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 storms ever to hit the Gulf Coast of Florida.

With over 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 Wednesday night or early Thursday morning, Reuters reported.

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 10 feet or more to much of Florida's Gulf Coast.

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

Milton packed maximum sustained winds of 260 kilometres per hour, 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.

At 10pm Central Daylight Time, the eye of the storm was 650 kilometres southwest of Tampa, moving northeast at 19kph.

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.

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."

Over a dozen coastal counties issued mandatory evacuation orders, including Tampa's Hillsborough County, Pinellas County and Lee County.

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

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