Tropical Storm Helene formed Tuesday in the Caribbean Sea and could strengthen into a major hurricane while moving north toward the U.S., 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 U.S. National Hurricane Center said. A tropical storm warning was issued for parts of the Florida Keys.

The storm was 170 miles (275 kilometers) 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 U.S., federal authorities are positioning generators, food and water, along with search-and-rescue and power restoration teams, White House spokesperson Jeremy Edwards said.

Florida Gov. 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, fueling 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 center 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 plowing 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," DeSantis said 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 closed schools, airports and government offices as strong winds knocked out power in some areas of Grand Cayman, while heavy rain and waves as high as 10 feet (3 meters) unleashed flooding.

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, piles of uncollected garbage and chronic power outages.

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

Up to 8 inches (20 centimeters) of rain is forecast for western Cuba and the Cayman Islands with isolated totals of 12 inches (30 centimeters). Up to 6 inches (15 centimeters) of rain is expected for the eastern Yucatan, with isolated totals of more than 8 inches (20 centimeters).

Heavy rainfall also was forecast for the southeastern U.S. starting Wednesday, threatening flash and river flooding, according to the National Hurricane Center. Up to 8 inches (20 centimeters) of rain was forecast for the region, with isolated totals of 12 inches (30 centimeters).

A storm surge of up to 15 feet (5 meters) was forecast from Ochlockonee River, Florida, to Chassahowitzka, and up to 10 feet (3 meters) from Chassahowitzka to Anclote River and from Indian Pass to Ochlockonee River.

Authorities in Mexico offered free ferry service to move tourists off the low-lying coastal island of Holbox. The offer was made Tuesday by Mara Lezama, the governor of the Caribbean coast state of Quintana Roo, home to resorts like Cancun, Playa del Carmen and Tulum. Holbox is known among tourists for its tranquil, shallow waters and flocks of flamingos. It is easily flooded.

Lezama said forecasters predict the storm is expected to move just offshore and not hit land in Mexico.

The Atlantic hurricane season runs from June 1 to Nov. 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 barreled into Mexico's southern Pacific coast, blowing tin roofs off houses, triggering mudslides and toppling scores of trees, officials said Tuesday.

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Coto reported from San Juan, Puerto Rico, and Hollingsworth from Mission, Kansas. Associated Press journalists Andrea Rodríguez in Havana and Mark Stevenson in Mexico City contributed to this report.

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