Alligator Point (United States) (AFP) - A powerful hurricane was barreling towards Florida on Thursday, with officials warning of “unsurvivable” conditions and a potentially catastrophic storm surge high enough to swamp a two-story 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 expected around 11.00 pm (0300 GMT), the US National Hurricane Center (NHC) said.

It was packing winds of 130 miles (215 kilometers) per hour as it churned over the Gulf’s warm waters towards the Big Bend area south of 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 have closed, with parts of St Petersburg, downtown Tampa, Sarasota, Treasure Island and other cities on Florida’s west coast already flooded.

About 525,000 homes and businesses were without power, according to tracking site PowerOutage.us.

“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 story 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.

- ‘I’m stuck with them’ -

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.

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

Most residents have bolted, 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 mega storm, blew through the Florida panhandle.

At a gas station in Panacea, John Luper said he was reluctantly staying put because his mother and brother were refusing to flee to higher ground.

“They’re not going to leave,” he said, filling jerry cans with fuel. “I’m stuck with them.”

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 went out across northern Florida, Georgia and the Carolinas.

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

- ‘Multi-state event’ -

“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 mobilized 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,” DeSantis said.

Helene could become the most powerful hurricane to hit the United States in over a year – and almost certainly the biggest, at nearly 500 miles across.

Scientists 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.