The National Hurricane Center said Wednesday that Tropical Storm Hermine had formed from a system swirling in the Gulf of Mexico.
The Miami center said a hurricane hunter plane had determined that a tropical depression strengthened into the named storm and that Hermine had top sustained winds of 40 mph (65 kph).
It said Hermine was centered about 415 miles (665 kilometers) west-southwest of Tampa, Florida, and was drifting at 2 mph (4 kph) toward the north.
The center said the tropical storm should turn more toward the northeast with increasing speed on Thursday and was on a track that would approach the northwest Florida coast Thursday afternoon.
Florida Governor Rick Scott declared a state of emergency for 42 counties ahead of the expected landfall. He said the declaration would ease access to disaster resources and funding and allow the state to seek federal assistance.
Meanwhile, the National Park Service said it would close Georgia's Cumberland Island to visitors ahead of the tropical storm system that is expected to eventually approach the state's Atlantic coast.
The Park Service said in a news release the island would close Thursday afternoon and wouldn't reopen until Saturday morning. The barrier island is home to roughly 15 miles of federally protected wilderness. The decision came as the busy Labor Day weekend was approaching.
Cumberland Island is reachable only by boat and ferry service will shut down during the closure.
The National Hurricane Center placed the southern half of Georgia's 100-mile coast under a tropical storm watch.
In Hawaii, merchants boarded up shop windows Wednesday along Hilo Bay, and shoppers snatched supplies of food and water from grocery store shelves as what could be the first hurricane to hit Hawaii in a quarter-century neared the island.
The National Weather Service said Hurricane Madeline had weakened but remained on track to hit Hawaii's Big Island early Thursday.
Officials urged residents to expect hurricane conditions and to take steps to protect themselves and their property.