WILMINGTON, North Carolina (Reuters) - The eastern United States ramped up its alert on Friday ahead of Hurricane Irene and New York City ordered evacuations of vulnerable residents as the broad, menacing storm closed in on the Atlantic coast.
As 55 million Americans on the eastern seaboard braced for the weekend onslaught from the nearly 600 mile-wide hurricane, President Barack Obama said its impact could be "extremely dangerous and costly" for a nation that still remembers destructive Hurricane Katrina in 2005.
Hundreds of thousands of residents and vacationers were evacuating from Irene's path, starting in east North Carolina where the hurricane, now packing winds of 100 miles per hour, is expected to make landfall on Saturday.
Tropical storm winds were already arriving along the coast of the Carolinas, the National Hurricane Center said.
A quarter of a million New Yorkers were ordered to leave homes in low-lying areas as authorities prepared for dangerous storm surge and flooding on Sunday in the city and farther east on Long Island.
Some New York hospitals in flood-prone areas were already evacuating patients.
"We've never done a mandatory evacuation before and we wouldn't be doing it now if we didn't think this storm had the potential to be very serious," New York Mayor Michael Bloomberg told a news conference.
Those affected were ordered to evacuate by 5 p.m. (2100 GMT) on Saturday afternoon.
Federal and state leaders, from Obama downward, urged the millions of Americans in the hurricane's path to prepare and to heed evacuation orders if they received them.
"All indications point to this being a historic hurricane," Obama said.
Coastal communities from the Carolinas to New England stocked up on food and water and tried to secure homes, vehicles and boats. States, cities, ports, hospitals, oil refineries and nuclear plants activated emergency plans.
"We've been through about four or five (hurricanes), but this looks like it'll be the worst," Henry Burke, a vacation homeowner in Atlantic Beach, North Carolina, told Reuters.
"If you are in the projected path of this hurricane, you have to take precautions now .... don't wait, don't delay," Obama said, speaking from the island of Martha's Vineyard off the Massachusetts coast where he is vacationing.
"We all hope for the best but we have to be prepared for the worst," added Obama, who will cut short his vacation by a day and head back to Washington Friday night.
As U.S. authorities ramped up preparations to cope with a potential major natural disaster on the densely populated East Coast, U.S. airlines cut at least 1,000 flights and were moving airplanes out of Irene's path.
Officials are taking every precaution with Irene because they remember all too well how Hurricane Katrina in 2005 swamped New Orleans, killing up to 1,800 people and causing $80 billion in damage.
WIND FIELD "HUGE"
Irene weakened early on Friday to a Category 2 hurricane from a 3 on the five-step Saffir-Simpson scale, but it still was carrying winds of up to 100 miles per hour.
It was expected to remain a hurricane as it sweeps up the mid-Atlantic coast from Saturday but the Miami-based hurricane center said it could dip below hurricane strength before reaching New England. But its impact would not vary much.
At 5 p.m. (2100 GMT), Irene's center was churning northward 265 miles south-southwest of Cape Hatteras, North Carolina.
Irene, the first hurricane of the 2011 Atlantic season, had already caused as much as $1.1 billion in insured losses in the Caribbean this week, catastrophe modeling company AIR Worldwide said, with more losses expected to come.
New York City's mass transit system, which serves 8.5 million riders a weekday, will shut around noon on Saturday ahead of Irene's arrival, Governor Andrew Cuomo said in a statement.
The NHC said hurricane force winds extended outward up to 90 miles from Irene's center, while tropical storm force winds extended out to 290 miles, giving the storm a vast wind field width of nearly 600 miles.
"The wind field is huge," U.S. National Hurricane Center Director Bill Read told Reuters Insider.
In earlier comments, NHC chief Read said Irene, which will be the first significant hurricane to affect the populous U.S. Northeast in decades, would lash the eastern seaboard with tropical storm-force winds and a "huge swath of rain" from the Carolinas to New England.
He said North Carolina would start seeing tropical storm conditions on Friday afternoon. Cities like Washington, Baltimore, Philadelphia and New York could experience heavy rain and wind and power outages from the weekend.
"WATCHING THAT BIG WHITE SWIRL"
Homeland Security Secretary Janet Napolitano urged East Coast residents not to delay precautions. "The window of preparation is quickly closing," Napolitano said.
"This is a big, bad storm," North Carolina Governor Bev Perdue told CNN.
Maryland Governor Martin O'Malley told the TV network: "Anyone who thinks this is just a normal hurricane and they can stick it out is being ... selfish and stupid."
Wall Street firms scrambled to raise cash into early next week in case Irene causes major disruption in trading. Bond trading volume dropped precipitously by noon on Friday.
Traders were "watching that big white swirl" on their television sets, said Guy LeBas, chief fixed income strategist at Janney Montgomery Scott in Philadelphia.
Northeast oil, natural gas and power facilities also made preparations.
Brent crude oil futures rose in choppy trade on Friday as Irene targeted the U.S. East Coast and traders weighed comments by Federal Reserve Chairman Ben Bernanke on the economy.
North Carolina, Virginia, Maryland, Delaware, New Jersey, New York and Connecticut have declared emergencies.
Irene will be the first hurricane to hit the U.S. mainland since Ike pounded Texas in 2008.
In Washington, Irene forced the postponement of Sunday's dedication ceremony for the new memorial honoring civil rights leader Martin Luther King Jr. Tens of thousands of people, including President Barack Obama, had been expected to attend.
Flooding from Irene killed at least one person in Puerto Rico and two in Dominican Republic. The storm knocked out power in the Bahamian capital, Nassau, and blocked roads with trees.
(Reporting by Jane Sutton, Tom Brown, Manuel Rueda in Miami, Daniel Trotta, Basil Katz, Richard Leong, Joan Gralla, Lynn Adler, Ben Berkowitz in New York; Jeremy Pelofsky and Vicki Allen in Washington, Laura MacInnis and Alister Bull on Martha's Vineyard; Writing by Pascal Fletcher; Editing by Philip Barbara)