Officials in the US state of Florida are pleading with residents to heed mandatory evacuation orders or face life-threatening conditions, as the enormous Hurricane Helene advanced across the Gulf of Mexico towards the United States.

Hurricane Helene swamped parts of Mexico and has already brought tropical storm conditions to Florida, where one person was killed on Thursday evening after a sign fell on their car on a highway in Tampa amid violent winds and rain, according to the Florida Highway Patrol.

“EVERYONE along the Florida Big Bend coast is at risk of potentially catastrophic storm surge,” the National Hurricane Center (NHC) said on social media.

The NHC has upgraded Helene, which was expected to make landfall in Florida’s Big Bend region at about 11pm local time (3am GMT, Friday), to an “extremely dangerous” Category 4 with sustained winds near 130 miles per hour (209km/h).

“We’re expecting to see a storm surge inundation of 15 to 20 feet [4.5 to 6 metres] above ground level,” NHC director Mike Brennan said in a video briefing.

“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,” he said.

Satellite image shows Hurricane Helene churning through the Gulf of Florida, US, on September 26, 2024, in this screengrab from a video [Handout/NASA via Reuters]

Brennan said that waves accompanying the hurricane’s arrival “can destroy houses, move cars, and that water level is going to rise very quickly”.

Driving rain has flooded roadways, closed schools and airports and left about 698,700 homes and businesses without power in Florida, where a state of emergency has been declared.

Florida state authorities are providing buses to evacuate people from the Big Bend area, home to about 832,000 people, and taking them to shelters in the state capital, Tallahassee.

More than 55 million people in the US have been placed under some form of weather alert from Hurricane Helene.

States of emergency have also been declared in Georgia, North and South Carolina, Virginia and Alabama, as the NHC warned that much of the southeast could experience power outages, toppled trees and intense flooding.

In the southern Appalachian mountains, the National Weather Service has warned the region could be hit with landslides and flooding 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 said.

Only three Gulf hurricanes since 1988 – Irma in 2017, Wilma in 2005, and Opal in 1995 – have been bigger than Helene’s predicted size, according to Colorado State University hurricane researcher Phil Klotzbach, The Associated Press news agency reports.