The head of the Federal Emergency Management Agency says "the time for preparing and talking is about over."
Craig Fugate says it's now time to act, before Hurricane Sandy moves to shore and collides with two other weather systems, potentially threatening some 50 million people.
Tens of thousands are being told to evacuate coastal areas of Delaware, New Jersey, Connecticut and other vulnerable spots along the East Coast.
New York City is shutting down its subways, buses and trains tonight, and closing its schools tomorrow. Mayor Michael Bloomberg has also ordered the evacuation of low-lying neighborhoods in the city, including lower Manhattan.
Airlines are canceling thousands of flights and Amtrak is scaling back train service in the Northeast Corridor.
Forecasters expect Sandy to come ashore late tomorrow or early Tuesday, most likely in New Jersey, bringing high winds and coastal flooding. Then it's expected to meet up with a storm moving in from the west and cold air streaming down from the Arctic.
Forecasters say the resulting megastorm could blow down trees and power lines and dump heavy rain or snow over 800 miles, from the East Coast to the Great Lakes. Parts of West Virginia, Virginia, Kentucky and North Carolina could get up to 2 feet of snow.
Broadway is taking the threat of the mammoth storm seriously, with theater owners canceling all Sunday evening and Monday shows long before a drop of rain fell in Times Square.
Sundays are big days for Broadway. Many shows have both a matinee and evening performance. But forecasts called for rain late Sunday or early Monday and subway service is to be halted, so all Sunday evening performances were scrubbed. Off-Broadway shows were also canceled.
Airlines cancelled more than 3,000 flights as a result of Hurricane Sandy as of Sunday morning, with hubs along the East Coast bearing the brunt of the disruptions.
For Monday, 2,499 flights are cancelled, with 774 cancellations at Newark, followed by 428 at Dulles in Washington and 355 cancellations at Philadelphia.
New York City mass transit is shutting down Sunday evening, and public schools in the city will close Monday. New York City Mayor Michael Bloomberg ordered residents to evacuate some low-lying areas Sunday.
The New York Stock Exchange plans normal operations Monday as Hurricane Sandy makes its way up the Atlantic.
According to the Exchange, weather last caused the NYSE to shut down on March 27, 1985 for Hurricane Gloria. Trading on the Big Board also was suspended after the attacks on Sept. 11, 2011 for three days afterward.
Hurricane Sandy is heading north from the Caribbean, where it has left nearly five dozen dead, to meet a winter storm and a cold front, plus high tides from a full moon, and experts say the rare hybrid storm that results will cause havoc over 800 miles from the East Coast to the Great Lakes.