By Letitia Stein

TAMPA, Fla. (Reuters) - Winds from strengthening Hurricane Hermine lashed at Florida's northern Gulf Coast late on Thursday, forcing residents to evacuate some coastal areas and stock up on provisions ahead of what the state's governor warned would be a lethal storm.

Hermine, expected to be the first hurricane to make landfall in the state in more than a decade, also posed a Labor Day weekend threat to states along the northern Atlantic Coast that are home to tens of millions of people.

On Thursday afternoon, the National Hurricane Center extended a tropical storm watch to Sandy Hook, New Jersey.

Hermine became the fourth hurricane of the 2016 season around midafternoon when its sustained winds reached 75 mph (120 kph). Located about 85 miles (135 km) south of Apalachicola, Florida at 5 p.m., it was expected to make landfall early on Friday.

Hermine could dump as much as 20 inches (51 cm) of rain in some parts of the state. Ocean storm surge could swell as high as 12 feet (3.6 meters). Isolated tornadoes were forecast.

After battering coastal Florida, Hermine is expected to weaken and move across the northern part of the state into Georgia, then southern U.S. coastal regions on the Atlantic.

The governors of Georgia and North Carolina on Thursday declared emergencies in affected regions. In South Carolina, the low-lying coastal city of Charleston was handing out sandbags.

Florida Governor Rick Scott declared a state of emergency in 51 of Florida's 67 counties, and at least 20 counties closed schools. Mandatory evacuations were ordered in parts of five counties in northwestern Florida and voluntary evacuations were in place in at least three more counties.

"This is life threatening," Scott told reporters on Thursday afternoon. "You can rebuild a home. You can rebuild property. You cannot rebuild a life."

In coastal Franklin County, people on barrier islands and low-lying areas on the shore were being evacuated.

"Those on higher ground are stocking up and hunkering down," said Pamela Brownlee, the county's director of emergency management.

The last hurricane to make landfall in Florida was Wilma in 2005, the hurricane center said.

Towns, cities and counties were hastily preparing shelters for people and pets and placing electric line repair crews on standby ahead of the storm.

The storm was expected to affect many areas inland of the Gulf Coast. In Leon County, which includes the state capital of Tallahassee, more than 30,000 sandbags were distributed.

At Maximo Marina in St. Petersburg, Florida, dock master Joe Burgess watched anxiously as waters rose 6 inches (15 cm) over the dock at high tide on Thursday afternoon, before slowly receding.

"If we get hit with a real storm head on, all the provisions you can make aren't going to matter out here," he said, ready to use a chainsaw to cut beams on covered slips if rising water pushed boats dangerously close to the roof. "It'd be pretty catastrophic."

HEAVY RAIN

On its current path, the storm also could dump as much as 10 inches (25 cm) of rain on coastal areas of Georgia, which was under a tropical storm watch, and the Carolinas. Forecasters warned of "life-threatening" floods and flash floods there.

Still, many people in Florida, whose population has swelled since the last hurricane struck, saw Hermine less as a threat than entertainment.

Manatees on the Bay, a restaurant and bar on the waterfront in the Tampa Bay region, was offering storm drink specials including beer and shots.

"We thought about doing a hurricane," said owner Perry Murphree, referring to the name of a popular sweet cocktail. "But I don't want to tempt fate."

(Additional reporting by Brendan O'Brien in Milwaukee, Laila Kearney in New York and Jon Herskovitz in Austin, Texas; Editing by Bill Trott and Cynthia Osterman)