Updated at 12:10 a.m. ET Friday
Isaias has become a hurricane as it approaches Florida.
The National Hurricane Center said late Thursday that the storm was threatening the Bahamas and had winds of 80 mph. It was about 70 miles east-southeast of Great Inagua Island, moving northwest at about 18 mph.
The storm's center is forecast to move near South Florida on Saturday. The hurricane center said "strengthening is forecast during the next day or so" with Isaias predicted to become a hurricane on Friday or Friday night.
Earlier Thursday, it slammed the Dominican Republic with heavy rainfall and strong winds.
A tropical storm watch is listed for the east coast of Florida from Ocean Reef to Sebastian Inlet and for Lake Okeechobee.
The center has warned of the risk of winds, heavy rainfall and storm surge this weekend along Florida's east coast and spreading northward along the U.S. East Coast through early next week.
"The details of the track and intensity forecast remain uncertain, and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. East Coast should monitor the progress of Isaias and updates to the forecast," it said.
In a briefing on Thursday afternoon, Florida Gov. Ron DeSantis said that while the storm remains off shore, it is "an evolving situation." He urged residents to make sure they had safety plans as well as seven days' worth of food, water and medicine.
Isaias drenched Puerto Rico and will drop 4 to 8 inches of rain on the Bahamas, the Dominican Republic, northern Haiti and Turks and Caicos, the National Weather Service said, with some areas receiving up to 12 inches. Cuba is also expected to get 1 to 2 inches of rain, with up to 4 in some areas.
The weather service said heavy rains may begin to affect eastern Florida over the weekend.
"This rain could result in isolated flash and urban flooding, especially in low-lying and poorly drained areas," it said.
Isaias is projecting hurricane-force winds outward for up to 30 miles, and tropical storm-force winds for up to 240 miles.
Concerns about the storm's impact on Florida prompted officials to suspend coronavirus testing at state facilities for several days, from Friday through at least Tuesday.