Tropical Storm Isaac, which formed Tuesday in the Atlantic Ocean, is expected to become a hurricane over the next few days and drench a string of Caribbean nations, including Puerto Rico, Haiti and possibly Cuba.

At 11 p.m., the National Hurricane Center also extended its forecast cone to include South Florida, with Isaac potentially approaching the coast late Sunday or early Monday — but uncertainty remained about its path and potential power.

Robbie Berg, a hurricane specialist at the center, said some conflict remained in the computer models, with one pushing it on a more westerly track that could keep Isaac south of Cuba. The forecast track could change over the next few days.

“It really is a little bit too soon to say whether it will affect Florida,’’ Berg said.

Isaac was moving west at 18 mph with maximum winds of 40 mph. Forecasters expect the storm to reach hurricane strength by Thursday as it approaches Puerto Rico and continue west until Friday, when Isaac would be off the coast of Hispaniola. Puerto Rico, the Dominican Republic and Haiti could see heavy rains, flooding and mud slides.

Isaac’s direction from there will be determined by the timing and strength of a high-pressure ridge to its north and a trough digging down from the southeastern U.S. that could steer it more north, Berg said. Conditions favor Isaac’s development and a stronger storm would be more likely to push north, he said. But interaction with the mountains of Haiti or Cuba also could weaken the system or even rip it apart.

Tropical storm warnings were in effect for Martinique, Dominica, Guadeloupe, St. Martin, St. Kitts, Nevis, Antigua, Montserrat, Anguilla, Saba and St. Maarten. A tropical storm watch is in effect for Puerto Rico, the British Virgin Islands and the U.S. Virgin Islands.