Hurricane Sandy is expected to bring life-threatening storm surge flooding to the Mid-Atlantic coast and could wreak havoc for days across 800 miles of the United States, impacting tens of millions of people in the eastern third of the country.
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 "Perfect Storm," some meteorologists say.
"This storm that is going to be impacting the mid-Atlantic and parts of the Northeast...is going to be destructive, historic, and unfortunately life threatening," AccuWeather's Bernie Rayno said.
The first rainfall from the megastorm is expected today and forecasters warn it could bring inland flooding around Maryland and Pennsylvania and up to two feet of snow in West Virginia.
Sandy remained at a Category 1 strength today, with 75 mph winds being measured. The storm was moving northeast at 10mph, according to the National Hurricane Center.
A sea buoy that is 158 miles east from Cape Hatteras in N.C. reported wave heights of 32 feet every 13 seconds moving north, showing how much water the high energy storm was capable of pushing ahead of it.
New York City transit officials are preparing for a shutdown of 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.
"Lower Manhattan is the most vulnerable place to a storm surge," said New York City Mayor Michael Bloomberg.
States of emergency were declared from North Carolina to Connecticut. Coastal communities in Delaware were ordered to evacuate by 8 p.m. tonight.
"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. Chris 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 of run extenstion 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 hundreds of flight cancellations, with more scheduled for Monday as airlines prepare for the storm.
As of this morning, United Airlines had canceled more than 300 of the day's flights, according to data from FlightAware.
The other legacy carriers – Delta, US Airways and American – were so far sticking to their Sunday flight schedule as of 7 a.m. JetBlue and Virgin America has canceled 51 and 24, respectively.
ABC News' Sydney Lupkin and Genevieve Shaw Brown contributed to this report