Hermine is expected to make landfall in Florida around midnight on Thursday night, the first hurricane to hit the state since 2005.
Governor Rick Scott declared a state of emergency for 51 counties as residents braced for the dangerous storm.
Wind gusts reached 70mph (110kmph) on Thursday, the National Hurricane Center (NHC) said.
"This is life threatening. We have not had a hurricane in years," Governor Scott said.
He added that 8,000 members of the Florida National Guard were prepared to be deployed in the wake of the storm.
Mr Scott ordered evacuations in five counties in Florida's northwest and called for voluntary evacuations in three other coastal counties.
Weather officials predict Hermine will cross Georgia and the Carolinas, and could bring heavy rains along the East Coast over Labour Day Weekend.
Georgia Governor Nathan Deal also has declared a state of emergency for 56 counties ahead of the storm.
The system also poses a risk of tornadoes near the central Florida coast, according to the NHC.
The last hurricane to strike Florida was Wilma in 2005, which made landfall in the US the same year as Katrina.
Tropical Storm Hermine has strengthened into a hurricane, threatening to bring a dangerous storm surge to Florida.