The storm had strengthened into a category two hurricane early on Thursday and had maximum sustained winds of 169km/h, according to Michael Brennan, the director of the US National Hurricane Center.
Helene was forecast to continue gaining power on its northward path toward Florida's Big Bend region, where it was expected to make landfall in the evening, possibly as a category four storm with sustained winds in excess of 209km/h.
Officials pleaded with residents in the path of the storm to heed mandatory evacuation orders or face life-threatening conditions.
"We've all heard the adage, play stupid games win stupid prizes. Somebody is going to win a stupid prize because they're not going to get out and we're not coming."- Sheriff Bob Gualtieri— Pinellas County Sheriff's Office (@SheriffPinellas) If you are in evacuation zone A, get out now. pic.twitter.com/kMa2vroT5gSeptember 26, 2024
Helene's surge - the wall of seawater pushed on land by hurricane-force winds - could rise to as much as 6.1 metres in some spots, as tall as a two-storey house.
"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 has already lashed Georgia, South Carolina, central and western North Carolina and portions of Tennessee.
Atlanta, hundreds of kilometres north of Florida's Big Bend, was under a tropical storm warning.
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.
Airports in Tampa, Tallahassee and St Petersburg all suspended operations on Thursday.
Governor Ron DeSantis warned north Florida residents to flee before time runs out.
"You have time to get to a shelter but you've got to do it now," he said at a morning news briefing.
"Every minute that goes by brings us closer to having conditions that are going to be simply too dangerous to navigate."
Helene is expected to still be 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.
