Tropical Storm Helene was rapidly strengthening in the Caribbean Sea and expected to become a hurricane Wednesday while moving north along Mexico's coast toward the U.S., prompting residents to evacuate, schools to close and officials to declare emergencies in Florida and Georgia.

The storm is forecast to be “near hurricane strength” when it passes near Mexico's Yucatan Peninsula early Wednesday, the U.S. National Hurricane Center said, and to “intensify and grow in size” as it moves north across the Gulf of Mexico. Heavy rainfall was forecast for the southeastern U.S. starting Wednesday, with a “life-threatening storm surge” along the entire west coast of Florida, according to the center.

Helene is expected to become a major hurricane — a Category 3 or higher — on Thursday, the day it's set to reach Florida's Gulf Coast, according to the hurricane center. The center has issued hurricane warnings for part of Mexico’s Yucatan Peninsula and Florida's northwestern coastline, where large storm surges of up to 4.5 meters were expected.

Mexico is still reeling from former Hurricane John battering its other coast. John hit the country's southern Pacific coast late Monday, killing two people, blowing tin roofs off houses, triggering mudslides and toppling scores of trees, officials said Tuesday.

John grew into a Category 3 hurricane in a matter of hours Monday and made landfall about 130 kilometers east of the resort of Acapulco, near the town of Punta Maldonado, with maximum sustained winds of 193 kph before weakening to a tropical storm after moving inland.

Helene, which formed Tuesday in the Caribbean, is expected to move over deep, warm waters, fueling its intensification. People in regions under hurricane warnings and watches should be prepared to lose power and should have enough food and water for at least three days, forecasters warned.

Hurricane watches — which are a step down from warnings — were also in effect for parts of western Cuba and Florida, including the Tampa Bay area, the National Hurricane Center said.

“It’s going to be a very large system with impacts across all of Florida,” said Larry Kelly, a specialist at the hurricane center.

Several counties on Florida's west and northwestern coasts have issued evacuation orders. Multiple school districts, including in the areas around Tampa and the state capital Tallahassee, plan to close schools or reduce hours starting Wednesday.

Some residents started filling sandbags ahead of anticipated flooding and began leaving areas on the coast.

President Joe Biden declared an emergency in Florida and deployed Federal Emergency Management Agency teams to Florida and Alabama to support local first responders. Federal authorities were positioning generators, food and water, along with search-and-rescue and power restoration teams, the White House said.

Florida Gov. Ron DeSantis also issued an emergency for most of the state's counties, while Georgia Gov. Brian Kemp declared an emergency in his state as well.

The storm is anticipated to be unusually large and fast-moving, meaning storm surges, wind and rain will likely extend far from the storm’s center, the hurricane center said. States as far inland as Tennessee, Kentucky and Indiana could see rainfall.

Heavy rains and big waves already lashed the Cayman Islands on Tuesday. Officials there 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 3 meters unleashed flooding. Authorities urged people to stay indoors as the storm moved away later Tuesday and said crews would fan out to assess damage.

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

Helene is the eighth named storm of the Atlantic hurricane season, which began June 1. Since 2000, eight major hurricanes have made landfall in Florida, according to Philip Klotzbach, a Colorado State University hurricane researcher.

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 before the season ends Nov. 30, with four to seven major hurricanes of Category 3 or higher.