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

A tropical storm warning was issued for parts of the Florida Keys.

The storm was 170 miles (275 kilometres) south-east 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 by Thursday.

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 on 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 centre.

“Don’t be fooled by the way the storm looks at the moment. We are expecting it to rapidly intensify.”

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, she said.

The storm is expected to move over deep, warm waters that would fuel its intensification.

“This system will become quite large and powerful before landfall,” the centre 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 north-eastern 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.

The centre warned of “inland penetration of strong winds over parts of the south-eastern US after landfall.”

Ms Bucci said states including Tennessee, Kentucky and Indiana could see rainfall associated with the storm.

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, along with the Isle of Youth.

A storm surge watch was in effect for Florida’s Tampa Bay, Charlotte Harbour 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.

The National Weather Service in Tallahassee, Florida, urged people to take potential evacuations seriously.

“10-15ft of surge is NOT survivable,” it wrote on the social media platform X, formerly Twitter.

Mr DeSantis said the storm is reminiscent of 2018’s Hurricane Michael, a Category 5 hurricane that rapidly intensified and caught many 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.

“We are going to see significant impacts no matter what happens,” Mr DeSantis said on Tuesday.

The sun shone Tuesday in Tarpon Springs, Florida, but residents already filled sandbags as they braced for potential flooding.

Officials in the Cayman Islands shuttered schools and airports as forecasters warned of heavy wind and rain and waves of up to 10 feet (3 metres).

Many in Cuba worried about the storm, whose tentacles are expected to reach the capital of Havana, which is struggling with a severe shortage of water and piles of uncollected garbage.

Overall, roughly 600,000 people in Cuba are experiencing water shortages, including more than 130,000 in Havana alone. Chronic power outages also persist.

Helene is expected to slip through waters separating Cuba from Mexico’s Yucatan Peninsula early on Wednesday and then head north across the Gulf of Mexico.

The Atlantic hurricane season runs from June 1 to November 30.

The National Oceanic and Atmospheric Administration has predicted an above-average Atlantic hurricane season this year because of record-warm ocean temperatures.

It forecast 17 to 25 named storms, with four to seven major hurricanes of Category 3 or higher.

In the Pacific, former Hurricane John killed two people after it barrelled into Mexico’s southern Pacific coast, blowing tin roofs off houses, triggering mudslides and toppling scores of trees, officials said on Tuesday.