ATLANTA (Bloomberg)—The U.S. East Coast has three days to get ready for a possible strike by Hurricane Earl, which has delayed flights in the Caribbean and halted ship loading at a refinery in the U.S. Virgin Islands.

Earl strengthened today to a Category 3 hurricane on the five-step Saffir-Simpson scale with winds of 125 mph about 120 miles east-northeast of San Juan, Puerto Rico, at 3 p.m. Eastern, according to a National Hurricane Center advisory. Earl is the season's second major hurricane.

Earl's projected track runs parallel to the East Coast. The storm may strike land anywhere from North Carolina northward, said Rick Knabb, hurricane expert for the Weather Channel in Atlanta.

“Along the mid-Atlantic and East Coast you basically have today, Tuesday, and Wednesday to prepare,” Mr. Knabb said in a telephone interview. “It could still be a powerful hurricane, and even if it doesn't come directly ashore it could be close enough for tropical storm-force winds.”

Earl is the Atlantic's fifth storm and second to form in the past nine days, which is when the most active part of the hurricane season begins. Trailing Earl is an area of disturbed weather that the hurricane center gives a 90% chance of becoming a tropical cyclone. Forecasters are watching a third system just off Africa.

“I think that people definitely realize now that we're turning into a hyperactive season, and the threats made before the season are starting to be realized,” said Jim Rouiller, a senior energy meteorologist at Planalytics Inc., a commercial forecaster in Berwyn, Pa.

Before the season, academic, government and commercial forecasters all predicted an above-average number of storms in 2010. A typical year has 11 storms with winds of at least 30 mph, according to the hurricane center.

The hurricane prompted warnings in the U.S. Virgin Islands, parts of Puerto Rico, St. Maarten and St. Martin, Saba and St. Eustatius, as well as Anguilla, St. Barthelemy and the British Virgin Islands.

Hovensa L.L.C.'s St. Croix refinery in the U.S. Virgin Islands is operating while the island ports are shut as Earl passes to the east.

The refinery sends finished products including heating oil and gasoline to the U.S. Gulf and East coasts, according to the company's website.

Forecasters cautioned of hurricane-like conditions within the warning area and said hoteliers and residents should have completed “preparations to protect life and property.”

As a result of the conditions, British Airways P.L.C. said its only flight today from the United Kingdom to Antigua was canceled. Antigua-based Leeward Islands Air Transport canceled “several flights” to the Caribbean island, according to the airline's website.

The storm may generate a surge of 5 feet above normal tide levels and produce rainfall of 8 inches in the affected islands and 12 inches in isolated higher elevations, the hurricane center said.

North Carolina, Massachusetts

Hurricane-force winds extend as much as 60 miles out from the storm and tropical storm-force winds as far as 185 miles, the center said. The storm is expected to turn toward the U.S. mid-Atlantic coast tomorrow.

Five-day storm tracks show hurricane-force winds from Earl brushing the North Carolina coast and Massachusetts shores Sept. 2-3 and then striking the Canadian Maritimes.

“We're so close to the coast now, all residents have to watch and be vigilant, because once it comes it is going to come quick and it is going to come hard,” Mr. Rouiller said.

Mr. Rouiller said the farther Earl moves to the west before turning north the more it is likely the hurricane will come ashore in the United States. It is moving west-northwest at 15 mph.

“Earl still represents a serious concern for the entire East Coast,” Mr. Rouiller said. “The tracking guidance may be off by a couple of hundred miles on days three and four.”

Weather forecasters urged beachgoers to stay out of rough storm surf, which has affected parts of the East Coast because of Hurricane Danielle.

Danielle continued to weaken over the cooler open North Atlantic, the center said today in another advisory. Danielle was packing sustained winds of 75 mph and was about 420 miles south-southeast of Cape Race, Newfoundland.

Danielle surpassed Hurricane Alex last week as the season's most intense storm. Alex had 105 mph winds when it hit northeastern Mexico on June 30 and dissipated over land.

Behind Earl looms what is likely to become storm Fiona. Another front just off Africa that may well become Hurricane Gaston, Mr. Rouiller said.