The centre of Hurricane Debby is expected to bring potential record-setting rains, catastrophic flooding and life-threatening storm surge to the Big Bend coast of Florida before it moves slowly across the northern part of the state and stalls over the coastal regions of Georgia and South Carolina.

The National Hurricane Center of Miami says Debby is located about 72 kilometres west northwest of Cedar Key, Fla., with maximum sustained winds of 128 km/h.

The storm is moving north at 16 km/h. The hurricane centre says Debby is expected to make landfall around midday Monday. A tornado watch is in effect for parts of Florida and Georgia until 6 a.m. Monday.

Debby is the fourth named storm of the 2024 Atlantic hurricane season after Tropical Storm Alberto, Hurricane Beryl and Tropical Storm Chris, all of which formed in June.

Forecasters warned heavy amounts of rain from Debby could spawn catastrophic flooding in Florida, South Carolina and Georgia.

"Right now, we are trying to secure everything from floating away," said Sheryl Horne, whose family owns the Shell Island Fish Camp along the Wakulla River in St. Marks, Fla., where some customers moved their boats inland.

The sparsely populated Big Bend region in the Florida Panhandle also was hit last year by Hurricane Idalia, which made landfall as a Category 3 hurricane.

"I am used to storms and I'm used to cleaning up after storms," Horne said.

Debby was expected to move eastward over northern Florida and then stall over the coastal regions of Georgia and South Carolina, thrashing the region with potential record-setting rains totalling up to 76 centimetres beginning Tuesday.

Officials also warned of life-threatening storm surge along Florida's Gulf Coast, with 1.8 to three metres of inundation expected Monday between the Ochlockonee and Suwannee rivers.

Historic rainfall possible for Georgia, South Carolina

"There's some really amazing rainfall totals being forecast and amazing in a bad way," Michael Brennan, director of the hurricane centre, said at a briefing. "That would be record-breaking rainfall associated with a tropical cyclone for both the states of Georgia and South Carolina if we got up to the 30-inch [76-centimetre] level."

Flooding impacts could last through Friday and are expected to be especially severe in low-lying areas near the coast, including Savannah, Georgia; Hilton Head, South Carolina; and Charleston, South Carolina. North Carolina officials were monitoring the storm's progress.

Officials in Savannah said the area could see a month's worth of rain in four days if the system stalls over the region.

"This is going to a significant storm," Savannah Mayor Van. R. Johnson said during a news conference.

Roads flooded on island off Sarasota

Debby's outer bands grazed the west coast of Florida, flooding streets and bringing power outages. Sarasota County officials said most roadways on Siesta Key, a barrier island off the coast of Sarasota, were under water. The hurricane centre had predicted the system would strengthen as it curved off the southwest Florida coast, where the water has been extremely warm.

At a briefing Sunday afternoon, Florida Gov. Ron DeSantis warned the storm could lead to "really, really significant flooding that will happen in north central Florida."

The storm would follow a similar track to Hurricane Idalia but would "be much wetter. We are going to see much more inundation," he said.

A hurricane warning was issued for parts of the Big Bend and Florida Panhandle areas, while tropical storm warnings were posted for Florida's west coast, the southern Florida Keys and Dry Tortugas. A tropical storm watch extended farther west into the Panhandle.

Tropical storms and hurricanes can trigger river flooding and overwhelm drainage systems and canals. Forecasters warned of 15 centimetres to 30 centimetres of rain and up to 46 centimetres in isolated areas of Florida.

Storm surge expected to hit Gulf Coast, including Tampa Bay

Flat Florida is prone to flooding even on sunny days, and the storm was predicted to bring a surge of 0.6 to 1.2 metres along most of the Gulf Coast, including Tampa Bay, with a storm tide of up to 2.1 metres farther north in the Big Bend region.

DeSantis declared a state of emergency for 61 of Florida's 67 counties, with the National Guard activating 3,000 guard members. Utility crews from in and out of state were ready to restore power after the storm, he said in a post on X.

In Tampa alone, officials gave out more than 30,000 sandbags to barricade against flooding.