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Tropical Storm Isaac is headed toward the Dominican Republic and Haiti, continuing its march across the Caribbean after unleashing heavy rain on parts of Puerto Rico.
Isaac also poses a threat to Florida, where it could pass near the state's Gulf Coast on Monday (local time) 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.
"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.
Isaac is forecast to remain a tropical storm after crossing the Dominican Republic and Haiti and then passing over Cuba into the Florida Straits.
Many forecast models show it eventually taking a track into the Gulf of Mexico, strengthening into a hurricane and possibly making landfall near Alabama, the Florida Panhandle, Louisiana or Mississippi on Tuesday.
"Isaac will likely restrengthen when it moves over the Florida Straits and the eastern Gulf of Mexico," the U.S. National Hurricane Centre said.
But the centre warned it was "important not to focus on the exact track because of forecast uncertainties and the fact that Isaac has a large area of tropical storm force winds."
It said Isaac was centered about 235km south-southeast of the Dominican Republic's capital, Santo Domingo, late Thursday night.
The storm had top sustained winds of 75kmh. The Miami-based hurricane centre said Isaac could become a hurricane on Friday as it nears Hispaniola, the island shared by the Dominican Republic and Haiti, but weaken as it moved over land.
Isaac was expected to dump between 20cm and 30cm of rain over parts of Hispaniola, with total accumulations up to 50cm 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 after Isaac passed south of the island.
The NHC said Isaac was moving westward at 30kmh.
"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.
Some forecast models predict a final landfall in the Florida Panhandle, in the northwest corner of the state, and several showed the storm passing near Florida's Gulf Coast where Tampa is located.
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 130kmh.
In Haiti, Red Cross workers toured crowded tent camps of 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.
Some camp residents will likely 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 (2 metres) of water in places, destroying roads and bridges and virtually cutting it off from the rest of the country.
Over the open Atlantic, Tropical Storm Joyce weakened into a depression. It was expected to regain tropical storm strength on Monday while on a path that may take it close to Bermuda.