Tampa, Fla., the site of the Republican National Convention, remains vulnerable in the event of a storm surge, bound by water to the south and west that could put much of the city under water. NBC's Kerry Sanders reports.
By NBC News, Weather.com and wire reports
Updated at 1:15 a.m. ET: Tropical Storm Isaac brought rain and gusty winds to Puerto Rico on Thursday as it passed south of the U.S. territory. It is forecast to remain a tropical storm after crossing into the Dominican Republic and Haiti and then passing over Cuba into the Florida Straits, the National Hurricane Center said.
Forecasters said it was too soon to gauge Isaac's potential impact on Tampa on Florida's Gulf Coast, where the Republican National Convention is to run from Monday through Thursday.
Related: Track Tropical Storm Isaac
Some computer models showed Isaac shifting slightly to the west and eventually moving parallel to Florida's western coastline. Others forecast the storm to make landfall in South Florida and then move inland.
Isaac was centered late Thursday evening about 145 miles southwest of the Dominican Republic's capital of Santo Domingo, the National Hurricane Center said. The storm had top sustained winds of 45 miles per hour.
The storm is 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 Hurricane Center said.
The Bahamas issued tropical storm warnings for its southeastern areas; Cuba issued tropical storm warnings for several provinces, including Guantanamo.
The largest threat was the potential for extremely heavy rainfall over the islands near Isaac's path, weather.com reported.
The Red Cross was ready in Haiti to help evacuate people to shelters and camps but was in a "wait and see" mode, Jerry Anderson, senior director of international services, told NBC News.
Residents and visitors of the northern Caribbean, Yucatan Peninsula, southeastern United States and the central/eastern Gulf Coast should watch the progress of Isaac closely over the next week or more, weather.com reported.
With nearly 400,000 people still living in evacuation tents, a hurricane or even a tropical storm could lead to deaths and more damage to the already fragile country. NBC's Mark Potter reports.
Get the latest on this story from weather.com
As the storm approached, Puerto Rico Governor Luis Fortuno declared a state of emergency, canceled classes, closed government agencies and activated the National Guard.
The government also froze prices on basic necessities such as food, batteries and other supplies and prepared emergency shelters at schools and other facilities.
Heavy rainfall, flooding and mudslides will be threats in all of the northern Caribbean islands regardless of how strong the system becomes, weather.com reported.
Isaac may also threaten U.S. energy interests in the Gulf of Mexico, weather experts said.
From weather.com: Isaac's looming US threat
At the Guantanamo Bay U.S. Naval Base in southeast Cuba on Wednesday, authorities said Isaac 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 flights to evacuate the base of lawyers, paralegals, interpreters, journalists, rights monitors and family members of 9/11 victims who had traveled there for the hearings.
In Key West, Fla., Mayor Craig Cates told NBC News that officials were monitoring Isaac but hadn't made any decisions about evacuating because the storm hadn't yet strengthened. All agencies were preparing in case an evacuation would be needed if the storm gets strong, he said.
The tropical storm may also cause damage in Tampa, Fla., where the GOP convention will take place. Weather Channel meteorologist Jim Cantore reports.
From weather.com: Track Isaac's path
Coordinating Meteorologist Tom Moore at The Weather Channel said it was difficult to predict how Isaac would affect Tampa when the Republican National Convention to nominate Mitt Romney gets under way on Monday.
Because the storm was tracking farther south than earlier predictions, it could track to the west side of the Gulf of Mexico over the weekend, he said. That would leave Tampa facing rain and thunderstorms with20-30 mph winds gusting up to 40 mph on Monday.
Tropical Storm Isaac churns over Caribbean, could threaten GOP convention
Orange juice prices rise
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.
The threat to Florida triggered a nearly 6 percent jump in orange juice prices on Wednesday as they surged to a six-week high in 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.
Lurking behind Isaac, the hurricane center said Thursday another tropical depression grew into Tropical Storm Joyce, the 10th named storm of the Atlantic hurricane season.
Located about 1,280 miles east of the Leeward Islands, it was packing winds of 40 mph and was moving northwest at 14 mph on Thursday afternoon.
Forecasts predicted it will eventually veer toward the open Atlantic and away from the Caribbean. No coastal watches or warnings were in effect for Joyce.
Reuters and weather.com contributed to this report.
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