An increasingly powerful hurricane threatening "catastrophic," dangerous storm surges and flooding was forecast to smash into Florida's Gulf coast on Thursday, as thousands of residents evacuated towns along the US state's shoreline.

Helene strengthened into a hurricane mid-morning Wednesday in the Gulf of Mexico and is "expected to bring life-threatening storm surge, damaging winds, and flooding rains to a large portion of Florida and the Southeastern United States," the National Hurricane Center in Miami said in its latest bulletin.

Maximum sustained winds have increased to 85 miles (137 kilometers) per hour with higher gusts, as the storm moves north at 12 mph.

"Strengthening is forecast, and Helene is expected to be a major hurricane when it reaches the Florida Big Bend coast Thursday evening," the NHC added.

The storm now has the potential to roar ashore as an intensely powerful Category 4 hurricane, on the five-level Saffir-Simpson scale, potentially with sustained winds of 130 miles per hour, the center said.

"A catastrophic and deadly storm surge is likely along portions of the Florida Big Bend coast, where inundation could reach as high as 20 feet (six meters) above ground level, along with destructive waves," according to the NHC.

The storm also has potential to "penetrate well inland," it added. Several states are in the warning cone, and Atlanta, a Georgia metropolis hundreds of miles from the Gulf Coast and whose region is home to five million people, is forecast to experience close to tropical storm-force winds and heavy rain into Friday.

President Joe Biden was briefed on the storm Wednesday.

"The entire Biden-Harris Administration stands ready to provide further assistance to Florida, and other states in the path of the storm, as needed," the White House said in a statement.

Florida Governor Ron DeSantis has issued a state of emergency for nearly all of Florida's 67 counties, including Miami-Dade. He has mobilized the National Guard and positioned thousands of personnel to prepare for possible search and rescue operations and power restoration.

"The impacts are going to be far beyond the eye of the storm," DeSantis said as he urged Floridians to rush preparations to completion and evacuate if ordered.

Helene earlier lashed Mexico's Yucatan peninsula, home to multiple tourist hotspots, including Cancun.

By Wednesday afternoon, the storm's outer bands were buffeting the southern Florida Keys.

Sixteen Florida counties have announced mandatory partial evacuation orders, while two have ordered the evacuation of all residents.

DeSantis said at least 62 health care facilities, from hospitals to nursing homes, have already begun evacuations.

Whole state bracing

A "direct impact" was likely in the Tallahassee region, where coastal communities already looked like ghost towns by Wednesday afternoon.

In Crawfordville, potentially in the storm's direct path, wheelchair-bound residents of the Eden Springs Nursing and Rehab Center were being placed on coach buses for evacuation.

Other locals were seen loading up on gas and supplies, filling sandbags and boarding up homes and businesses.

Communities across a wide swath of northwest Florida—including Tampa Bay, an area of more than three million residents—faced the dangerous threats of storm surge, heavy rain and fierce winds.

In St. Petersburg, adjacent to Tampa, cars lined up at supply donation or distribution centers while people filled sandbags.

Teacher Lorraine Major, seen making her own preparations, has lived in Florida her whole life. "You get used to it," she said of the multiple storms and hurricanes that batter her state every year.

"But these last couple of years, the hurricanes are getting really, really bad," the 44-year-old told AFP in St. Petersburg.

In nearby Clearwater, resident Jasper MacFarland laid sand bags at his house entrance.

"I expect the water to come up and I just don't want it to get in the house," he said.

A 250-mile stretch of coastline, essentially from Tampa Bay to just shy of Panama City, on the Florida panhandle is under hurricane warning.

If forecasts are confirmed, Helene would become the most powerful hurricane to hit the US in more than a year.

Category 3 Hurricane Idalia hit northwestern Florida in August 2023.

Researchers say climate change likely plays a role in the rapid intensification of storms, because there is more energy in a warmer ocean for them to feed on.

© 2024 AFP