Hurricane Milton has unleashed a tornado in Florida as the monster storm thunders towards the US with the potential to “forever change” communities in its path.

The tornado was filmed crossing the I-75 highway in the state’s south as authorities urged people to “seek shelter NOW!”.

Forecasters warned that tornadoes and other severe weather were likely as Milton approached.

The hurricane is expected to be a major and extremely dangerous storm when it makes landfall late Wednesday or early Thursday (local time).

Residents had one final day to flee or hunker down before the “catastrophic” storm was predicted to hit, triggering a life-threatening storm surge.

Milton weakened to a category four system on Thursday morning (AEDT), with 233km/h winds, and is expected to cross as category three.

Vice President Kamala Harris warned it would be a storm of “historical proportions” and urged Floridians to “take this seriously”.

The Federal Emergency Management Agency said Milton could have lasting effects that could change communities.

“You need to prepare for catastrophic impacts,” FEMA administrator Deanne Criswell said.

“This is going to be a serious storm, one that could forever change communities that are still recovering from Helene.”

With more than a million people in coastal areas under evacuation orders, those fleeing for higher ground clogged highways and petrol stations ran out of fuel.

After the mayor of Tampa Bay warned residents at ground zero to leave or they were “going to die”, a police chief followed up by asking people who stayed to write their name on their leg for identification purposes.

“If you don’t leave, you’re on your own,” Holmes Beach police chief William Tokajer said on CNN.

“Take a pen and write your name and social security number on your leg so that we have a contact if we find you, because this is not going to end well.”

Steady rain fell and winds began to gust in the Tampa Bay area Wednesday morning (local time) ahead of the hurricane.

But some residents insisted they would stay even after millions were ordered to evacuate. Stragglers face grim odds of surviving, officials said.

The Tampa Bay region hasn’t had a direct hit from a major hurricane in more than a century.

“This is it, folks,” said Cathie Perkins, emergency management director in Pinellas County, which sits on the peninsula that forms Tampa Bay.

“Those of you who were punched during Hurricane Helene, this is going to be a knockout. You need to get out, and you need to get out now.”

Waves crash along St Pete Pier in St Petersburg, Florida. Photo: Getty

Major bridges in Tampa Bay would close in the afternoon, she said, and public shelters were open for evacuees.

Residents should be relieved at signs Milton’s centre might come ashore south of Tampa, Perkins said.

“Everybody in Tampa Bay should assume we are going to be ground zero,” she said.

The normally busy interstate leading into downtown Tampa was mostly free of vehicles early on Wednesday. There were a few cars on side streets.

Drivers hoping to top up tanks were hard-pressed to find petrol stations that weren’t closed or boarded up. Many had plastic-wrapped their fuel pumps to keep nozzles from whipping around in hurricane-force winds.

In Hillsborough County, where Tampa is, Sheriff Chad Chronister urged residents in a Facebook video to finalise their plans: “My message is simple. We’re approaching that 11th hour. If you need to get someplace safe for whatever reason, the time to do so is now.”

Milton bears down on the Gulf Coast in Sarasota, Florida. Photo: Getty

In Charlotte Harbour, about two blocks from the water, clouds swirled and winds gusted as Josh Parks packed his Kia sedan with clothes and other belongings early on Wednesday.

Two weeks ago, Helene’s surge brought about 1.5 metres of water to the neighbourhood. Its streets remain filled with waterlogged furniture, ruined plasterboard and other debris.

Parks, an auto technician, planned to flee to his daughter’s home inland and said his roommate had already gone.

“I told her to pack like you aren’t coming back,” he said.

Milton is targeting communities still reeling two weeks after Helene flooded streets and homes in western Florida along its devastating march that left at least 230 dead across the US South.

In many places along the coast, municipalities raced to collect and dispose of debris before Milton’s winds and storm surge. It is projected to reach as high as 3.6 metres in Tampa Bay and up to 4.5 metres further south.

The storm is expected to retain hurricane strength as it crosses central Florida on Thursday toward the Atlantic Ocean.

-with AAP