Powerful Hurricane Irene could pose a big threat to the densely populated northeast United States, including New York, as it swings up the eastern seaboard from Saturday on its current forecast track, the top U.S. government hurricane forecaster said on Wednesday.

National Hurricane Center Director Bill Read gave the warning as Irene, now a major Category 3 storm with winds of 115 miles per hour, roared through the Bahamas on a path that will take it to the U.S. East Coast by the weekend.

As it swept over the southeastern Bahamas, its winds tore off home roofs and knocked out power, authorities reported.

Some tourists fled Nassau and other resorts in the low-lying Atlantic archipelago, while others hunkered down in hotels, and cruise lines canceled their Bahamas stops for the next few days.

Forecasters see the first hurricane of the 2011 Atlantic season, turning north from Thursday to miss Florida and Georgia and clip North Carolina's Outer Banks region on Saturday afternoon. Irene is then seen taking a coast-hugging track up the U.S. mid-Atlantic and New England coastline.

States from the Carolinas northward were on alert and evacuations were already underway in some of North Carolina's most exposed Outer Banks barrier islands, such as Ocracoke.

The exact center of the storm may actually stay pretty close to the coastline during the day on Saturday and then become a big threat for New England and perhaps Long Island toward 96-100 hours out on Sunday, Read said.

And be advised, it's going to be a very large circulation as it moves north of the Carolinas, he added on a conference call with U.S. Federal Emergency Management Agency Administrator Craig Fugate.

Earlier on Wednesday, Irene strengthened over the Bahamas to a major Category 3 hurricane on the five-step Saffir-Simpson intensity scale, posing a high risk of injury and death from flying and falling debris.

Even if the center of the storm stays offshore as it tracks up the mid-Atlantic coast, the hurricane could lash cities including Washington and New York with winds and rain and cause coastal flooding and power outages.

Irene will be a large storm, impacting areas far from the storm center track, hurricane expert Jeff Masters of private forecaster Weather Underground wrote in his blog.

As forecasts of more than four days can have a margin of error of up to 200 miles, U.S. emergency officials have warned the entire eastern U.S. coast to be on the alert

East Coast residents were already jittery after a 5.8 magnitude earthquake rattled Washington and cities from the Carolinas to Canada on Tuesday.

At 11 a.m. EDT, Irene was located about 285 miles southeast of Nassau.