Florida bracing for 'unsurvivable' Hurricane Helene

MIAMI - 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 on Thursday evening, ahead of landfall expected around 11pm, the US National Hurricane Center (NHC) said. 

It was packing winds of 215km/h 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. 

The NHC warned of up to 51cm 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.