Hurricane Hermine has made landfall in Florida's Gulf coast just east of St Marks with 80mph winds.
It is the first hurricane to directly strike the Sunshine State in more than a decade, roaring in from the north-west Gulf Coast with 80mph (129kph) winds and heavy rain.
The US National Hurricane Centre said Hermine made landfall at about 1.30am EDT (5.30am Irish Time) today.
Projected storm surges of up to 12ft (3.7m) threatened the coastline and expected rainfall of up to 10in (25.4cm) carried the danger of flooding through the storm's path, including the state capital, Tallahassee, which has not been hit by a hurricane since Kate in 1985.
Hermine is expected to drop back down to a tropical storm before pushing into Georgia, the Carolinas and up the East Coast with the potential for torrential rain and deadly flooding.
Hurricane Hermine gained strength as it roared towards Florida's Gulf Coast, as residents were warned to flee inland if necessary amid a "life-threatening situation".
As people braced for the first direct hit on the state from a hurricane in over a decade, the former tropical storm's top sustained winds rose from 75mph in the afternoon to 80mph by nightfall.
Hermine gained new fury as it bore down on the coast, and forecasters warned it would probably gain even more punch before slamming ashore.
Its landfall was expected in the Big Bend area - the mostly rural and lightly populated corner where the Florida peninsula meets the Panhandle.
Hermine will then drop back down to a tropical storm and push into Georgia, the Carolinas and up the East Coast with the potential for drenching rain and deadly flooding.
Florida Governor Rick Scott warned of the danger of strong storm surge, high winds, downed trees and power outages, and urged people to move to inland shelters if necessary and make sure they had enough food, water and medicine.
"This is a life-threatening situation," he said. "It's going to be a lot of risk. Right now, I want everybody to be safe."
He said 6,000 National Guardsmen in Florida are ready to mobilise after the storm passes. The governors of Georgia and North Carolina declared states of emergency.
The last hurricane to strike Florida was Wilma, a powerful Category 3 storm that arrived on October 24 2005. It swept across the Everglades and struck heavily populated south Florida, causing five deaths in the state and an estimated $23bn in damage.