RALEIGH, N.C. — A powerful Hurricane Earl threatened to sideswipe much of the East Coast just ahead of Labor Day, worrying countless vacationers who planned to spend the traditional last week of summer at the beach.

The Federal Emergency Management Agency warned people along the Eastern Seaboard to prepare for possible evacuations and islanders in the Turks and Caicos hunkered down in their homes Tuesday as the Category 4 hurricane steamed across the Caribbean with winds of 135 mph.

Earl was expected to remain over the open ocean before turning north and running parallel to the East Coast, bringing high winds and heavy rain to North Carolina’s Outer Banks by late Thursday or early Friday. From there, forecasters said, it could curve away from the coast somewhat as it makes it way north, perhaps hitting Massachusetts’ Cape Cod and the Maine shoreline on Friday night and Saturday.

“My guests are calling and they don’t know what to do and I don’t know what to tell them,” said Dave Dawson, owner of the oceanfront Cape Hatteras Motel in Buxton, N.C.

Forecasters cautioned that it was still too early to tell how close Earl might come to land. But not since Hurricane Bob in 1991 has such a powerful storm had such a large swath of the East Coast in its sights, said Dennis Feltgen, spokesman for the National Hurricane Center.

“A slight shift of that track to the west is going to impact a great deal of real estate with potential hurricane-force winds,” Feltgen said.

Even if Earl stays well offshore, it will kick up rough surf and dangerous rip currents up and down the coast through the Labor Day weekend, a prime time for beach vacations, forecasters said.

The approaching storm troubled many East Coast beach towns that had hoped to capitalize on the BP oil spill and draw visitors who normally vacation on the Gulf Coast.

On Monday, Earl delivered a glancing blow to several small Caribbean islands, tearing roofs off homes and knocking out electricity to people in Anguilla, Antigua and St. Maarten. In Puerto Rico, nearly 187,000 people were without power and 60,000 without water, Gov. Luis Fortuno said. Cruise ships were diverted and flights canceled across the region. But there were no reports of deaths or serious injuries.

On Tuesday, gusty winds from Earl’s outer fringes whipped palm fronds and whistled through doors in the Turks and Caicos Islands as tied-down boats seesawed on white-crested surf.

Islanders gathered to watch big waves pound a Grand Turk shore as the wind sent sand and salt spray flying.

“We can hear the waves crashing against the reef really seriously,” Kirk Graff, owner of the Captain Kirks Flamingo Cove Marina, said by telephone as he watched the darkening skies. “Anybody who hasn’t secured their boats by now is going to regret it.”

In the U.S., FEMA administrator Craig Fugate said state and local authorities may need to order evacuations along the Eastern Seaboard later this week if the storm does not veer away as expected.

“Today is the day to make sure you have your plan completed and your supplies in place,” he said.

As of late Tuesday afternoon, Earl was centered about 150 miles east of Grand Turk island — and 1,000 miles southeast of Cape Hatteras, N.C. — as it headed northwest at 14 mph. Close on its heels was Tropical Storm Fiona, which was not expected to reach hurricane strength for at least several days.

Carl Hanes of Newport News, Va., kept an eye on the weather report as he headed for the beach near his rented vacation home in Avon, N.C. He, his wife and their two teenage children were anticipating Earl might force them to leave on Thursday, a day ahead of schedule.

“We’re trying not to let it bother us,” Hanes said before enjoying the calm surf.

In Rehoboth Beach, Del., Judy Rice said she has no plans to leave the vacation home where she has spent most of the summer. In fact, the Oak Hill, Va., resident plans to walk around town in the rain if it comes.

“I kind of enjoy it actually. You know, it’s battling the elements,” Rice said. “I have seen the rain go sideways, and, yeah, it can be scary, but I have an old house here in Rehoboth, so it’s probably more important that I am here during a storm than anywhere.”

In the Florida Panhandle, which has struggled all summer to lure back tourists scared away by the Gulf oil spill, bookings were up 12 percent over last year at the Sandestin Golf and Beach Resort. The resort is nowhere near Earl’s projected path, and spokeswoman Laurie Hobbs said she suspects the increase in reservations was partly because of a discount the hotel is offering and partly because of the hurricane.

“Weather drives business,” she said. “They go to where the weather is best.”

If Earl brings rain farther inland, it could affect the U.S. Open tennis tournament, being played now through Sept. 12 in New York City.

“We’re keeping our eye on it very closely,” said United States Tennis Association spokesman Chris Widmaier. “It’s still a little early to tell how it will track and we’re hoping it will stay off the coast.”

On the coast of southern Maine, about 15 people have already canceled Labor Day reservations at Burnette’s Campground in York because of concerns about Earl, said owner David Woods.

But a Labor Day weekend washout won’t have the impact it would have had a decade ago.

“Labor Days have changed in the past 10 years,” he said. “It used to be the big bang end to summer, but now from the 15th of August until the first of September, it sort of dwindles off.”