripped bare of bread, bottled water and milk.
Customers at a Home Depot in Willow Grove, a suburb of Philadelphia, were greeted by big hand-written signs saying, "No generators." The store received a shipment of 50 generators on Friday evening and sold them all within minutes of opening on Saturday, said January Introcaso, one of the store managers.
The store also sold out of flashlights and batteries. It was a similar story at other hardware stores in the area, with candles, tarps and rope also in demand.
Sandy also threatened to disrupt air travel in the region.
"There's no avoiding a significant storm-surge event over a large area. We just can't pinpoint who's going to get the worst of it," National Hurricane Center Director Rick Knabb said.
President Barack Obama on Saturday morning convened a call with Homeland Security Secretary Janet Napolitano, FEMA Administrator Craig Fugate and other officials to ensure federal resources would be ready to aid state and local agencies responding to the expected severe weather. FEMA is prepositioning water, meals, blankets and other resources should they be needed at support bases along the East Coast, the White House said.
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.
"This is not a coastal threat alone," Fugate told reporters, warning of the potential for flooding in Maryland and Pennsylvania, as well as more than 2 feet of snow in West Virginia and southwest Pennsylvania. "This is going to go well inland," he added.Forecasters: Sandy 'one for record books'
Subway could close
In New York, authorities were considering closing down the city's buses, subways, commuter railroads, bridges and tunnels as early as 7 p.m. on Sunday, when the last commuter trains would depart, with the entire system to be closed down by 3 a.m. Monday, officials said.
Delaware Governor Jack Markell ordered a mandatory evacuation of an estimated 50,000 people in coastal communities. New Jersey's Cape May County ordered an evacuation of its barrier islands, home to some popular beach resorts, by Sunday afternoon.
Evacuations were ordered along some coastal areas, including Atlantic City and its casinos beginning Sunday afternoon.
In a public advisory notice issued at 11 p.m. ET Saturday, the National Hurricane Center said Sandy packed hurricane-force winds of 75 mph, up from earlier reported maximum sustained wind speed of 70 mph. High wind warnings were in effect for the Mid-Atlantic states, Southern New England and portions of Southeastern Virginia.
"Hurricane force winds extend outward up to 105 miles from the center... and tropical storm force winds extend outward up to 520 miles," the NHC said.
Sandy was about 360 miles southeast of Charleston, S.C., and was moving northeast at 14 mph, up from 11 mph earlier in the day, the center said.
Many airlines are posting hurricane-related travel tips online. Some airlines announced fee waivers for customers traveling on certain dates between some airports expected to be affected by the storm.
Customers trying to call airlines to rebook travel by phone over the weekend were reporting waits of several hours.
Airline hurricane statements:
Don Morelli, a meteorologist with WSI, a sister company of The Weather Channel, said major flight delays and airport closings starting Saturday evening and lasting through Tuesday were likely.
"The criteria for closing an airport is around 58, 60 miles an hour, which is easily going to be reached for much of the major hubs from D.C. northward to New York City and even into Logan [in Boston]," Morelli said. "Major delays [are] going to be very, very widespread right through mid-week, so [it’s] not a good week to be traveling across the Northeastern U.S."
Coming in the hectic run-up to the U.S. presidential election on November 6, the storm presented a challenge to the campaigns of Obama and his Republican challenger, Mitt Romney.
Obama is canceling campaign stops in Virginia and Colorado early next week because of the hurricane and will instead monitor the storm from the White House, his spokesman said on Saturday. Vice President Joe Biden canceled a Saturday trip to Virginia Beach.
Romney was rescheduling all of his campaign events planned for Virginia on Sunday and flying to Ohio instead.Sandy is set to deliver potential election surprise
Power outages caused by the storm could continue through the election, NBC meteorologist Bill Karins warned.
"After the storm hits, expect the clean-up 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."Your images of Sandy's fury Video: Carolinas, East Coast on watch for Hurricane Sandy
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.Story: Busy Atlantic storm season tied to lack of El Nino
"It's almost a weeklong, five-day, six-day event," Jim Cisco, a forecaster at the National Weather Service's prediction office, told NBC News Friday. "It's going to be a widespread serious storm."
NBC News' Ian Johnston, Miguel Llanos, Andrew Mach and Jim Gold, The Associated Press and Reuters contributed to this report.