FEMA administrator Craig Fugate urged people in Sandy's path to take the storm seriously and to heed any evacuation orders.
"The time for preparing and talking is about over. People need to be acting now," Fugate said.
New York City transit officials shut down the subway system, the largest rapid transit system in the world at 7 p.m. Sunday. Sandy could potentially create a storm surge capable of overtopping the Manhattan flood walls, filling the subway tunnels with water.
New York City Mayor Michael Bloomberg ordered the evacuation of areas of lower Manhattan and the Rockaways.
"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," Bloomberg said at a news conference. "This is a serious and dangerous storm."
New York City Schools will also be closed Monday, Bloomberg said.
Given its size and expected duration of two to three days, Sandy could turn out to be comparable to 1991's Hurricane Grace, also known as the "Perfect Storm," and a cyclone that struck near the Appalachians in November 1950, FEMA administrator Craig Fugate said. But, Fugate said, officials don't try to make historical comparisons until after a storm hits.
Power companies are being proactive before Sandy makes landfall, trimming trees and putting equipment place to hopefully minimize the number of people left without power after the storm.
Last year, Hurricane Irene left 7 million homes without power in the same area Sandy is expected to batter with wind and rain.
"The best thing is to be prepared, and I think that's where we are. We're prepared for what the worst will bring," said Vince Maione, who has been with Atlantic City Electric, a company serving south New Jersey, for 28 years.
As of this morning, 6,500 flights in the U.S. had been cancelled, according to Flight Aware.
People scheduled to fly to or from the eastern third of the country are encouraged to check their flight status.
ABC News' Russell Goldman, Sydney Lupkin and Genevieve Shaw Brown contributed to this report