"An Air Force Reserve Hurricane Hunter aircraft indicates Dennis has become a hurricane over the central Caribbean Sea," said forecaster Stacy Stewart of the National Hurricane Center in West Miami-Dade County.
Forecasters said it could become a Category 3 hurricane by Friday, when it strikes Cuba, and reach the Panhandle or elsewhere on the Gulf Coast early next week as a strong hurricane.
"The official forecast . . . calls for a major, dangerous hurricane in the northwestern Caribbean Sea and the eastern Gulf of Mexico," said hurricane forecaster Lixion Avila.
The storm's projected path kept the potent core well west of South Florida and the Florida Keys, but Dennis was expected to grow in size as well as intensity and the region could be swept by Dennis' outlying wind and rain this weekend.
Local forecasters predicted periods of heavy rain and gusty wind in South Florida from Thursday night through Sunday. They warned of dangerous boating conditions and a high risk of rip currents.
"It looks like we'll mostly be spared, but it will be a pretty crummy weekend," said Jim Lushine, the National Weather Service's severe weather expert for South Florida.
It may be far worse for other Floridians, including many still recovering from last year's unprecedented quartet of hurricanes.
The five-day forecast suggested that Dennis could strike the mainland close to Pensacola as a major Category 3 hurricane early Monday. The same area was bulldozed last September by Hurricane Ivan, also a Category 3 storm on the five-category Saffir-Simpson scale.
The Florida National Guard began elevating its level of readiness.
"We are prepared for the challenges of Dennis or a future hurricane," said Lt. Col. Ron Tittle, the guard's spokesman.
Experts emphasized, however, that long-range forecasts contain large margins of error. They advised everyone living along the upper and eastern Gulf Coast to keep a close eye on future forecasts.