Forecasters warned that the New York City region could face the worst of Hurricane Sandy as it bore down on the U.S. East Coast's largest cities Monday, forcing the shutdown of financial markets and mass transit, sending coastal residents fleeing and threatening high winds, rain and a wall of water up to 11 feet (3.35 metres) tall. It could endanger up to 50 million people for days.

Sandy strengthened before dawn and stayed on a predicted path toward New York, Washington, Baltimore and Philadelphia— putting it on a collision course with two other weather systems that would create a superstorm with the potential for havoc over 800 miles (1,280 kilometres) from the East Coast to the Great Lakes. Up to 3 feet (0.9 metres) of snow were even forecast for mountainous parts of West Virginia.

Airports closed, and authorities warned that the time for evacuation was running out or already past. Many workers planned to stay home as subways, buses and trains shut down across the region under the threat of flooding that could inundate tracks and tunnels. Utilities anticipated widespread power failures.

The centre of the storm was positioned to come ashore Monday night in New Jersey, meaning the worst of the surge could be in the northern part of that state and in New York City and on Long Island. Higher tides brought by a full moon compounded the threat to the metropolitan area of about 20 million people.

"This is the worst-case scenario," said Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration.

As rain from the leading edges began to fall over the Northeast on Sunday, hundreds of thousands of people from Maryland to Connecticut were ordered to leave 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, New Jersey, where the city's 12 casinos shut down for only the fourth time ever.