Helene hit Florida packing sustained winds of around 210 km/h, the National Hurricane Center said late on Thursday, making it a powerful category four storm.
Even before it made landfall, the storm had flooded the Gulf Coast and knocked out power for at least one million customers in the state.
Officials pleaded with residents in the path of the storm to heed mandatory evacuation orders or face life-threatening conditions.
Hurricane Helene has hit Florida packing sustained winds of around 210 km/h. (AP PHOTO)
Helene's surge - the wall of seawater pushed on land by hurricane-force winds - could rise to as much as six metres in some spots, as tall as a two-storey house, the centre's director, Michael Brennan, said in a video briefing.
"A really unsurvivable scenario is going to play out" in the coastal area, Brennan said, with water capable of destroying buildings and carrying cars pushing inland.
Strong rain bands were whipping parts of coastal Florida, and rainfall had already lashed Georgia, South Carolina, central and western North Carolina and portions of Tennessee. Atlanta, hundreds of miles north of Florida's Big Bend, was under a tropical storm warning.
Florida Governor Ron DeSantis told reporters late on Thursday the hurricane had already caused one fatality. He gave no details.
In Pinellas County, which sits on a peninsula surrounded by Tampa Bay and the Gulf of Mexico, roads were already filling with water before noon.
Officials warned the storm's impact could be as severe as last year's Hurricane Idalia, which flooded 1500 homes in the low-lying coastal county.
Videos posted on the county's social media site showed some swamped beachside roads and water rising over boat docks.
Roads and piers were being swamped before Hurricane Helene's arrival. (AP PHOTO)
Airports in Tampa, Tallahassee and St Petersburg all suspended operations on Thursday.
Helene is expected to remain a full-fledged hurricane as it rolls through the Macon, Georgia, area on Friday, forecasters said.
It could bring 30cm of rain or more, potentially devastating the state's cotton and pecan crops, which are in the middle of harvesting season.
"The current forecast for Hurricane Helene suggests this storm will impact every part of our state," Georgia Governor Brian Kemp said.
After making landfall across the Florida coast, Helene is expected move more slowly over the Tennessee Valley on Friday and Saturday, the NHC said.
Storm surge was forecast to reach up to six meters in the Big Bend area of Florida's Panhandle region where the storm came ashore.
Numerous evacuations were ordered along Florida's Gulf Coast, including Sarasota and Charlotte counties.
In Taylor County, the Sheriff's Department asked residents who decided not to evacuate to write their names and dates of birth on their arms in ink - so that they could be identified in the case they lost their lives in the hurricane.
