Officials issued dire warnings, pleading with residents in coastal areas along the hurricane's path to evacuate ahead of catastrophic winds and a potentially deadly storm surge, the wall of seawater pushed on land by hurricane-force winds, that could rise to 20 feet (6.1 meters) in some spots.

"This is not a survivable event for those in coastal or low- lying areas," said Jared Miller, the sheriff of Wakulla County, where Helene is forecast to make landfall. "Please heed the evacuation orders in place as time is running out to do so."

Helene roared across the Gulf of Mexico, picking up power from the warm ocean water. It is forecast to make landfall in Florida's panhandle Thursday evening, packing sustained wind speeds of up to 156 miles per hour (251 km per hour), forecasters said.

"For those in the path, that unfortunately means catastrophic wind impacts," National Hurricane Center Deputy Director Jamie Rhome said.

WALL OF WATER

Storm surge was forecast to reach 15 to 20 feet (4.6 to 6.1 meters) in the Big Bend area of Florida's panhandle where the storm is expected to come ashore, Rhome said.

More than 40 million people in Florida, Georgia and Alabama were under hurricane and tropical storm warnings, the hurricane center said.

Numerous evacuations were ordered along Florida's Gulf Coast, including Sarasota and Charlotte counties, and dozens of counties have announced school closures, including Hillsborough and Pinellas counties.

Pinellas County officials ordered evacuations of long-term healthcare facilities, including nursing homes, assisted living centers and hospitals near the coast. The county sits on a peninsula surrounded by Tampa Bay and the Gulf of Mexico.

John Dailey, the mayor of Tallahassee, Florida's capital city that is in the direct path of Helene, said the hurricane could be the strongest storm to ever make a direct hit on his city.

Helene could produce "unprecedented damage like nothing we have ever experienced before as a community," Dailey told reporters on Wednesday.

TORRENTIAL RAINS

Helene was expected to dump up to 15 inches (38.1 cm) of rain in some isolated spots after making landfall in Florida, causing considerable flash and urban flooding, the hurricane center said.

Rhome said about half of lives lost in hurricanes typically came from flash flooding caused by torrential rain, often among people who drive into flooded roads and are swept away. He urged those in affected areas to use extreme caution.

Rhome added that the expected hurricane-force wind impact area stretched around 180 miles (290 km) north from the Florida panhandle to southern Georgia.

"You need to prepare for prolonged (energy) outages, those trees are going to come down in strong winds, block roads," Rhome said.