At least three people have died as a result of the powerful storm
Tampa is braced for a potentially life-threatening storm surge as Hurricane Irma moves up through the state of Florida.
The city appeared to avoid a direct hit from the hurricane, which has claimed three lives in the US so far, but heavy rain and flooding is still a major threat.
Around 5.7 million homes and businesses in the state are without power.
Irma has now weakened to a tropical storm as it heads towards southern Georgia, and forecasts also suggest that South Carolina and Alabama are in its path.
Sustained winds of more than 200km/h were reported when the storm first made landfall in the Florida Keys on Sunday, but by Monday had dropped to around 130km/h.
Irma's eye is moving up the coast and earlier made landfall for a second time at Marco Island before heading towards Naples, one of America's wealthiest cities.
Meteorologist Steve Hamilton said flash flooding could become a problem as the storm moved north and heavy rains fell on mountainous area, and that the extreme conditions would continue for a few more days.
"The storm surge occurs in what we tend to call the dirty side of the storm," he said.
"The wind literally pushes the water up against the shoreline . That is a storm surge, basically like a wall of water, like a pancake of water."
There are fears storm surges of 3-4.5m (10-15ft) could engulf houses in low-lying coastal areas and cause devastating flooding.
A tweet from the National Hurricane Centre read: "Urgent warning about the rapid rise of water on the SW FL coast with the passage of #Irma's eye. MOVE AWAY FROM THE WATER!"
Areas of southeastern Florida, including Miami, have already suffered flooding.
Rainfall of two to four inches (5-10cm) an hour was reported as the storm brought down two cranes. Streets were also flooded in the city's financial district.
The storm is forecast to hit states such as Georgia and South Carolina on Monday afternoon, before later moving into Alabama - albeit in a far weaker state.
Officials urged Floridians not to drive on flooded roads, with the National Weather Service's division in Melbourne tweeting: "TURN AROUND, DON'T DROWN!"
Police said a man in the Florida Keys was killed in a truck crash.
Monroe County Sheriff's Office said his body was discovered by first responders who were patrolling during lulls in the bad weather on Saturday afternoon.
Another two people, both law enforcement officials, died in a head-on crash in Hardee County.
Meanwhile, a woman in Miami had no choice but to deliver her own baby - with medical personnel coaching her on the phone because the emergency services were unable to reach her.
The National Weather Service (NWS) said alerts for tornadoes were in place across southern Florida until midnight on Monday (5am Irish time).
Locals were warned that "fast-moving tornadoes can develop with little warning in hurricane rain bands".
The NWS has urged those exposed to Irma to stay indoors, hunker down away from windows, and to use "whatever you have to try and protect yourself from flying debris".
President Donald Trump has declared a major disaster in Florida, which will allow emergency funds to pour into the state.
Speaking at his Camp David presidential retreat, Mr Trump said: "The bad news is this is some big monster, but I think we're very well coordinated.
"I hope there aren't too many people in the path. You don't want to be in that path, that's a path you don't want to be in and we tried to warn everybody.
"For the most part, they've left, but that's a bad path to be in."
The President said Irma would "cost a lot of money", but added: "Right now we're worried about lives, not cost."
Florida governor Rick Scott said: "This has been a challenging week for our state... Take this deadly storm seriously. Stay safe, be prepared, listen to local evacuation advisories."
For those trying to flee and seek refuge inland, the changing forecast has caused concern.
"Not only did we go west, but so did Irma. She's tracking us, that feisty minx," said Chris Cardona, who has fled his mobile home near Miami with his wife Laurie.
A total of 6.3 million people in Florida were ordered to evacuate - about a third of the state's population.
Hurricane Irma is up to 640km wide, leaving the entire Florida peninsula exposed.
The storm killed at least 26 people as it swept across the Caribbean towards America, and caused widespread destruction on islands including Antigua and Barbuda.