Tropical Storm Helene forms in Caribbean. It's expected to become a hurricane and threaten Florida

Tropical Storm Helene formed Tuesday in the Caribbean Sea and could strengthen into a major hurricane while moving north toward the US, forecasters said.

Heavy rains and big waves already lashed the Cayman Islands, and some Florida residents filled sandbags days ahead of anticipated flooding.

Hurricane watches were issued for parts of Cuba, Mexico and a stretch of the Florida coastline, including Tampa Bay, the US National Hurricane Center said. A tropical storm warning was issued for parts of the Florida Keys.

The storm was 170 miles (275 kilometres) southeast of the western tip of Cuba and had sustained winds of 45 mph (75 kph). It was expected to strengthen into Hurricane Helene on Wednesday as it approached the Gulf Coast and could become a major hurricane Thursday.

In the US, federal authorities are positioning generators, food and water, along with search-and-rescue and power restoration teams, White House spokesperson Jeremy Edwards said.

Florida Governor Ron DeSantis declared a state of emergency Monday ahead of the storm's arrival on the state's Gulf Coast as soon as late Thursday and increased the counties included in the high-risk area Tuesday from 41 to 61.

"Now is the time to start preparing. If you're in an evacuation zone, you should evacuate," said Lisa Bucci, a specialist at the hurricane center. "Don't be fooled by the way the storm looks at the moment."

The storm is expected to move over deep, warm waters, fuelling its intensification. People in regions under watches and warnings should be prepared to lose power and should have enough food and water for at least three days, Bucci said.

Helene, the eighth named storm of the Atlantic hurricane season, could strengthen into a major Category 3 hurricane — with winds of at least 111 mph (178 kph) — before approaching the northeastern Gulf Coast. Since 2000, eight major hurricanes have made landfall in Florida, according to Philip Klotzbach, a Colorado State University hurricane researcher.

Given the anticipated large size, storm surge, wind and rain will extend far from the centre of the expected storm, especially on the eastern side. States as far inland as Tennessee, Kentucky and Indiana could see rainfall associated with the storm, Bucci said.

A tropical storm warning was in effect for Florida's Dry Tortugas; the lower Florida Keys west of the Seven Mile Bridge; Grand Cayman; Rio Lagartos to Tulum, Mexico; and the Cuban provinces of Artemisa, Pinar del Rio, and the Isle of Youth.

A storm surge watch was in effect for Florida's Tampa Bay, Charlotte Harbor, and from Indian Pass south to Flamingo. A tropical storm watch was issued for the middle Florida Keys from the Seven Mile Bridge to the Channel 5 Bridge; Flamingo to south of Englewood; and from west of Indian Pass to the Walton Bay County line.

DeSantis said the storm is reminiscent of 2018's Hurricane Michael, a Category 5 hurricane that rapidly intensified and caught residents off guard before ploughing a destructive path across the western Florida Panhandle. Communities still rebuilding from previous storms could get battered again, DeSantis warned.

Helene is expected to slip between Cuba and Mexico's Yucatan Peninsula early Wednesday and then head north across the Gulf of Mexico.

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