The US National Hurricane Centre on Thursday afternoon upgraded Tropical Storm Hermine to a hurricane after maximum sustained winds increased to 120 kilometres per hour as the spinning storm continued to bear down on Florida's Gulf Coast.
The storm was expected to make landfall Thursday night or early Friday.
The agency said "potentially life-threatening storm surge" was expected along the north-western Florida coast in an area known as the Big Bend.
Hermine is the fourth hurricane recorded in the Atlantic in 2016. The last storm to hit Florida at hurricane strength was Wilma in 2005, killing five people.
Governor Rick Scott declared a state of emergency in 42 counties. In a news conference he warned of the dangers of possible tornadoes and fallen power lines.
"This storm is life threatening," he said.
The weather agency issued a tropical storm warning for coastal locations extending from north-eastern Florida through Georgia as far north as Oregon Inlet in North Carolina.