(AP) -- 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 ashore 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.

Meanwhile New York City's mass transit system is being shut down, schools are closing and hundreds of thousands of people are being ordered to flee their homes.

Officials Sunday predicted the storm could pack a seawater surge of anywhere from 6 to 11 feet. That might swamp parts of lower Manhattan, flood subway tunnels and knock out underground power, phone and high-speed Internet lines in America's financial capital.

Still, the New York Stock Exchange plans to open Monday. And some flood-zone residents say they're not going anywhere, even though Mayor Michael Bloomberg is urging them to go.

He has announced a mandatory evacuation affecting about 375,000 people in low-lying areas from the Rockaways to Battery Park City.