Fast-moving Hurricane Helene was advancing across the Gulf of Mexico toward Florida on Thursday, threatening a “catastrophic” storm surge in northwestern parts of the state as well as tornadoes, damaging winds, rains and flash floods hundreds of miles inland across much of the southeastern US, forecasters said.

Helene strengthened into a major Category 4 hurricane on Thursday as it barrelled across the Gulf of Mexico on a path to Florida, according to the US National Hurricane Centre.

Tropical storm-force winds have already started hitting the state in advance. Hurricane warnings and flash flood warnings extended far beyond the coast up into south-central Georgia.

The governors of Florida, Georgia, the Carolinas and Virginia have all declared emergencies.

In Florida, Governor Ron DeSantis said that models suggest Helene will make landfall further east, lessening the chances for a direct hit on the capital city of Tallahassee, whose metro has a population of around 395,000.

The shift has the storm aimed squarely at the sparsely-populated Big Bend area, where rain began to blow in on Thursday morning along the coastal US Highway 98, which winds through fishing villages and vacation hideaways in the region where Florida’s panhandle and peninsula meet.

Shuttered gas stations dotted the two-lane highway, their windows boarded up with plywood.

Mandatory evacuation orders stretched from the panhandle south along the Gulf Coast in low-lying areas around Tallahassee, Gainesville, Cedar Key, Lake City, Tampa and Sarasota.

The National Weather Service office in Tallahassee forecast storm surges of up to 20 feet (6 meters) and warned they could be particularly “catastrophic and unsurvivable” in Florida’s Apalachee Bay.

It added that high winds and heavy rains also posed risks.

“This forecast, if realised, is a nightmare surge scenario for Apalachee Bay,” the office said. “Please, please, please take any evacuation orders seriously!”

Helene was about 255 miles (405 kilometres) southwest of Tampa on Thursday morning and moving north-northeast at 14 mph (22 kph) with top sustained winds of 105 mph (165 kph). Forecasters said it should become a Category 3 or higher hurricane, meaning winds would top 110 mph (177 kph).

President Joe Biden approved an emergency declaration on Thursday for Georgia after issuing one for Florida earlier in the week. Federal authorities have positioned generators, food and water, along with search-and-rescue and power restoration teams.