FLORIDA, United States (AFP) — A powerful hurricane was barrelling towards Florida on Thursday, with officials warning of “unsurvivable” conditions and a potentially catastrophic storm surge high enough to swamp a two-storey house.

Tens of thousands of people were without power and roads were already flooded ahead of what is expected to be one of the largest Gulf of Mexico storms in decades.

Fast-moving Helene strengthened to an “extremely dangerous” Category 4 hurricane Thursday evening, ahead of landfall, the US National Hurricane Center (NHC) said.

It was packing winds of 130 miles (215 kilometres per hour as it churned over the Gulf’s warm waters towards the Big Bend area south of Florida’s capital city Tallahassee.

“Everyone along the Florida Big Bend coast is at risk of potentially catastrophic storm surge,” the NHC said on social media.

Tampa and Tallahassee airports were closed, with parts of St Petersburg, downtown Tampa, Sarasota, Treasure Island and other cities on Florida’s west coast already flooded.

About 125,000 homes and businesses were without power.

“We’re expecting to see a storm surge inundation of 15 to 20 feet above ground level,” NHC director Mike Brennan said. “That’s up to the top of a second storey building. Again, a really unsurvivable scenario is going to play out here in this portion of the Florida coastline.”

The accompanying waves “can destroy houses, move cars, and that water level is going to rise very quickly”, Brennan added.

Evacuate or hunker down

In Alligator Point, a coastal town on a picturesque peninsula in the storm’s path, David Wesolowski was taking no chances.

“I just came to button up a few things before it gets too windy,” the 37-year-old real estate agent told AFP as he boarded up his house on stilts.

“If it stays on course, this is going to look different afterwards, that’s for sure,” he said, before taking his family to higher ground in Tallahassee.

Meanwhile, Patrick Riickert refused to budge from his small wooden house in Crawfordville, a town of 5,000 people a few miles inland.

As in Alligator Point, most residents have bolted and it looked like a ghost town, but Riickert, his wife and five grandchildren were “not going anywhere”, the 58-year-old insisted.

“I am going to hunker down” and ride out the hurricane, as he did in 2018 when deadly Hurricane Michael, a Category 5 megastorm, blew through the Florida panhandle.

The NHC warned of up to 20 inches (51 cm) of rain in some spots, and potentially life-threatening flooding as well as numerous landslides across the southern Appalachians.

The National Weather Service said the region could be hit extremely hard, with floods not seen in more than a century.

“This will be one of the most significant weather events to happen in the western portions of the area in the modern era,” it warned.

Tornado warnings, meanwhile, went out across northern Florida, Georgia and the Carolinas.

Georgia’s sprawling capital Atlanta was forecast to experience tropical storm-force winds and flash flooding from up to 12 inches of rain.

And Tennessee — more than 300 miles from the Gulf Coast — braced for tropical storm conditions statewide.

More than 55 million Americans were under some form of weather alert or warning from Hurricane Helene.

Life-threatening impacts

“This is going to be a multi-state event with the potential for significant impacts from Florida all the way to Tennessee,” Federal Emergency Management Agency administrator Deanne Criswell told reporters.

Vice-President Kamala Harris said the White House was watching.

“The president and I, of course, are monitoring the case and the situation closely, and we urge everyone who is watching at this very moment to take this storm very seriously,” she told reporters.

Florida Governor Ron DeSantis mobilised the National Guard and ordered thousands of personnel to ready for search-and-rescue operations.

He warned that the powerful storm would be dangerous, and urged everyone to take precautions.

“We can’t control how strong this hurricane is going to get. We can’t control the track of the hurricane, but what you can control is what you can do to put yourself in the best chance to be able to ride this out in a way that’s going to be safe.”

Helene could become the most powerful hurricane to hit the United States in over a year — and almost certainly the biggest.

Hurricane specialist Michael Lowry called Helene “extreme”, noting its tropical storm winds of 39 mph or higher stretched nearly 500 miles across.

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