* Isaac forecast to become hurricane on Thursday
* Tampa mayor plays down threat to Republican Convention
* Storm cancels Guantanamo hearings in 9/11 case
By Tom Brown
MIAMI, Aug 22 (Reuters) - Tropical Storm Isaac swirled over the Caribbean on Wednesday and was forecast to become a hurricane as it moved on a track that would put it off the coast of Florida on Monday, the opening day of the Republican National Convention in Tampa.
Isaac was already dumping heavy rains on the Leeward Islands on Wednesday afternoon and hurricane watches were in effect for many places, including Puerto Rico and the U.S. and British Virgin Islands.
Isaac could also potentially threaten U.S. energy interests in the Gulf of Mexico, weather experts said. It was centered about 75 miles (120 kilometres) east-northeast of Dominica early on Wednesday afternoon, the U.S. National Hurricane Center (NHC) said.
Isaac had top sustained winds of 45 miles per hour (72 kph) and was forecast to become a hurricane by Thursday, as it neared the coast of Hispaniola, the island shared by the Dominican Republic and flood-prone Haiti. Computer forecast models showed the storm moving west-northwest across the island early Friday.
There was still a lot of uncertainty about the storm's path after its projected passage over Cuba on Saturday and Sunday. But computer models showed it making landfall somewhere in South Florida by late Sunday or early Monday.
At the Guantanamo Bay U.S. Naval Base in southeast Cuba on Wednesday, authorities said Isaac had forced the postponement of pretrial hearings that were to begin on Thursday for five prisoners accused of plotting the Sept. 11 attacks.
The U.S. military was preparing evacuation flights for Thursday for the lawyers, paralegals, interpreters, journalists, rights monitors and family members of 9/11 victims who had traveled to the base for the hearings.
Forecasters said it is far too soon to gauge Isaac's potential impact on Tampa on Florida's Gulf Coast, where the Republican Convention is set to run from Monday through Thursday.
But Lixion Avila, a senior hurricane specialist at the Miami-based NHC, suggested it would be foolish for anyone to think Tampa -- where Republicans will nominate Mitt Romney as their presidential candidate -- was out of harm's way.
"With the convention or without the convention, I can tell you this is Aug. 22, hurricane season, and normally anywhere in Florida or the Gulf of Mexico we should monitor any system that forms," he said.
Tampa Mayor Bob Buckhorn, a Democrat, said he wasn't really worried about Isaac, however.
"We're watching it. We're tracking it. I think we're going to be OK but we'll be prepared in the event it heads this way," Buckhorn told CNN.
"We hope it moves further away from us, but if it doesn't it's still going to be a great convention," he said.
Hurricane expert Jeff Masters of private forecaster Weather Underground said Tampa had a 9 percent chance of getting hit with tropical storm-force winds for a 24-hour period ending on the morning the Republican Convention kicks off. But that could make the storm a non-event in terms of the convention itself.
"I put the odds of an evacuation occurring during the convention in the current situation at 3 percent," Masters said in his blog on the weatherunderground.com website.
OJ PRICE JUMP
Florida has not been hit by a major hurricane since 2005 and no one is forecasting that Isaac will strengthen into anything more than a weak Category 1, with top sustained wind speeds of about 80 mph (129 kph).
Still, the threat to Florida triggered a jump in orange juice prices on Wednesday, as they surged to a six-week high in early trading in New York.
Florida produces more than 75 percent of the U.S. orange crop and accounts for about 40 percent of the world's orange juice supply, making it key to volatility in orange juice futures trading.
The Atlantic hurricane season runs from June 1 through Nov. 30, and August has traditionally been an active month in the six-month period. Friday is the 20th anniversary of Hurricane Andrew, which caused widespread damage when it came barreling ashore south of Miami on Aug. 24, 1992.
Lurking behind Isaac, the NHC said another tropical depression formed over the eastern tropical Atlantic on Wednesday, about 860 miles (1,385 km) west-southwest of the Cape Verde Islands. It was packing winds of 35 mph (56 kph) and will take the name Joyce if it becomes a tropical storm.
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