Hurricane Sandy, now a 900-mile megastorm, is forecasted to make landfall late Monday night in Atlantic City, N.J., bringing with it life-threatening storm surges, forceful winds and rainfall that could cripple transportation and leave millions without power.
The size and power of the storm are almost without equal as several systems will combine to wreak havok on a large section of the nation--from North Carolina to New England as far west as the Great Lakes.
Waves 20 to 25 feet are possible on the south side of Lake Michigan Monday night into Wednesday, along with beach erosion and flooding especially southeast of Chicago around Gary, Ind., and Michigan City with waves there as high as 33 feet.
On the East Coast, a storm surge is expected along a 600-mile stretch of the Atlantic along with rainfall in places of 6 to 10 inches and even more.
"We want to prepare people for the worst," New Jersey Gov. Chris Christie said Sunday, warning that some residents could be without power for more than a week.
Christie urged people in the path of Hurricane Sandy to "remain calm and listen to instructions."
Tens of thousands of people in coastal areas have been ordered to evacuate their homes before Hurricane Sandy pounds the eastern third of the United States.
States of emergency were declared from North Carolina to Connecticut. Coastal communities in Delaware were ordered to evacuate by 8 p.m. tonight.
As of 5 p.m. EST today, Sandy was moving toward the northeast at nearly 15 mph, slightly faster than it was measured earlier today. It is expected to take a turn to the north and then northwest, bringing the center of the storm near the mid-Atlantic coast Monday night.
Sandy is expected to bring potentially life-threatening storm surges on the coast ranging from several feet to potentially as high as 11-feet in the Long Island Sound area of New York, said Rick Knabb, director of the National Hurricane Center.
"The size of the storm is going to carve a pretty large swath of bad weather," Knabb said. "This is not just a coastal event."
Sandy will meet up with cold front coming from the northwest and a high pressure system from Greenland, fueling it with enough energy to make it more powerful than the so-called "Perfect Storm" in 1991, meteorologists say.
The first rainfall from the megastorm is expected today and forecasters warn it could bring inland flooding around Maryland and Pennsylvania. A blizzard warning was issued for portions of West Virginia, where Sandy could bring up to two feet of snow.
Sandy remained at a Category 1 strength today, with 75 mph winds being measured. The storm was moving northeast at 10 mph, according to the National Hurricane Center.
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 are shut down the subway system, the largest rapid transit system in the world at 7 p.m. tonight. Sandy can 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.
"While the predicted track of Hurricane Sandy has shifted a number of times over the last 24 hours, it has become clear that the state will be affected by high winds, heavy rainfall, and flooding, especially along the coastline for a several day period," said Gov. Jack Markell of Delaware. "These factors, along with the potential for power outages, have convinced me that the prudent thing to do is have people leave most of our coastal communities."
Given its size and expected duration of two to three days, Hurricane 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 of 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.
Gov. Christie warned New Jersey residents they could be without power for a week to ten days. He said he is concerned residents may try to put generators indoors or run extension cords in a haphazard way to get electricity.
"That's a good general New Jersey rule: If it looks stupid, it is stupid," Christie said.
Sunday also brought more than 1,000 flight cancelations, with 5,559 expected for Monday and 613 cancelled for Tuesday, Flight Aware reported. The most affected airport today was Newark with 305 cancellations.
People scheduled to fly to or from the eastern third of the country are encouraged to check their flight status.
ABC News' Sydney Lupkin and Genevieve Shaw Brown contributed to this report