TAMPA, Fla. — Tropical Storm Debby was strengthening rapidly Sunday and was expected to become a hurricane as it churned through the Gulf of Mexico toward Florida, bringing heavy bands of rain to that state and with it the threat of devastating floods to the southeast Atlantic coast later in the week.

The storm was likely to become a strong Category 1 hurricane before making landfall around midday Monday in the Big Bend area of Florida, the National Hurricane Center in Miami said. A tornado watch was also in effect for parts of Florida and Georgia until 6 a.m. Monday.

“Right now, we are trying secure everything from floating away,” said Sheryl Horne, whose family owns the Shell Island Fish Camp along the Wakulla River in St. Marks, Florida, where some customers moved their boats inland. The Big Bend region 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 totaling up to 30 inches (76 centimeters) beginning Tuesday. Officials also warned of life-threatening storm surge along Florida’s Gulf Coast, with 6 to 10 feet of inundation expected Monday between the Ochlockonee and Suwannee rivers.

“There’s some really amazing rainfall totals being forecast and amazing in a bad way,” Michael Brennan, director of the hurricane center, 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 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. The word historic cannot be underscored here,” Savannah Mayor Van. R. Johnson said during a press conference.

The hurricane center said in a 8 p.m. update that Debby was located about 100 miles west of Tampa, Florida, with maximum sustained winds of 70 mph. The storm was moving north at 12 mph. Tropical storm force winds extended up to 140 miles from the center.

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.

At a briefing Sunday afternoon, Florida Gov. Ron DeSantis warned that the storm could lead to “really, really significant flooding that will happen in North Central Florida.”

He said it would follow a similar track to Hurricane Idalia but would “be much wetter. We are going to see much more inundation.”