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 have been issued for parts of Cuba, Mexico and a stretch of the Florida coastline, including Tampa Bay, the U.S. National Hurricane Center said in Miami. A tropical storm warning has been issued for parts of the Florida Keys.

The storm was 150 miles southeast of the western tip of Cuba and had sustained winds of 50 mph. It was expected to strengthen into a hurricane Wednesday as it approached the Gulf Coast and could be a Category 3 storm, with winds of at least 111 mph, by Thursday.

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 as soon as late Thursday and increased the counties 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 hurricane specialist at the center. “Don’t be fooled by the way the storm looks at the moment. We are expecting it to rapidly intensify.”

She said 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.

Helene is the eighth named storm of the Atlantic hurricane season, which ends Nov. 30.