Irene nears hurricane strength off Puerto Rico

SAN JUAN, Puerto Rico (Reuters) - Tropical Storm Irene strengthened as it barreled across the Caribbean toward Puerto Rico on Sunday on a course that could take it to Florida later in the week.

Irene, the ninth named storm of the 2011 Atlantic hurricane season, could strengthen into a hurricane before hitting Puerto Rico early on Monday, forecasters at the U.S. National Hurricane Center said.

It would be the first hurricane of the busy -- but so far not destructive -- 2011 Atlantic hurricane season.

"We have to take this seriously," Puerto Rico Governor Luis Fortuno said Sunday evening.

At 11 p.m., Irene's top winds had strengthened to 70 miles per hour (110 km per hour), up from 50 mph earlier in the day. That was just under the 74 mph threshold to become a hurricane.

Irene was moving west-northwest. Its center was about 50 miles southeast of San Juan, Puerto Rico, but Irene was a large storm and conditions had begun to deteriorate over the eastern part of the island, the forecasters said.

The storm was expected to weaken on Monday as it moved over the Dominican Republic, then strengthen again on Tuesday as the it emerged over water and moved toward the Bahamas.

Irene pelted the Leeward Islands with heavy rain and squalls as it crossed from the Atlantic Ocean into the northeastern Caribbean Sea on Sunday.

Hurricane watches and warnings were in effect in the U.S. Virgin Islands, Puerto Rico, the Dominican Republic and the central Bahamas. Tropical storm watches and warnings were in effect for the Virgin Islands, Haiti, the Turks and Caicos Islands and the southeastern Bahamas.

The storm sprang to life on Saturday night. Puerto Rico lifted a ban on Sunday morning shopping, allowing stores to open so residents could stock up on canned food, bottled water and other necessities. Prices were frozen and alcohol sales were halted until after the storm passes.

Schools and government offices were closed for Monday in the U.S. territory of 3.9 million people. Shelters were opened but few people had sought refuge in them by nightfall.

Fortuno rushed back to Puerto Rico on Sunday from North Carolina, where he was named chairman of the Southern Governors Association. He expressed concern that the storm's erratic path was leading people to ignore its potential harm.

He urged residents in flood zones to go quickly to government storm shelters and those with secure homes to stay inside and off the roads. The island was expected to get up to 10 inches of rain, making driving hazardous.

"This could put your life in danger," Fortuno said.

WATCHING IRENE'S PATH

In the Dominican Republic, authorities warned of abnormal waves up to 15 feet high. Weeks of heavy rainfall have already caused deadly flooding in the Dominican Republic and authorities said they may issue evacuation orders for vulnerable areas on Monday.

Residents of the southeastern United States were urged to monitor Irene's progress as the storm headed their way.

Computer forecast models showed Irene moving northwest over the Dominican Republic and then heading toward the Florida peninsula.

Depending on its eventual path and possible turns, Irene might still pose a threat to U.S. oil and gas installations in the Gulf of Mexico, but forecasters say it is too early to predict with certitude.

An early northward turn would bring it near the Georgia-South Carolina coast late in the week but a later turn could take it over central Cuba and into the Gulf of Mexico.

Tropical Storm Harvey hit Belize in Central America on Saturday and weakened to a tropical depression as it moved inland. It crossed into Mexico's Bay of Campeche on Sunday and forecasters said it could briefly strengthen back into a tropical storm before hitting the southern coast of Mexico.

Mudslides and flooding could affect agricultural output in Central America, but this year's coffee and sugar harvests are largely complete.

(Reporting by Reuters in San Juan, with additional reporting by Pascal Fletcher and Jane Sutton in Miami and Manuel Jimenez in Santo Domingo; editing by Bill Trott)