* Earl is second major hurricane of 2010 Atlantic season
* Maximum winds clocked at 135 mph (215 kph)
* Earl's fringes could pummel US east seaboard by Thursday
* Tropical Storm Fiona forms in Atlantic (Updates with Earl strengthening, Fiona forming)
By Pascal Fletcher
MIAMI, Aug 30 (Reuters) - Hurricane Earl strengthened into a powerful Category 4 storm on Monday after lashing the northeast Caribbean islands, and was expected to swipe the U.S. East Coast in the next few days, the U.S. National Hurricane Center said.
But the Miami-based forecasters said it was too early to say which part of the U.S. eastern seaboard might be impacted by Earl, the second major hurricane of the 2010 Atlantic season.
Earl had sustained winds of 135 mph (215 kph) and could strengthen in the next two days, the forecasters said.
The hurricane was moving west-northwest on a curving track that the National Hurricane Center said would take it near Cape Hatteras, North Carolina, on Thursday and Friday.
A direct hit could not be ruled out, and Earl was expected to bring drenching rain, dangerous seas and surf and gusting wind to the Atlantic Coast from North Carolina to New England and Canada, said Alex Sosnowski, a senior meteorologist for private forecaster AccuWeather.
"How nasty the weather gets in this region will depend on the exact track of Earl and its proximity to the coast," Sosnowski said in a posting on the AccuWeather website.
If Earl swings farther west than expected, heavy rain could sweep the Interstate 95 corridor from North Carolina to Washington, D.C., Philadelphia and New York City, he said.
On its current path, Earl posed no threat to the Gulf of Mexico, where major U.S. oil and gas installations are located.
Hovensa LLC said operations were normal at its 500,000 barrel-per-day refinery on the island of St. Croix but that the refinery's harbor and all other ports in the U.S. Virgin Islands had been closed because of Earl. [IDnN30225026)
At 5 p.m. EST (2100 GMT), the hurricane's center was 110 miles (180 km) northeast of San Juan, Puerto Rico.
Earlier, the hurricane buffeted the northernmost Leeward Islands of the Caribbean with fierce winds, driving rain and pounding waves as it passed.
The world's three largest cruise lines -- Carnival Corp (CCL.N), Royal Caribbean (RCL.N) and Norwegian Cruise Line APOLO.UL (GENH.SI) -- changed their Caribbean itineraries and rerouted at least seven ships to avoid the storm.
POWER OUTAGES, TREES TOPPLED
Residents on the island of St. Martin/St. Maarten, its two halves respectively administered by France and the Netherlands, said Earl's passage caused power outages and toppled trees.
"Now the wind is really blowing, incredibly strong ... I've seen a lot of tree damage ... I would certainly assume roofs off, I'm watching mine very carefully," Steve Wright, general manager of the Grand Case Beach Club in Grand Case, St. Martin, told Reuters.
"It's nothing that we haven't seen before but I'm surprised at the ferocity of the winds right now," he said.
There were no immediate reports of casualties.
In Antigua, some flooding in low-lying areas was reported. After the hurricane passed, Antigua and Barbuda Governor General Dame Louise Lake-Tack declared a national holiday to allow residents of the twin-island state to mop up.
The forecasters said hurricane conditions would gradually subside over Puerto Rico on Monday evening, while the Turks and Caicos Islands and the southeast Bahamas would get tropical storm conditions as Earl passed east of them in the next few days.
The ports of the U.S. Virgin Islands and the Puerto Rican ports of Vieques, Culebra, Fajardo, and San Juan were closed, the U.S. Coast Guard said. Government offices and schools in eastern Puerto Rico were shut.
Elsewhere in the Atlantic, the hurricane center said Tropical Storm Fiona had formed about 890 miles (1,435 km) east of the Leeward Islands. Fiona had top winds of 40 mph (65 kph) and was moving west at 24 mph (39 kph) on a course that was expected to take it northeast of the Leeward Islands on Wednesday and east of Bermuda by Sunday.
None of the forecast models took Fiona into the Gulf of Mexico.
In the North Atlantic, Hurricane Danielle, which was a major Category 4 storm last week, weakened to a tropical storm as its sustained winds fell to 70 mph (110 kph). It was expected to weaken further and lose its tropical characteristics later in the day. It was about 425 miles (685 km) south southeast of Cape Race, Newfoundland.
(For a factbox on 2010 hurricane forecasts, please see [ID:nN24250094])
(For more information about hurricanes and weather models, see: here; www.nhc.noaa.gov/ and www.skeetobiteweather.com/)
(Reporting by Pascal Fletcher, Tom Brown and Jane Sutton in Miami; Writing by Pascal Fletcher and Jane Sutton, Editing by Stacey Joyce)