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With maximum sustained winds of 80 mph, Hurricane Isaias is moving northwest from about 45 miles southeast of Great Inagua Island, according to the National Hurricane Center.
Central and southeastern Bahamas are under a hurricane warning, while the east coast of Florida including Miami, Ft. Lauderdale and West Palm Beach have a tropical storm watch in effect.
Rainfall will be the main concern over the next few days. South and east-central Florida could see 2 to 4 inches with isolated totals of 6 inches, NHC said.
Dominican Republic, northern Haiti, the Bahamas and Turks and Caicos could see 4 to 8 inches. This could lead to flash flooding, mudslides and potential riverine flooding.
The storm slammed Puerto Rico on Thursday, leaving 300,000 to 400,000 customers without power, National Weather Service San Juan meteorologist Gabriel Lojero told CNN. The 5 to 10 inches of rain brought flash floods and triggered mudslides on the island, Lojero said. One woman went missing after her car was swept away, he said.
"A lot of neighborhoods were submerged under water," Lojero said.
A major hurricane possibility during a pandemic
Florida closed some state-supported Covid-19 testing sites on Thursday in anticipation of the storm.
Drive-thru and walk-up testing sites were to close beginning at 5 p.m., the Florida Division of Emergency Management (FDEM) said in a statement Wednesday, "to keep individuals operating and attending the sites safe."
FDEM later said testing sites would remain open in 11 counties, nearly all of which are on the west coast or the Panhandle, based on the National Hurricane Center's last advisory.
The testing sites will remain closed until it's safe to reopen, though the FDEM anticipates all sites being opened by 8 a.m. August 5 at the latest.
Miami-Dade County Mayor Carlos Giménez told CNN on Thursday he would be concerned about keeping evacuees socially distanced if a major hurricane hit the state amid the ongoing pandemic.
"Look, if we have a major hurricane here, then we're going to have to evacuate a number of people and then we're going to have to ... try to keep them separated as much as possible," he said. "That's a concern."
"When you're not testing is also a concern," he added. "But the greater danger, the immediate danger has to be taken care of first, and that's getting our people out of harm's way."
Isaias -- the ninth named storm of 2020 -- is forecast to strengthen over the next 24 to 36 hours and become a hurricane sometime Friday or Friday night, the National Hurricane Center said Thursday afternoon.
There is uncertainty where and how strong the storm will be when it nears Florida
Where Isaias will hit and how intense it will be was still uncertain Thursday evening. Some forecast models show a weak storm hitting the southern coast of Florida, while others show a much stronger storm lashing the east side of the state and moving toward the Carolinas.
Isaias' official track does not bring the storm over Florida, but within 75 miles on Saturday and Sunday as it moves northward over the Bahamas. This would bring tropical storm conditions to the eastern portion of the Florida peninsula.
The NHC's forecast for Friday does show the hurricane directly over Grand Bahama Island, which was devastated by Hurricane Dorian less than a year ago.
How the storm interacts over the Hispaniola could impact the intensity of the storm. The NHC said Thursday morning that the storm would likely weaken as it passed over Hispaniola, but it could gradually strengthen as it moves away from the Greater Antilles.
"The eventual track will determine Isaias's strength and potential future development," CNN meteorologist Chad Myers said. "A track mainly over water will let the storm get stronger. A path more over land and the mountains of Hispaniola and Cuba will help to tear it apart."
However, once it gets over warmer waters, the storm could strengthen quickly like what was seen with Hurricane Hanna last weekend. That's something many models struggled to pick up on.
"We should have a better idea of how strong Isaias will become near the US after reconnaissance aircraft sample the storm and after it passes Hispaniola later today," the NHC said Thursday afternoon.
Why it was originally called Potential Tropical Cyclone Nine
Now that it has been given the name Isaias -- pronounced (ees-ah-EE-as) -- it is the earliest storm to begin with an "I" on record. The previous record was set on August 7, 2005, part of the busiest season to date.
This continues the record-breaking pace of the 2020 Atlantic hurricane season. Hurricane Hanna smashed the record for the earliest storm with an "H" name by 11 days.
There were tropical storm warnings issued before the storm even formed. The reason it was called "Potential Tropical Cyclone Nine" is because the storm did not have a round center of circulation, says Myers.
Instead, it was very elongated. "When a circular center finally formed, that is when they began to call it a tropical storm."
By calling it a potential tropical cyclone, it allowed countries to issue watches and warnings.
CNN's Joe Sutton contributed to this report.
