Hurricane Ian's most damaging winds have begun hitting Florida's southwest coast, lashing the state with heavy rain and pushing a devastating storm surge after strengthening to the threshold of the most dangerous Category 5 status.

Fueled by warm waters in the Gulf of Mexico, Ian grew to a catastrophic Category 4 hurricane overnight with top winds of 250km/h, according to the National Hurricane Center.

The storm trudged on a track that would have it making landfall north of the heavily populated Fort Myers area, which forecasters said could be inundated by a storm surge of up to 5.5m.

People walk where water is receding out of Tampa Bay due to a negative surge ahead of Hurricane Ian. Photo / Steve Helber, AP

"This is going to be a nasty nasty day, two days," Florida Gov. Ron DeSantis said, stressing that people in Ian's path along the coast should rush to the safest possible shelter and stay there.

Ian menaced Florida after bringing destruction Tuesday to western Cuba, where two people were reported dead and the storm brought down the country's electricity grid.

A classic American car drives past utility poles tilted by Hurricane Ian in Pinar del Rio, Cuba. Photo / Ramon Espinosa, AP

Ian's centre was about 80km west of Naples at noon Wednesday, as it churned toward the coast at 15km/h. Ian's plodding pace meant the storm was expected to spend a day or more crawling across the Florida peninsula, dumping flooding rains of 300-450mm across a broad area including Tampa, Orlando and Jacksonville in the state's northeast corner.

• Hurricane Ian smashes into Cuba, Florida evacuating
• Hurricane Ian is headed for Florida, set to arrive within 24 hours

Catastrophic storm surges could push 3.6m of water or more across more than 400km of coastline, from Bonita Beach to Englewood, the hurricane centre warned.

"It's going to get a lot worse very quickly. So please hunker down," DeSantis said.

Ivan Mendoza begins to repair damage at his mobile home in Davie, Florida, as Hurricane Ian rapidly intensified. Photo / Joe Cavaretta, South Florida Sun-Sentinel via AP

Off the coast on Sanibel Island near Fort Myers, swirling water-covered residential streets and was halfway up mailbox posts by mid-morning. Seawater rushed out of Tampa Bay, leaving parts of the muddy bottom exposed, and waves crashed over the end of a wooden pier at Naples.

More than 2.5 million people were under mandatory evacuation orders but, by law, no one could be forced to flee. The governor said the state has 30,000 linemen, urban search and rescue teams, and 7000 National Guard troops from Florida and elsewhere ready to help once the weather clears.

Florida residents rushed ahead of the impact to board up their homes, stash precious belongings on upper floors and join long lines of cars leaving the shore.

Utility trucks are staged in a rural lot in The Villages of Sumter County, Florida, in readiness for Hurricane Ian. Photo / Stephen M. Dowell, Orlando Sentinel via AP

Some chose to stay and ride out the storm. Jared Lewis, a Tampa delivery driver, said his home has withstood hurricanes in the past, though not as powerful as Ian.

"It is kind of scary, makes you a bit anxious," Lewis said. "After the last year of not having any, now you go to a Category 4 or 5. We are more used to the 2s and 3s."

Forecasters predicted Ian would make landfall more than 160km south of Tampa and St. Petersberg, likely sparing the densely populated Tampa Bay area from its first direct hit by a major hurricane since 1921.

An airplane overturned by a likely tornado produced by the outer bands of Hurricane Ian at North Perry Airport in Pembroke Pines. Photo / Wilfredo Lee, AP

Officials warned residents that Tampa could still experience powerful winds and up to 500mm of rain.

"Please, please, please be aware that we are not out of danger yet," Tampa Mayor Jane Castor said in a video on Twitter. "Flooding is still going to occur."

During the night, Ian went through a natural cycle when it lost its old eye and formed a new one. The timing was bad for the Florida coast, because the storm got stronger and larger — 193km/h to 250km/h — with landfall just a few hours away.

The size of the storm also grew, with tropical storm force winds extending 280km from the hurricane's centre.

"With the higher intensity you're going to see more extensive wind damage," University of Miami hurricane researcher Brian McNoldy said. "The larger wind field means that more people will experience those storm-force winds."

The most damaging winds could hit a coastline where the population has jumped sevenfold since 1970, according to the US Census.

Vinod Nair wasn't taking any chances. He drove inland from the Tampa area Tuesday with his wife, son, dog and two kittens to a hotel in Orlando, where only tropical storm force winds were expected.

"You can't do anything about natural disasters," Nair said. "We live in a high-risk zone, so we thought it best to evacuate."

The eye of Hurricane Ian approaching the southwest coast of Florida. Photo / GOES-East GeoColor satellite via AP

Ash Dugney warily watched ocean water being sucked out below a Tampa Bay pier Wednesday morning. He said he didn't trust Tampa's storm drainage system to keep his corner tuxedo rental business safe from flooding that he said has happened even during mild storms.

"I don't care about the wind and the rain and the stuff like that, I just care about the flooding," Dugney said, adding that he moved essentials out of the shop and moved other items up to above waist-high level.

Flash floods were possible across all of Florida. Hazards include the polluted leftovers of Florida's phosphate fertiliser mining industry, more than 1 billion tons of slightly radioactive waste contained in enormous ponds that could overflow in heavy rains.

Forecasters placed roughly 193km of central Florida's east coast under a hurricane warning Wednesday, signaling that Ian may remain a hurricane longer than previously expected as it moves inland.

Isolated tornadoes were spinning off the storm well ahead of landfall. One tornado damaged small planes and a hangar at the North Perry Airport, west of Hollywood along the Atlantic coast.

More than 200,000 homes and businesses were without electricity, and Florida Power and Light warned those in Ian's path to brace for days without power.

The federal government sent 300 ambulances with medical teams and was ready to truck in 3.7 million meals and 3.5 million litres of water once the storm passes.

"We'll be there to help you clean up and rebuild, to help Florida get moving again," President Joe Biden said Wednesday. "And we'll be there every step of the way. That's my absolute commitment to the people of the state of Florida."

Parts of Georgia and South Carolina also could see flooding rains and some coastal surge into Saturday. Georgia Gov. Brian Kemp preemptively declared an emergency, ordering 500 National Guard troops on to standby. — AP