The National Hurricane Center says Hermine has strengthened to a hurricane, with maximum sustained winds increasing to near 75 mph.
Hermine's upgrade from tropical storm makes it the fourth hurricane of 2016 in the Atlantic basin.
It's expected to make landfall along Florida's Gulf coast around 2 a.m. Friday, Gov. Rick Scott said.
Hillsborugh and Pinellas Counties are under a tornado watch through 11 p.m. as the hurricane approaches land in the Big Bend.
Scott says Hurricane Hermine is potentially life-threatening, and he's urging Gulf Coast residents to take precautions immediately.
At a news conference Thursday, Scott said officials expect storm surges, flooding, power outages, high winds and downed trees when as the storm comes ashore.
Scott says people in the area should take action now to protect themselves and ensure they have enough food, medicine and water.
For a list of closures and flooding in the Tampa Bay area, click here.
The governor also says 6,000 National Guard members are ready to mobilize once the storm has passed.
“The most frequent hazards that cause loss of life have to do with water: storm surge and inland flooding,” said Rick Knabb, director of the National Hurricane Center. “That can occur well to the east of where the center comes ashore.”
Heavy winds could arrive by evening commute time from Apalachicola to Cedar Key down to Tampa, he said.
Tropical Storm Warnings now extend from the Nature Coast to Florida's First Coast, including the Jacksonville metro area. Hurricane Warnings are in effect from Apalichicola to Cedar Key, extending inland to Tallahassee.
In the Tampa Bay area, the greatest threat from flooding is during high tides at 3 p.m. and 3 a.m., according to the Tampa Emergency Operations Center.
The area could get between six and 10 inches of rain, with 20 inches possible in isolated areas north of Interstate 4.
Pinellas and Sarasota counties are under a coastal flood advisory. Tides could climb to 2-3 feet above normal.
Scott is ordered state government offices in 51 counties to close.
The order included the state capital Tallahassee and home to tens of thousands of state workers. The city, which is located roughly 35 miles from the coast, has not had a direct hit by hurricane in 30 years.
Residents in some low-lying communities of Florida are being asked to evacuate.
The Tallahassee Democrat newspaper reports emergency management officials in Franklin County have issued a mandatory evacuation notice for people living on St. George Island, Dog Island, Alligator Point and Bald Point. Residents in other low-lying areas prone to flooding area also being asked to evacuate.
The last hurricane to strike Florida was Hurricane Wilma, which entered the state from along southwest Gulf Coast as a major Category 3 storm on Oct. 24, 2005. It swept across the Everglades and struck highly-populated south Florida, causing 5 deaths in the state and an estimated $23 billion in damage, according to the National Oceanic and Atmospheric Administration.
