Florida residents have been boarding up their homes and packing up their vehicles as Hurricane Ian draws nearer, carrying high winds and torrential rain.
It approaches the state after slamming into Cuba - leaving the entire country without power, swamping fishing villages and forcing mass evacuations.
In its most recent update, the US National Hurricane Center (NHC) warns areas most at risk from the "life-threatening" storm are between the Naples and Sarasota regions.
Hurricane Ian was upgraded to a Category 4 storm today, meaning it risks maximum sustained winds of 140 miles per hour (220 km per hour) and more than a foot of rain in some areas.
More than 2.5 million Floridians have been told to evacuate amid warnings from the NHC of "catastrophic wind damage" at the centre of the storm.
Heavy rainfall is expected to spread through the Florida peninsula throughout Thursday, bringing "widespread, life-threatening catastrophic flooding" in the centre of the state, forecasters say.
Considerable flooding has also been predicted in southern and northern Florida, as well as southeastern Georgia and coastal South Carolina.
Ian pummelled Cuba on Tuesday and was expected to crash ashore into Florida on Wednesday evening south of Tampa Bay.
By late Tuesday night, tropical storm-force winds generated by Ian extended through the Florida Keys island chain to the southernmost shores of the state's Gulf Coast, according to the hurricane centre.
The NHC also issued storm surge warnings for much of western Florida's shoreline, predicting coastal flooding of up to 12 feet from wind-driven high surf.
"The time to evacuate is now. Get on the road," Florida's director of emergency management, Kevin Guthrie, said during a news briefing on Tuesday evening.
Governor Ron DeSantis warned evacuation would become difficult for those who waited much longer to flee because increasing winds would soon force authorities to close highway bridges.
"You need to get to higher ground, you need to get to structures that are safe," DeSantis said, adding that widespread power outages would leave millions without electricity.
US Federal Emergency Management Agency chief Deanne Criswell said she worried that too few Florida residents were taking the threat seriously.
She said: "I do have concerns about complacency. We’re talking about impacts in a part of Florida that hasn’t seen a major direct impact in nearly 100 years. There’s also parts of Florida where there’s a lot of new residents."
Nearly 60 Florida school districts have cancelled classes due to the hurricane, while more than 175 evacuation centres were opened statewide, many of them school buildings converted to shelters.
Commercial airlines reported more than 2,000 storm-related US flight cancellations, with the St. Petersburg-Clearwater International Airport and Tampa International Airport shut down on Tuesday.
If Ian strikes the Tampa area, it would be the first hurricane to make landfall there since the 1921 Tarpon Springs storm.
It also may prove one of the costliest, with data modelling service Enki Research projecting storm-related damages ranging from $38 billion to more than $60 billion.
Ian moved across the southeastern edge of the Gulf of Mexico headed for Florida after ravaging Cuba with violent winds and flooding.
The Florida coastal zone at highest risk for US landfall is home to miles of sandy beaches, scores of resort hotels and numerous mobile home parks, a favourite with retirees and vacationers alike.
"We're right on the water, along a canal, so ... this could be devastating," said Melissa Wolcott Martino, a 78-year-old retired magazine editor in St. Petersburg.
She and her husband were loading valuables and pets into their car for a drive to their son's home north of Tampa on Tuesday.
Some of the city's residents, including 50-year-old software engineer Vanessa Vazquez, said they planned to ride out the storm despite the evacuation warnings.
"I'm staying put," Vazquez said. "I have four cats and I don't want to stress them out. And we have a strong house."