Hurricane Irma is blasting up the west coast of Florida and bearing down on the city of Tampa, with dangerous flooding along long stretches of coast.
It has been downgraded from category three to one but still has maximum sustained winds of 85mph (137km/h).
More than 3.4 million homes in the state are without power, and parts of the city of Miami are under water.
Three storm-related deaths have been reported and several areas have brought in curfews.
A wind gust of 96mph was reported in Clearwater Beach and one of 91mph recorded in Egmont Channel.
Irma cut a devastating track across 10 Caribbean countries and territories, killing at least 28 people there.
President Donald Trump has approved a major disaster declaration and emergency federal aid for Florida, describing the hurricane as a “big monster”.
“We may have been a little bit lucky in that it went on the west, and it may not have been quite as disruptive, but we’re going to see, it’s going to play out over the next five or six hours,” he added.
At 05:00 GMT, the centre of the hurricane was about 15 miles (20km) south-west of Lakeland, a town 32 miles north-east of Tampa.
Some 3m people live in the Tampa Bay area. The region has not been hit by a major hurricane since 1921.
The NHC said Irma would become a tropical storm over “far northern Florida or southern Georgia on Monday”.
“On the forecast track, the centre of Irma will continue to move over the western Florida peninsula through Monday morning and then into the south-eastern United States late Monday and Tuesday.”
The NHC earlier tweeted that people in the area of Naples and Marco Island should move away from the coast as storm surges of up to 15ft (4.5m) were possible.
There has been some flooding in the centre of Naples but it is believed to be fresh water.
How did it arrive in Florida?
Irma made landfall on Marco Island off Florida’s west coast at 15:35 local time (19:35 GMT) on Sunday, with winds of up to 120mph.
After leaving the coast of Cuba, Irma had barrelled through the Florida Keys, a chain of low-lying islands to the south, on Sunday morning.
The whole of the southern tip of Florida has seen high winds, driving rain and storm surges.
About 2ft of water has been seen in Miami’s financial district, where one major street resembled a river, but the worst damage is expected on the west coast. Two cranes have collapsed in high winds in the city.
How have residents felt the impact?
Media reports say a man was killed on Saturday in the Keys when his truck crashed into a tree as the outer bands of the storm arrived.
There are fears of further casualties in the Middle and Upper Keys, the Miami Herald reports.
Some 6.3 million people in the state were told to evacuate before the storm.
As many as 4.4 million homes and businesses have lost power, Reuters news agency reports, quoting Florida Power & Light and other utilities.
There is major disruption to transport, with Fort Lauderdale-Hollywood International Airport and Miami International Airport closed for Monday.
Amid fears of debris and reports of looting, curfews have been imposed in areas such as Miami-Dade County.