NEW YORK — From Washington to Boston, big cities and small towns buttoned up Sunday against the onslaught of a superstorm that could endanger 50 million people in the most heavily populated corridor in the nation, with forecasters warning that the New York area could get the worst of it — an 11-foot wall of water.
‘‘The time for preparing and talking is about over,’’ Federal Emergency Management Administrator Craig Fugate said as Hurricane Sandy made its way up the Atlantic on a collision course with two other weather systems that could turn it into one of the most fearsome storms on record in the United States. ‘‘People need to be acting now.’’
Forecasters said the hurricane could blow ashore Monday night or early Tuesday along the New Jersey coast, then cut across into Pennsylvania and travel up through New York State on Wednesday.
Airlines canceled more than 5,000 flights and Amtrak began suspending train service across the Northeast. New York, Philadelphia, Washington, and Baltimore moved to shut down their subways, buses, and trains and said schools would be closed Monday. And all nonessential government offices closed in the nation’s capital.
As rain from the leading edges of the monster hurricane began to fall over the Northeast, hundreds of thousands of people from Maryland to Connecticut were ordered to evacuate low-lying coastal areas, including 375,000 in lower Manhattan and other parts of New York City, 50,000 in Delaware, and 30,000 in Atlantic City, where the city’s 12 casinos were forced to shut down for only the fourth time ever.
‘‘I think this one’s going to do us in,’’ said Mark Palazzolo, who boarded up his bait-and-tackle shop in Point Pleasant Beach, N.J., with the same wood he used in past storms, crossing out the names of Hurricanes Isaac and Irene and spray-painting ‘‘Sandy’’ next to them. ‘‘I got a call from a friend of mine from Florida last night who said, ‘Mark, get out! If it’s not the storm, it’ll be the aftermath. People are going to be fighting in the streets over gasoline and food.’ ”
Authorities warned that the nation’s biggest city could get hit with a surge of seawater that could swamp parts of lower Manhattan, flood subway tunnels, and cripple the network of electrical and communications lines that are vital to the nation’s financial center.
The hurricane was expected to hook inland during the day Monday, colliding with a wintry storm moving in from the west and cold air streaming down from the Arctic.
Forecasters said the combination could bring close to a foot of rain in places, a potentially lethal storm surge of 4 to 11 feet across much of the region, and punishing winds that could cause widespread power outages that last for days. The storm could also dump up to 2 feet of snow in Kentucky, North Carolina, and West Virginia.
Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration, said that given Sandy’s east-to-west track into New Jersey, the worst of the storm surge could be just to the north, in New York City, on Long Island, and in northern New Jersey.
Forecasters said that because of giant waves and high tides made worse by a full moon, the metropolitan area of about 20 million people could get hit with an 11-foot wall of water.
‘‘This is the worst-case scenario,’’ Uccellini said.
Mayor Michael Bloomberg of New York warned: ‘‘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. This is a serious and dangerous storm.’’
New Jersey’s famously blunt governor, Chris Christie, was less polite: ‘‘Don’t be stupid. Get out.’’
New York called off school Monday for the city’s 1.1 million students and announced it would suspend all train, bus, and subway service Sunday night. The New York Stock Exchange announced it will shut down its trading floor Monday but continue to trade electronically.
In Washington, President Obama promised the government would ‘‘respond big and respond fast’’ after the storm hits.