Floridians on Wednesday had one final day to evacuate or hunker down ahead of the Category 5 Hurricane Milton, potentially one of the most destructive ever to hit the Gulf Coast of Florida.

With more than one million people in coastal areas under evacuation orders, those fleeing for higher ground clogged highways and gas stations ran out of fuel, further rattling a region still recovering from the devastating impacts of Hurricane Helene less than two weeks ago.

The storm was on a collision course for the Tampa Bay metropolitan area, home to more than three million people, though forecasters said the path could vary before the storm makes landfall late on Wednesday night or early Thursday morning.

The storm is on a rare west-to-east path through the Gulf of Mexico and is likely to bring a deadly storm surge of three metres or more to much of Florida's Gulf Coast.

Officials from US President Joe Biden to Tampa Mayor Jane Castor warned people in evacuation zones to get out or risk death.

Michael Tylenda, who was visiting his son in Tampa, said he was heeding that advice.

"If anybody knows anything about Florida, when you don't evacuate when you're ordered to, you can pretty much die," Tylenda said. "They've had a lot of people here stay at their homes and they end up drowning. It's just not worth it. You know, the house can be replaced. The stuff can be replaced. So it's just better to get out of town."

Milton packed maximum sustained winds of 260 kph, the US National Hurricane Centre said, putting it at the highest level on the five-step Saffir-Simpson scale.

While wind speeds could drop and downgrade Milton to a lesser category, the size of the storm was growing, putting ever more coastal areas in danger.

Milton was expected to maintain hurricane strength as it crosses the Florida peninsula, posing storm surge danger on the state's Atlantic Coast as well.

About 2.8 percent of US gross domestic product is in the direct path of Milton, said Ryan Sweet, chief US economist at Oxford Economics.

Airlines, energy firms and the Universal Studios theme park were among the companies beginning to halt their Florida operations as they braced for disruptions.

Milton became the third-fastest intensifying storm on record in the Atlantic, growing from a Category 1 to a Category 5 in less than 24 hours.

"These extremely warm sea surface temperatures provide the fuel necessary for the rapid intensification that we saw taking place to occur," said climate scientist Daniel Gilford of Climate Central, a nonprofit research group.

"We know that as human beings increase the amount of greenhouse gasses in the atmosphere, largely by burning fossil fuels, we are increasing that temperature all around the planet."

More than a dozen coastal counties issued mandatory evacuation orders, including Tampa's Hillsborough County. Pinellas County, which includes St. Petersburg, ordered the evacuation of more than 500,000 people.

Lee County said 416,000 people lived in its mandatory evacuation zones.

Mobile homes, nursing homes and assisted living facilities also faced mandatory evacuation.

Bumper-to-bumper traffic choked roads leading out of Tampa on Tuesday, when about 17 percent of Florida's nearly 8,000 gas stations had run out of fuel, according to fuel markets tracker GasBuddy. (Reuters)