(Reuters) - Tropical Storm Isaac unleashed heavy rain and winds off Puerto Rico and the Virgin Islands as it moved across the Caribbean on Thursday and could strengthen into a hurricane before tearing across the Dominican Republic and Haiti.
Isaac also posed a threat to Florida, where it is forecast to pass near the state's Gulf Coast on Monday as the Republican National Convention starts in Tampa.
Authorities have not ruled out the possibility of postponing or relocating the Republican convention if the storm takes direct aim at Tampa. But Craig Fugate, director of the Federal Emergency Management Agency, said the convention was not his biggest concern, at least for now.
"People are spending a lot of time talking about that," Fugate said of the convention. "I wish they'd be talking about making sure people in the (Florida) Keys are getting ready and that people in southwest Florida are getting ready," he told CNN.
The storm could also affect U.S. energy interests in the Gulf of Mexico, with analysts at Weather Insight, a Thomson Reuters company, giving it a 50 percent probability of moving into the heart of the oil and gas production region.
After passing through the Caribbean, Isaac is forecast to -strengthen again near Florida on Monday to a Category 1 hurricane, the lowest on the five-step Saffir-Simpson scale, but with an unclear path. Some computer models show the storm's track swinging farther west into the Gulf of Mexico.
"Significant uncertainty remains about the threat Isaac poses to Florida and other portions of the Gulf Coast," the U.S. National Hurricane Center said.
It said Isaac was centered about 190 miles south-southwest of San Juan, Puerto Rico, on Thursday evening and was moving westward at 16 miles per hour.
The storm had top sustained winds of 45 mph. The Miami-based hurricane center said Isaac could become a hurricane on Friday as it nears Hispaniola, the island shared by the Dominican Republic and Haiti.
Isaac was expected to dump between 8 and 12 inches of rain over parts of Hispaniola, with total accumulations up to 20 inches in some areas, the NHC said, posing a significant threat to Haiti, which is highly prone to flooding and mudslides because of its near-total deforestation.
Haiti, the poorest country in the Americas, still has about 350,000 people living in tents or makeshift shelters more than 2 1/2 years after a devastating earthquake that took more than a quarter of a million lives.
Authorities in the Dominican Republic began evacuating people living on the banks of rivers, streams and areas vulnerable to landslides in preparation for the approach of Isaac, whose effects were beginning to be felt with showers in the south of the country.
In the U.S. commonwealth of Puerto Rico, officials also braced for flooding as Isaac slowed down but swung farther south of the island than initially predicted.
"Our big worry is flooding," said Governor Luis Fortuno, who ordered schools and government offices to remain closed for a second day.
"We are preparing for the worst and hoping for the best," he said.
Many computer forecast models showed Isaac tracking west of the Florida Peninsula after passing over Cuba en route to a landfall farther north.
"Any slight westward versus eastward deviation makes a huge difference for Florida," said Phil Klotzbach, a research scientist who heads the respected hurricane forecast team at Colorado State University.
Several forecast a final landfall in the Florida Panhandle, in the northwest corner of the state, although one model put the storm moving almost directly over Tampa.
Republican convention planners said they would continue to monitor the storm closely while staying in close contact with the National Weather Service, Governor Rick Scott, local emergency officials and the campaign of presumptive Republican presidential nominee Mitt Romney.
Florida has not been hit by a major hurricane since 2005 and forecasts showed Isaac was not expected to strengthen beyond a weak Category 1, with top sustained wind speeds of about 80 mph.
In Haiti, Red Cross workers toured crowded tent camps home to Haitians left homeless by the 2010 quake to warn about Isaac.
Text messages were sent out to tens of thousands of people urging them to stay away from rivers and evacuate tent camps in case the storm hits.
Red Cross teams, equipped with shelter and sanitation kits, deployed to distribute emergency supplies, including cooking equipment, water chlorination kits, and plastic sheeting and wood for temporary shelters, said Florent Del Pinto, Haiti head of operations for the International Federation of the Red Cross and Red Crescent Societies.
Camp residents will likely receive orders to evacuate on Friday and move to government-designated shelters. "But there are not enough shelters for them all," said Del Pinto, adding the shelters - schools, churches and other concrete buildings - could only handle about 50 percent of the camp residents.
In 2004, Tropical Storm Jeanne killed hundreds and flooded the port city of Gonaives with 7 feet of water in places, destroying roads and bridges and virtually cutting it off from the rest of the country.
In addition to Isaac, the hurricane center said Tropical Storm Joyce formed on Thursday over the open Atlantic.
The 10th named storm of the Atlantic hurricane season, Joyce was about 1,280 miles east of the Caribbean's Leeward Islands and had maximum sustained winds of 40 mph. It was not forecast to gain hurricane strength.