New York City Mayor Michael Bloomberg said that residents who live in low-lying areas near the coast must leave their homes ahead of Hurricane Irene's expected arrival on Saturday.

Bloomberg said emergency shelters would be open at 4 p.m. and subway service would shut down across the city by Saturday afternoon.

"Waiting until the last minute is not a smart thing to do," Bloomberg said. "This is life-threatening."

New York has never entirely shut down its system in advance of a hurricane, and Bloomberg added that the mandatory evacuations are a first for the city.

About 270,000 residents could be directly affected by the evacuations, as low-lying areas include parts of the Rockaways, Coney Island and Battery Park City, The Associated Press reported.

Through the entire region, evacuation orders were also extended to at least 2.3 million people, including:

  • 1 million in New Jersey
  • 315,000 in Maryland
  • 300,000 in North Carolina
  • 200,000 in Virginia
  • 100,000 in Delaware

The hurricane warning extends over large swaths of the Atlantic coast, from North Carolina to New York. A hurricane warning means hurricane conditions are expected within 36 hours.

"This is probably the largest number of people that have been threatened by a single hurricane in the United States," said Jay Baker, a geography professor at Florida State University, in an interview with AP.

The warnings in New York came as Irene was downgraded slightly from a Category 3 to a less-severe Category 2 storm, but is likely to strengthen by the time it reaches the mainland.

While the evacuations are mandatory in many places, Bloomberg said that people will have to make the ultimate decision to leave.

"We do not have the manpower to go door-to-door and drag people out of their homes," he said. "Nobody's going to get fined. Nobody's going to go to jail. But if you don't follow this, people may die."

Earlier, U.S. President Barack Obama warned residents in at-risk areas to take precautions or evacuate their homes in advance of "what's likely to be an extremely dangerous and costly storm."

On Friday the slow-moving storm had maximum sustained winds of 155 kilometres per hour and was about 480 kilometres south of Cape Hatteras, North Carolina.

"I cannot stress this highly enough: if you are in the projected path of this hurricane you need to take precautions now," Obama said, speaking in Martha's Vineyard where he is on vacation with his family.

"Don't wait, don't delay. We all hope for the best but we have to be prepared for the worst, all of us have to take this storm seriously."

Obama said Federal Emergency Management Agency (FEMA) teams had been deployed up and down the East Coast, and millions of litres of water, millions of meals, as well as blankets, tents and supplies had been pre-positioned in coastal communities.

The Red Cross has also set up shelters to house residents who become displaced from their homes, he said.

"All indication point to this being an historic hurricane," Obama said.

Irene is expected to first hit North Carolina on Saturday, then turn up the coast through Washington, New York and Boston before reaching Maine and northern New Brunswick late Sunday and into Monday.

Canadian preparations

The storm's power, size and slow pace means it could deliver several days of heavy damage to the U.S. East Coast before a weakened version hits Canada, forecasters predict.

By the time the storm reaches Canada, it will likely be a strong tropical storm still capable of creating damage.

According to current projections, southern New Brunswick, eastern Quebec, P.E.I. and mainland Nova Scotia could be hit, Forecasters are warning of possible storm surges in the Bay of Fundy.

The Canadian Hurricane Centre in Halifax warned that Irene, which is 700 kilometres in diameter, will affect a wide chunk of Eastern Canada and will likely still pack a mean punch.

Chris Fogarty, from the Canadian Hurricane Centre, said that recent reports that the storm has been downgraded should be tempered with caution.

He said that while the downgraded storm "is a positive trend in terms of impact," he noted that sometimes, the dangerous area of the hurricane increase.

"In terms of its total energy, it may not be changing very much."

Fogarty said that areas in Atlantic Canada could still be hit with hurricane-force gusts.

Public Safety Minister Vic Toews said the government is closely monitoring the system as it tracks northward

"Citizens in the potentially affected areas should follow the direction of local and provincial emergency first responders."