The move prompted a warning from the National Hurricane Center in Miami that the storm will likely intensify to a Category 3 storm with winds of 115 mph over the Bahamas by Thursday.
INTERACTIVE: Track the hurricanes of 2011
"Residents of Florida and the southeastern U.S. should be keeping their eyes on this storm and have their hurricane supplies ready now," hurricane center spokesman Dennis Feltgen says. "No one wants to be standing in line at the hardware store when a storm hits."
More than a million people in Puerto Rico saw their power cut off Monday, as Irene passed about 105 miles west of San Juan. No injuries were reported. The storm also walloped the British Virgin Islands, bringing lightning that was the likely culprit in a fire that destroyed billionaire Richard Branson's Caribbean home. Guests, including the Academy Award-winning actress Kate Winslet, escaped the fire uninjured.
The center of the storm's 500-mile-wide projected path now is on track to hit South Carolina on Saturday, although Feltgen says its path could change. In the best case, a low-pressure center could push the storm away from the coast late in the week.
The National Weather Service said odds were higher that Irene would make landfall as a Category 1 hurricane rather than Category 3.
"No one should just look at the center of the track and think they are clear," Feltgen says. "A major storm covers a lot of area."
Warmer-than-normal Gulf Stream waters and light upper-air winds off the U.S. coast are expected to spur the intensification of the storm.
The hurricane center projects a 70% chance of three to five major storms this hurricane season, which started on June 1. No hurricane has hit U.S. coasts since Hurricane Ike in 2008, which killed 112 people in Galveston, Texas.
Florida and the Carolinas have largely escaped hurricanes since the heavy storms of 2005.
"That might breed complacency," says hurricane scientist Phil Klotzbach of Colorado State University in Fort Collins. "But we have the heart of the season right ahead of us."