Stores were packed with customers, homeowners were boarding up homes and tens of thousands began evacuating coastal areas as the Northeast prepared itself for a hurricane that President Barack Obama called a "serious and big storm."

The president said the government would "respond big and respond fast" after the storm hits. On Sunday, millions of people began hunkering down and making last-minute preparations for a storm expected to impact a third of the country.

"My main message to everybody involved is that we have to take this seriously," Obama said. He urged people to "listen to your local officials."

Federal Emergency Management Administrator Craig Fugate warned that Hurricane Sandy, headed up the Eastern Seaboard, was on a collision course with two other weather systems.

The time for preparing and talking is about over. People need to be acting now.

- Federal Emergency Management Administrator Craig Fugate

"The time for preparing and talking is about over," Fugate said. "People need to be acting now."

New York City and Philadelphia both announced subways, buses and trains would stop running Sunday night because of the risk of flooding, and New York said its 1.1 million-student school system would be closed on Monday. Mayor Michael Bloomberg also ordered the evacuation of part of lower Manhattan and other low-lying neighborhoods.

"If you don't evacuate, you are not only endangering your life, you are also endangering the lives of the first responders who are going in to rescue you," he said. "This is a serious and dangerous storm."

Tens of thousands of people along the coast in Delaware, New Jersey, Connecticut and other threatened areas were also under orders to clear out because of the danger of as much as a foot of rain, punishing winds of 80 mph or higher and a potentially deadly wall of water 4 to 11 feet high. Communities opened shelter across the region.

Sandy was headed north from the Caribbean, where it left nearly five dozen people dead, and was expected to hook left toward the mid-Atlantic coast and come ashore late Monday or early Tuesday, most likely in New Jersey, colliding with a wintry storm moving in from the west and cold air streaming down from the Arctic.

Forecasters warned that the resulting megastorm could wreak havoc over 800 miles from the East Coast to the Great Lakes. Parts of West Virginia, Virginia, Kentucky and North Carolina could get snow of about 2 feet or more in places.

Witlet Maceno, an emergency room nurse working at New York City's Mount Sinai Hospital, was headed home to Staten Island on Sunday morning after his overnight shift. He said he was going home to check on his parents, visiting from Atlanta, before he returned to work Sunday evening.

"I'm making sure they're OK, that they have water and food, and that the windows are shut tight," he said. "And I'm going to remove stuff outside that could go flying into the windows" of his street-level apartment.

The danger was hardly limited to coastal areas, with forecasters worried about inland flooding. They also warned that the rain could saturate the ground, causing trees to topple onto power lines and cause blackouts that could last for several days.

States of emergency were declared from North Carolina, where gusty winds whipped steady rain on Sunday morning, to Connecticut. Delaware ordered 50,000 people in coastal communities to clear out by 8 p.m. Sunday.

Officials in New York City were particularly worried about the possibility of subway flooding. The city closed the subways before Hurricane Irene last year, and a Columbia University study predicted that an Irene surge just 1 foot higher would have paralyzed lower Manhattan.

However, the New York Stock Exchange planned to open for trading as usual Monday, despite fears that flooding would damage the underground electrical network that is so vital to the nation's financial center.

Sandy was at Category 1 strength, packing 75 mph winds, about 270 miles southeast of Cape Hatteras, N.C., and moving northeast at 14 mph as of 2 p.m. Sunday, according to the National Hurricane Center in Miami. It was about 575 miles south of New York City. But the storm was so big that forecasters could not say with any certainty which areas would get the worst of it.

Bobbie Foote said she would heed an evacuation order Sunday for south Wilmington, Del., and would take shelter at her daughter's home in nearby Newark.

"My daughter insists that I leave this time," said Foote, a 58-year-old fitness coach. It will be the first time she has fled a storm threatening the apartment building that has been her home for at least 40 years in the working-class neighborhood near the Delaware River.

Foote said she stayed last year when flooding from the remnants of Hurricane Irene blocked streets at either end of the neighborhood. She said her daughter wouldn't stand for her getting trapped that way again. "She said I should never put myself in that predicament where I cannot get in or out of where I live," Foote said.

Amtrak began canceling train service Saturday night to parts of the East Coast, including between Washington and New York. Airlines started moving planes out of airports to avoid damage and added Sunday flights out of New York and Washington in preparation for flight cancellations on Monday.

The Virginia National Guard was authorized to call up to 500 troops for debris removal and road-clearing, while homeowners stacked sandbags at their front doors in coastal towns.

In Arlington, just outside Washington, D.C., a few shoppers strolled in and out of a supermarket. Cathy Davis said the supermarket was sold out of the water she wanted to purchase, but she wasn't doing much else to prepare. She figured she would bring her outdoor furniture inside later in the day, and might make some chili.

She said the storm did lead her to decide against decorating for Halloween.

"I was like, 'Eh, it will just be blown away anyway,'" she said. "What's the point?"

President Barack Obama was monitoring the storm and working with state and locals governments to make sure they get the resources needed to prepare, administration officials said.

In New Jersey, hundreds of coastal residents started moving inland.

Gov. Chris 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. City officials said they would begin evacuating the gambling hub's 30,000 residents at noon Sunday, busing them to mainland shelters and schools.

The storm also forced the presidential campaign to juggle schedules. Mitt Romney scrapped plans to campaign Sunday in Virginia and switched his schedule for the day to Ohio. First lady Michelle Obama canceled an appearance in New Hampshire for Tuesday, and Obama moved a Monday departure for Florida to Sunday night to beat the storm. He also canceled appearances in northern Virginia on Monday and Colorado on Tuesday.

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