Hurricane Irma is blasting up the west coast of Florida and is now bearing down on the city of Tampa.
Irma made landfall on Marco Island off Florida's west coast with winds of up to 120mph (192km/h), but has since been downgraded from category three to two.
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, as Irma moves northwards.
Irma currently has maximum sustained winds of 100mph (160km/h), the NHC says, with a wind gust of 96mph reported in Clearwater Beach and one of 91mph recorded in Egmont Channel.
The storm was earlier pummelling the area around the city of Fort Myers.
Irma has already devastated parts of the Caribbean, killing at least 28 people.
President Donald Trump has approved a major disaster declaration and emergency federal aid for Florida.
He described the hurricane as a "big monster", praising the federal agencies involved with the storm and saying he would go to the state "very soon".
"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," Mr Trump 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.
Irma is now losing strength, the NHC says, predicting that Irma will 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 storm made landfall at Marco Island at 15:35 local time (19:35 GMT).
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.
After leaving the coast of Cuba, Irma barrelled through the Florida Keys, a chain of low-lying islands to the south, on Sunday morning.
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.
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.
Are you in the region? Are you a holidaymaker unable to get a flight home or a resident who has been preparing for Hurricane Irma? If it is safe for you to do so, share your experiences by emailing firstname.lastname@example.org.