The storm, which was responsible for at least two deaths, was expected to become a hurricane by the time it reaches the Florida Keys late Sunday, the National Hurricane Center in Miami said. Scott, who visited the Broward County Emergency Operations Center, said the path "is not positive for our state," The Miami Herald reported.
The hurricane center said in its 11 p.m. advisory that Isaac was centered about 65 miles east-northeast of Camaguey, Cuba, and 340 miles east-southeast of Key West, Fla.
The storm was producing sustained winds of 60 mph up to 205 miles from its center and was heading northwestward at a 17 mph clip.
The U.S. forecasters said Isaac was expected to stay on its current track for the next two days, moving along or just north of the central coast of Cuba Saturday night and move near or over the Florida Keys Sunday or Sunday night, reaching the southeastern Gulf of Mexico early Monday.
Isaac is forecast to be at or near hurricane strength when it reaches the Florida Keys.
In Haiti, Isaac brought heavy rain and flooding in Port-au-Prince, the capital, and other towns. While the island, with its steep easily eroded hillsides sustained less damage than from previous storms, at least two deaths were reported, including a 10-year-old girl killed by a collapsing wall in Thomazeau, CNN reported.
Many houses in the Cite de Soleil neighborhood in Port-au-Prince were underwater.
"I wasn't able to save anything; everything's gone," a woman identified by the Herald as Marc Henri-Louis said.
In South Florida, tourists were encouraged to leave the Keys and emergency shelters were set up. Local officials in Miami-Dade and Broward counties ordered schools to be closed Monday.
Organizers of the Republican National Convention decided Saturday to postpone convention activities from Monday to Tuesday afternoon because of Isaac's threat.
Rain was expected to begin in earnest in the Tampa area Sunday and winds would top 20 miles per hour early in the week, the National Weather Service said.
NASA said in a written statement the storm forced postponement of the launch of a scientific space probe at Cape Canaveral to Aug. 30, and Vice President Joe Biden earlier scrubbed his planned campaign trip to Tampa.
A tropical storm warning was invoked for the west coast of Florida from Bonita Beach to Tarpon Springs. The warning for Haiti was discontinued.
A tropical storm watch was in place up to the Florida Panhandle to Indian Pass.
A hurricane warning was in effect for the Florida Keys, including Dry Tortugas, the west coast of Florida from Bonita Beach southward to Ocean Reef, and Florida Bay.
A hurricane watch was in effect for Florida's east coast from Golden Beach southward to Ocean Reef, as well as for Andros Island in the Bahamas.
Tropical storm warnings remained in place for the Bahamas, Turkos and Caicos Islands, and parts of Cuba, including Guantanamo.