A powerful hurricane was barreling toward Florida on Thursday, with officials warning of 鈥渦nsurvivable鈥� 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 鈥渆xtremely dangerous鈥� Category 4 hurricane Thursday evening, ahead of landfall expected around 11pm (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鈥檚 warm waters toward the Big Bend area south of Florida鈥檚 capital city Tallahassee.

鈥淓VERYONE along the Florida Big Bend coast is at risk of potentially catastrophic storm surge,鈥� 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鈥檚 west coast already flooded.

About 125,000 homes and businesses were without power.

鈥淲e鈥檙e expecting to see a storm surge inundation of 15 to 20 feet above ground level,鈥� NHC director Mike Brennan said. 鈥淭hat鈥檚 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.鈥�

The accompanying waves 鈥渃an destroy houses, move cars, and that water level is going to rise very quickly,鈥� Brennan added.

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

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

鈥淚f it stays on course, this is going to look different afterwards, that鈥檚 for sure,鈥� 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 鈥渘ot going anywhere,鈥� the 58-year-old insisted.

鈥淚 am going to hunker down鈥� 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.

鈥淭his will be one of the most significant weather events to happen in the western portions of the area in the modern era,鈥� it warned.

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

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

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

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

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

Vice President Kamala Harris said the White House was watching.

鈥淭he 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,鈥� 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.

鈥淲e can鈥檛 control how strong this hurricane is going to get. We can鈥檛 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鈥檚 going to be safe.鈥�

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

Hurricane specialist Michael Lowry called Helene 鈥渆xtreme,鈥� 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.