The hurricane centre forecast life-threatening conditions, including storm surges of up to 3m in some areas.
As it slowly moves north through the week, the storm may bring “potentially historic rainfall” of between 25cm and 50cm, and catastrophic flooding to Georgia and South Carolina, it said.
“This is going to be the story of this storm,” said Jamie Rhome, the deputy director of the hurricane centre. “It’s slow motion is going to dump historic amounts of rainfall – potentially over 20 inches. You’re talking about catastrophic flooding.”
The storm could be like Hurricane Harvey, which slammed into Corpus Christi, Texas, in August 2017. While downgraded into a tropical storm as it moved inland, it lingered over the state, dumping about 127cm of rain on Houston.
Harvey is rated as one of the wettest storms in US history, causing more than 100 deaths and US$125 billion in damage, primarily from flooding in the Houston metropolitan area.
Rhome said Debby is fuelled by exceptionally warm Gulf waters.
Climate scientists believe man-made global warming from burning fossil fuels has raised the temperature of the oceans, making storms bigger and more devastating.
Preparing for Debby, Florida governor Ron DeSantis called up 3,000 National Guard troops and placed most of the state’s cities and counties under emergency orders, while mandatory evacuations were ordered in parts of the Gulf Coast counties of Citrus, Dixie, Franklin, Levy and Wakulla.
DeSantis said there are more than 17,000 linemen and other electric workers ready to restore power.
The governors of Georgia and South Carolina also declared states of emergency ahead of the storm.
Heavy rain
Debby became a tropical storm late on Saturday. As of 5pm local time, Debby was about 193km west of Tampa and moving toward the Gulf Coast at 19kph, with maximum sustained winds of 100kph, the NHC said.
To become a hurricane, sustained winds must reach 74mph.
The centre of Debby will move across the eastern Gulf of Mexico through Sunday and reach the Florida Big Bend coast by midday on Monday, the hurricane centre added. Debby is then expected to move slowly across northern Florida and southern Georgia on Monday and Tuesday, it said.
The storm is expected to lose some strength after landfall but bring heavy rain as it crosses central Florida out to the Atlantic coast, before crawling up to Savannah, Georgia, and then onward to Charleston, South Carolina, early in the week.
Ocean surges forecast for Bonita Beach northward to Tampa Bay could send sea waves further inland than normal, damaging structures and endangering anyone in their path.
A tropical storm warning is in effect for the Florida and Georgia coasts from Ponte Vedra Beach to the Savannah River, including the Fort Myers area, which was crushed by Hurricane Ian in 2022.
The last hurricane to make a direct hit on the Big Bend region was Hurricane Idalia, which briefly gained Category 4 strength before making landfall as a Category 3 in August 2023, with winds of more than 201kph. The National Centers for Environmental Information estimates it did US$3.5 billion in damage.
Forecasters expect a large number of Atlantic hurricanes in the 2024 season, which began on June 1, with four to seven seen as major. That exceeds the record-breaking 2005 season that spawned the devastating Katrina and Rita hurricanes.
Only one hurricane, Beryl, has yet formed in the Atlantic this year. The earliest Category 5 storm on record, it struck the Caribbean and Mexico’s Yucatan peninsula before rolling up the Gulf Coast of Texas as a Category 1 storm, with sustained winds up to 153kph.