Tropical Storm Debby expected to rapidly strengthen to a Category 1 hurricane and could bring historic rainfall to Southeast

(CNN) — Tropical Storm Debby is now forecast to rapidly intensify into a hurricane before it makes landfall in Florida’s Big Bend region and bring potentially historic amounts of rainfall to parts of the Southeast, due to near-record warm waters in the Gulf of Mexico.

Authorities in Florida, Georgia and South Carolina are urging residents to prepare as the storm makes its way through the Gulf, with Debby now expected to undergo rapid intensification , which occurs when a storm’s top-end winds increase 35 mph in 24 hours. It could become a hurricane by Sunday evening after being upgraded to a tropical storm a day prior, according to the National Hurricane Center.

Debby has sustained winds of 65 mph — up 30 mph from a day ago — and is located about 125 miles west-southwest of Tampa, Florida, according to the National Hurricane Center’s 2 p.m. ET Sunday update, warning that the threats of heavy rain and storm surge could lead to flooding in the coming days.

Hurricane conditions are expected to arrive by Monday morning, with the outer bands of the storm system making their way on shore during the day Sunday. The storm is forecast to reach the coast of Florida’s Big Bend early Monday, at which point Debby is expected to then crawl across northern Florida and southern Georgia throughout the day and into Tuesday, the hurricane center said.

The main threat will be flooding, both from storm surges up to 10 feet and heavy rainfall. Freshwater flooding, which is caused by rainfall, has become the deadliest aspect of tropical systems in the last decade, according to research conducted by the National Hurricane Center — a threat made more dangerous as the world warms from fossil fuel pollution.

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The strengthening storm tracking up the Florida Peninsula’s western coast prompted county and state officials to issue a string of voluntary and mandatory evacuation orders as the hurricane center posted hurricane watches and warnings across several parts of the state, including near Tampa and the Big Bend region.

Tropical storm and storm surge watches have also been issued for coastal Georgia and parts of South Carolina. The cities of Savannah, Georgia, and Charleston, South Carolina, could both see a month’s worth of rain in a single day — and perhaps even an entire summer’s worth of rain over the course of the storm.

A tornado watch has also been issued for much of the Florida Peninsula and parts of southern Georgia until Sunday night, covering more than 13 million people, including the cities of Tallahassee, Jacksonville, Tampa, and Orlando.

Florida Gov. Ron DeSantis , Georgia Gov. Brian Kemp and South Carolina Gov. Henry McMaster have declared states of emergency for their states in advance of the storm’s arrival. DeSantis on Sunday said in a news conference that he’d activated the Florida National Guard so it would be poised to assist with humanitarian needs as well as search and rescue.

DeSantis called on residents to finish their preparations and to brace for power outages, “particularly in parts of the state like here in Tallahassee.”

“There’s going to be a lot of trees that are going to fall down. You’re going to have debris. You are going to have power interruption,” the governor said, “so just prepare for that.”

President Joe Biden on Sunday approved a disaster declaration for Florida, the White House announced, authorizing federal resources to respond to any disaster relief efforts.

Storm expected to intensify over Gulf

The slower Debby moves and the longer it sits over warm waters, the more likely the storm is to intensify. Studies have shown tropical systems are slowing down over time, making them more likely to produce greater rainfall totals over a given area.

Oceans are also getting warmer and supercharging storms , pumping them full of moisture. A 2022 study published in the journal Nature Communications found climate change increased hourly rainfall rates in tropical storms by 5 to 10% and in hurricanes by 8 to 11%.

“Conditions are favorable for strengthening over the Gulf of Mexico with warm sea surface temperatures and light shear. Intensification is likely to be slow during the first 12–24 hours, then proceed at a faster rate after the cyclone develops an organized inner core,” the National Hurricane Center said of Debby.

By early Monday, Debby is expected to move into the Apalachee Bay area of Florida as it moves northward over the Gulf , according to the Weather Prediction Center.

The Apalachee Bay area, which includes parts of Taylor, Jefferson, Wakulla, and Franklin counties, can expect to get drenched with heavy rain from Debby on Sunday, increasing the possibility of flash flooding in several spots, the hurricane center said.

In the meantime, county officials have urged residents in communities along Florida’s Gulf Coast to evacuate ahead of the storm. Mandatory evacuation orders are in effect for parts of Franklin, Citrus and Levy counties, with voluntary orders issued in Hernando, Taylor and Pasco counties.

“I am worried about the aftermath and seeing how much damage we get (and) how we are going to fix it,” Sue Colson, the mayor of Cedar Key in Levy County, told CNN Sunday. The city sits on the island of Way Key in the Gulf of Mexico, about four miles off the coast. She cited high amounts of anticipated rain as well as the threat of storm surge.

“That is always concerning when you are a low-lying island in the middle of the Gulf,” she said.

On Saturday, Florida Highway Patrol knocked on doors to tell residents to consider leaving, Colson said. Residents were continuing to finish their preparations on Sunday morning.

“I think everybody needs to make wise decisions for themselves and not endanger others by endangering yourself,” she said. “If you’re endangering yourself, you are endangering others, because then they have to rescue you.”

Heavy rain could linger for days

As a slow-moving Debby churns along the Georgia-Carolina coastline heading into the new week, it could lead to seemingly endless amounts of rain for days, with totals potentially reaching over 2 feet.

The heaviest rain amounts could even top 30 inches or more, depending on how long Debby meanders, with some forecast models showing the storm could linger through at least Thursday. “This rainfall will likely result in areas of considerable flash and urban flooding, with significant river flooding expected,” the National Hurricane Center said.

Such exceptional rainfall would challenge state records for rain from a tropical cyclone: In Georgia, the record is 27.85 inches from 1994’s Alberto, while South Carolina’s record is 23.63 inches from Florence in 2018.

A warmer atmosphere holds more moisture and can dump heavier rain. Warmer oceans can fuel stronger hurricanes, packing a punch with higher storm surge thanks to sea-level rise.

With an uptick in the intensity forecast comes an increase in forecasted storm surge, which occurs when ocean water is pushed inland by the onshore winds of a hurricane. Storm surge flooding above ground could rise to 6 to 10 feet along Florida’s Big Bend, and coastal Georgia and South Carolina could see surges reach 2 to 4 feet.

Tampa Bay is expecting 2 to 4 feet of storm surge. Marco Island and other areas of southwest Florida will see 1 to 3 feet of storm surge.

Warmer air and ocean temperatures fueled by human-induced climate change can lead to wetter tropical systems.

The North Florida region nestled between the Panhandle and the rest of the state’s peninsula took a devastating hit last August from Category 3 Hurricane Idalia , and now faces a new threat from Debby.

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