The U.S. National Hurricane Center NHC.L has had a tough time projecting the exact track and long-term intensity of the large and poorly organized storm, which has drawn close scrutiny because the Republican Party's presidential nominating convention begins on Monday in Tampa on Florida's Gulf coast.

The storm has been slow to strengthen and the NHC is no longer forecasting Isaac to rev up to hurricane force until long after it crosses Haiti, where thousands of people are still living in tents and makeshift shelters after a devastating earthquake more than 2-1/2 years ago.

Isaac's projected track has drifted westward since Thursday, potentially increasing the storm's threat to U.S. energy interests in the Gulf of Mexico.

But Jerry Paul, the senior meteorologist at Weather Insight, a Thomson Reuters company, said he was still giving Isaac no more than a 50 percent probability of moving into the heart of the oil and gas production region.

"So far, it doesn't look like it's just going to plow through the production region itself," he said.

The NHC said Isaac was centered 220 miles (355 km) southeast of the Haitian capital Port-au-Prince on Friday morning, packing top sustained wind speeds of 50 miles (80 km) an hour.

The storm was moving west at about 15 mph (24 kph) and its center was forecast to be near western Haiti about 2 a.m. EDT (0600 GMT) on Saturday.

Flash-flooding and mudslides, which are common in the poorest nation in the Americas, could add to the misery of about 350,000 quake survivors housed in tent cities and camps after the January 2010 disaster than killed more than 250,000 people.

"There are still too many people living in the camps," said France Hurtubise of the International Federation of Red Cross and Red Crescent Societies in Haiti. "There's a good chance that those might be destroyed with the passage of the cyclone," she said.

NHC meteorologist Rick Danielson said it was very difficult to determine the extent of the possible U.S. impacts from Isaac until the storm emerges from its projected passage over mountainous Cuba on Sunday and enters the Florida Straits.

TAMPA STILL THREATENED

Danielson said the Florida Keys, the island chain off the southernmost part of the state, were highly vulnerable, however.

Isaac's landfall as a hurricane could come anywhere along the Gulf Coast between the Florida Panhandle in the northwest of the state to New Orleans, Danielson said.

Tampa, on Florida central Gulf Coast, is still in the NHC's so-called "cone of uncertainty." That's the area that could feel the full force of Isaac, as it ramps up to projected hurricane force on Tuesday.

Danielson said the threat to Tampa was still difficult to gauge but it could potentially be hit by coastal flooding and driving winds or rain.

"There is still a full range of possible impacts on Tampa at this point," Danielson told Reuters.

Authorities are keeping a close eye on Isaac. But so far, at least, there are no plans to postpone the four-day convention where former Massachusetts Governor Mitt Romney will receive the Republican presidential nomination.

Isaac was expected to dump between 8 and 12 inches (20 to 30.5 cm) of rain over parts of Hispaniola, the island shared by the Dominican Republic and Haiti.

Total accumulations up to 20 inches (51 cm) were expected in some areas, the NHC said. That poses a significant threat to Haiti, where deadly flooding is legion because of the Caribbean country's near-total deforestation.

There was a widespread sense of resignation among many Haitians late on Thursday, as many people got their first word of the approaching storm and Prime Minister Laurent Lamothe spoke about the government's "limited means" to ensure public safety.

"We live under tents. If there's too much rain and wind, water comes in. There's nothing we can do," said Nicholas Absolouis, an unemployed 34-year-old mechanic in a camp for homeless people on the northern edge of the chaotic capital.

"If he's coming, he's already on the way," added Juliette Jean-Baptiste, 26, another resident of the camp. "Our tents leak already," she said.

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 85 mph (137 kph).

The NHC's Danielson stressed that the storm was quite large, however, and said it could affect U.S. Gulf Coast residents as far west as Texas. (Additional reporting by Susana Ferreira in Port-au-Prince and David Adams in Miami)

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