With forecasts showing Hurricane Sandy and another powerful storm system likely to collide with devastating effect somewhere on the Eastern Seaboard as early as Sunday evening, tens of millions of people began to make preparations, and authorities increased the urgency of their warnings.

Several states declared emergencies, and thousands of people were evacuated from low-lying areas. President Obama consulted with the leaders of the federal agencies that are monitoring the storm and will respond when it moves ashore. Governors in nine states deployed more than 60,000 National Guard troops to assist the local authorities.

Authorities said the weather conditions would deteriorate long before the center of the storm makes landfall, and that damage from widespread inland flooding, snow and wind could extend into the Ohio Valley, afflicting a third of the country.

"We're looking at impact of greater than 50 to 60 million people," said Louis Uccellini of the National Oceanic and Atmospheric Administration.

Gov. Chris Christie of New Jersey said that power could be out for more than a week if the forecasts hold, and he urged residents not to dismiss the warnings. He said. "We have to be prepared for the worst."

From Maine to North Carolina, people boarded up windows; stocked up on water, batteries, generators and food; and prepared to hunker down.

In New York City, Mayor Michael Bloomberg also urged residents to stock up and warned: "This is a large, unpredictable storm, so be prepared for possible outages."

Officials prepared for a possible shutdown of the subway and regional rail system and bridges in and out of the city.

Rick Knabb, the director of the National Hurricane Center, said the intensity of the storm was unlikely to change. A system known as a mid-latitude trough is moving across the country from the west. It is expected to draw in Sandy, giving it added energy. A burst of arctic air is expected to sweep down through the Canadian plains just as those two storms are converging. The full moon on Monday could cause even greater flooding because tides would be at their peak.

The hurricane was forecast to make landfall between Delaware and Long Island, with its path shifting hourly. But as the storm continue to churn north, it began to spread out, with tropical-force winds extending about 450 miles from its center. On Saturday, Sandy was about 300 miles east of Charleston, S.C.

The storm was so big, and the convergence of the three storms so rare, that "we just can't pinpoint who is going to get the worst of it," Knabb said.

Jeff Masters, director of the private service Weather Underground, said the storm could be bigger than the worst East Coast storm on record: the 1938 hurricane known as the Long Island Express, which killed nearly 800 people. He said, "Part hurricane, part nor'easter -- all trouble."

The AP contributed to this report.