Tropical Storm Ophelia strengthened into a hurricane 70 miles off the northeast Florida coast Thursday, churning up waves that caused beach erosion and drenching the Kennedy Space Center with rain.

Thursday night, Ophelia had top sustained winds of 75 mph, just over the threshold to be classified as a hurricane, forecasters at the National Hurricane Center said.

Forecasters said it was still unclear where Ophelia was headed.

If it hits Florida, it would become the third hurricane to strike the state this year and the seventh in the last 13 months.

Downpours from earlier storms had caused flooding in Flagler County, raising anxiety about the effect of more rain. Authorities shut down a mile-long stretch of beachfront road in Flagler Beach so transportation workers could shore it up with sand and boulders.

"The storm is eating up our dunes," said Carl Laundrie, communications manager for Flagler County.

Fourteen Navy ships at the Naval Station Mayport in Jacksonville headed to sea as a precaution.

Two shelters were readied in Flagler County. Neighboring Volusia County opened three shelters, but later closed them because only 12 people showed up.

"We know from last year these storms can do an about-face. We are not out of the woods unless this storm moves well away from Volusia's coast," said Dave Byron, spokesman for Volusia County. Volusia County schools were closed Thursday.

Ophelia is the 15th named storm of the season. At 8 p.m., it was centered about 75 miles east-northeast of Cape Canaveral, with hurricane-force winds stretching up to 15 miles from the center. The storm was stationary.

Hurricane specialist Jack Beven said Ophelia should start moving north or northeast -- away from land -- within a day or so. However, it could curl back early next week and slam north Florida or Georgia as a Category 1 hurricane.

Storm warnings or watches were posted for Florida's east coast from Sebastian Inlet to Fernandina Beach.

Elsewhere in the Atlantic, the tropical storm warning for Bermuda was discontinued as Hurricane Nate pulled away, while Hurricane Maria moved over the cold North Atlantic with 75-mph winds.