President Barack Obama today warned Americans to take the approaching storm Irene seriously and urged them to obey orders to evacuate from the path of what is likely to be an "extremely dangerous and costly" event.
"All indications point to this being a historic hurricane," Mr Obama said in a statement to reporters from the farm where he is on holidays on an island off the Boston coast.
Fifty-five million people are potentially in Hurricane Irene's path, from the Carolinas to Cape Cod on the US east coast, and tens of thousands are evacuating as cities including New York brace for the powerful storm to hit.
"I cannot stress this highly enough. If you are in the projected path of this hurricane, you have to take precautions now. Don't wait. Don't delay," Mr Obama said.
Mr Obama and his family will leave Martha's Vineyard tomorrow, well before Irene is projected to scour its way this far north on the New England coast. But the vacation had already been marked by natural disaster, after the island felt an earthquake in Virginia on Tuesday that shook Washington and New York.
Mr Obama has held regular conference calls with aides on the storm, in a determined effort to learn from the mistakes of his predecessor George Bush, who was slammed for an ineffectual response after Hurricane Katrina swamped New Orleans in 2005.
"All of us have to take this storm seriously ... If you are given an evacuation order, please follow it," Mr Obama said.
New York, the United States' biggest city with a population of more than eight million, ordered residents in low-lying areas to evacuate before the onslaught.
Hundreds of thousands of residents and vacationers were already evacuating from Irene's path, starting in eastern North Carolina where Irene is expected to make landfall tomorrow, packing winds of 100 miles per hour (155 km/hour).
Irene's wide swirling bands are lashing the seabord with winds and rain from the Carolinas to New England.
A quarter of a million New Yorkers may have to leave their vulnerable homes if, as predicted, Irene triggers dangerous storm surge and flooding in the city and farther east on Long Island.
"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 5pm (2100 GMT) tomorrow afternoon.
US federal and state leaders, from Mr Obama downward, urged the millions of Americans in the hurricane's path to prepare and to heed evacuation orders if they received them.
The National Hurricane Center said hurricane force winds extended outward up to 90 miles (150 km) from Irene's centre, while tropical storm force winds extended out to 290 miles (465 km), giving the storm a vast wind field width of nearly 600 miles (960 km).
"The wind field is huge," US director Bill Read told Reuters Insider.
At 2 pm (1800 GMT), its centre was churning northwards 300 miles (480 km) south-southwest of Cape Hatteras, North Carolina.
Residents stocked up on food and water and worked to secure homes, vehicles and boats. States, cities, ports, industries, oil refineries and nuclear plants scrambled to activate emergency plans.
"I filled my tank up with gas in case I need to leave in a hurry or something, and get a lot of food supplies, taking everything out of my yard ... anything that can fly into a window," said Patricia Stapleton of Newport, North Carolina.
Irene lashed the low-lying Bahamas yesterday and was expected to hit North Carolina tomorrow before heading up the coast to New York and beyond.
This morning, Irene had sustained winds of 185km/h and its centre was about 740km south-southwest of Cape Hatteras, North Carolina.
Coastal evacuations were under way in North Carolina and were ordered for beach resorts in Virginia, Delaware and Maryland. Airlines began to cut flights at eastern airports, made plans to move aircraft from the region and encouraged travellers to consider postponing trips.
US Homeland Security Secretary Janet Napolitano today warned citizens to expect extensive power outages from Hurricane Irene when it hits.
After hitting North Carolina, Irene is expected to weaken to a still-dangerous Category 2 storm on the five-step Saffir-Simpson scale, with winds of up to 180km/h.
"All the major metropolitan areas along the northeast are going to be impacted," National Hurricane Center Director Bill Read said. "Being a large hurricane, tropical storm-force winds will extend far inland."
Virginia governor Bob McDonnell urged residents to seek shelter by tonight, before the winds kick up. "Saturday is going to be a horrendous day for travel. There will be roads and bridges closed," he said.
Anticipating severe storm damage in North Carolina, US president Barack Obama declared an emergency yesterday, authorizing federal aid to support that state's response. The governors of North Carolina, Virginia, Maryland, Delaware, New Jersey, New York and Connecticut also declared emergencies.
Even if the centre of Irene stays offshore as it tracks up the coast, its heavy winds and rain could lash cities like Washington, Baltimore, Philadelphia and New York and knock out power, forecasters said.
Irene will be the first hurricane to hit the US mainland since Ike pounded Texas in 2008.
New York Mayor Michael Bloomberg said the most populous US city was bracing for storm conditions and flooding starting on Sunday.
He urged residents of vulnerable areas to move to safety today because the mass transit system, the nation's biggest with 8 million passengers a day, may have to shut if flooding or high winds endanger its buses, subways and commuter trains.
Many New Yorkers do not have cars, so mass transit could be vital in evacuations.
Long Island, the populous area that extends about 160km east into the Atlantic Ocean from New York City, could be hit hard if Irene stays on its current track.
In Washington, Irene forced the postponement of Sunday's dedication ceremony for the new memorial honouring civil rights leader Martin Luther King Jr. Tens of thousands of people, including Mr Obama, had been expected to attend.
Flooding from Irene killed at least one person in Puerto Rico and two in the Dominican Republic. The storm knocked out power in the Bahamian capital Nassau and blocked roadways with fallen trees.
Reuters