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.
After passing through the Caribbean, Isaac is forecast to -strengthen again near Florida on Monday to a Category 1 hurricane, the lowest on the five-step Saffir-Simpson scale, but with an unclear path. Some computer models show the storm's track swinging farther west into the Gulf of Mexico.
"Significant uncertainty remains about the threat Isaac poses to Florida and other portions of the Gulf Coast," the U.S. National Hurricane Center said.
It said Isaac was centered about 190 miles (305 km) south-southwest of San Juan, Puerto Rico, on Thursday evening and was moving westward at 16 miles per hour (26 kph).
The storm had top sustained winds of 45 mph (75 kph). The Miami-based hurricane center said Isaac could become a hurricane on Friday as it nears Hispaniola, the island shared by the Dominican Republic and Haiti.
Isaac was expected to dump between 8 and 12 inches (20 to 30.5 cm) of rain over parts of Hispaniola, with total accumulations up to 20 inches (51 cm) 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.