Tropical Storm Isaac headed towards the Dominican Republic and Haiti on Friday, rumbling slowly west across the Caribbean after unleashing heavy rain on parts of Puerto Rico.

In Haiti, Isaac was threatening thousands of people still living in tents after a devastating earthquake more than two years ago. The storm also posed a threat to Florida, where it could pass near the state's Gulf Coast on Monday as the U.S. 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. 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 was 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 advancing 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 Center (NHC) said.

But the center 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 165 miles south of the Dominican Republic's capital, Santo Domingo, and about 230 miles southeast of Port-au-Prince, the Haitian capital, as of Friday morning.

"Poorly organized Isaac jogs westward," it said in its latest advisory, putting its speed at 15 mph and forecasting a turn towards the west-northwest later in the day.

"The center of Isaac will move near or over Hispaniola today ... and move near or over southeastern Cuba on Saturday."

RISK OF HURRICANE

The storm had top sustained winds of 45 mph (75 km per hour) but the Miami-based hurricane center said Isaac could swell into a hurricane later on Friday as it nears Hispaniola, the island shared by the Dominican Republic and Haiti, but weaken as it moves over land.

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.

"These rains could cause life-threatening flash floods and mud slides," the center said in its advisory.

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.

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.

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.

NOT ENOUGH SHELTERS IN HAITI

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.

"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 predicted 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.

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 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 that 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 of people 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.

Over the open Atlantic, Tropical Storm Joyce remained a weak tropical depression on Friday. It was expected to regain tropical storm strength on Monday while on a path that may take it close to Bermuda.