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 began to evacuate or fill sandbags ahead of anticipated flooding.

Tropical Storm Helene is expected to unleash mudslides and flooding in Cuba before reaching major hurricane strength on Thursday as it hits Florida with life-threatening ocean surge, the U.S. National Weather Service said.

Helene is expected to rapidly intensify into a major and massive hurricane with 115-mph winds by the time it makes landfall Thursday.