Tropical storm Debby hit land as it arrived on Florida's Gulf Coast on Tuesday, threatening to dump another round of heavy rains on areas already reeling from flooding, US forecasters said.

Debby, which has been raking the coast with strong winds and rains since the weekend, swept over Steinhatchee in north-central Florida as it headed eastward to the Atlantic Ocean, where it is expected by Friday.

The Miami-based National Hurricane Center (NHC) said Debby was about 35 miles (55 kilometers) northwest of Cedar Key, Florida.

It was packing maximum sustained winds of 40 miles (64 kilometers) per hour, but had speeded up and was now moving toward the east at six miles (nine kilometers) per hour.

Parts of northern Florida were expected to get over two feet (61 centimeters) of rain. The storm also downed trees, upturned catamarans and drowned turtle nests along the Gulf, according to local media.

Authorities have closed several bridges and roads, including parts of Interstate 10, northern Florida's main interstate highway.

"Debby is expected to weaken to a tropical depression tonight," the NHC said in its latest bulletin, but it warned that "flooding will continue during the next day or two due to persistent onshore winds."

It also warned that "the combination of a storm surge and the tide will cause areas near the coast to be flooded by rising waters during the next high tide this evening (Tuesday)."

Florida Governor Rick Scott declared a state of emergency on Monday as power outages and flash floods heralded the storm's approach. Up to four feet of flooding was predicted for Apalachee Bay, while Tampa Bay could get up to three feet of water.

President Barack Obama called Scott as he traveled on his Air Force One jet "to ensure the state had no unmet needs" as it responds to extreme weather and flooding from Debby, White House spokesman Jay Carney said.

The president "expressed condolences" for the loss of life as well as extensive damage to homes. Carney said federal emergency authorities would be in close contact with the state, and that the Obama administration "stood ready to provide additional assistance if necessary."

At least one woman was reported to have died in a tornado that local media attributed to Debby, and state authorities warned that more deadly twisters were possible.

Alabama search and rescue workers were also searching for a man who may have drowned in rough surf on Sunday, according to local media.