Helene became a hurricane Wednesday after the huge storm rapidly strengthened in the Caribbean Sea and moved north along Mexico's coast on a path toward the U.S., prompting residents to evacuate, schools to close and officials to declare emergencies in Florida and Georgia.

The storm's centre was near Mexico's Yucatan Peninsula on Wednesday, the U.S. National Hurricane Center said, and it is expected to intensify and grow in size as it crosses 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 centre. 

The storm is so large that areas roughly 145 kilometres north of the Georgia-Florida line could expect hurricane conditions. States as far inland as Tennessee, Kentucky and Indiana could see rainfall. On Wednesday morning, winds of tropical storm force, at least 62 km/h, extended as far as 445 kilometers from Helene's centre.

"You are going to have a major hurricane plowing inland, and storms take a little time to decay once they're inland," said Brian McNoldy, an environmental researcher at the University of Miami.

Forecasters warned of possible tornadoes Wednesday night in western Florida and southern Alabama and said the tornado risk would increase Thursday, expanding across Florida and into Georgia and South Carolina.

Expected to be Category 3 or higher

Helene is expected to become a major hurricane — a Category 3 or higher, with winds above 177 km/h — on Thursday, the day it's set to reach Florida's Gulf Coast, according to the hurricane centre. The centre 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.

As residents of Florida's Big Bend — the curving stretch of Gulf coastline in the state's north — battened down their homes, many saw the ghost of 2018's Hurricane Michael. That storm rapidly intensified and crashed ashore as a Category 5 that laid waste to Panama City and parts of the rural panhandle.

"People are taking heed and hightailing it out of there for higher ground," said Kristin Korinko, a Tallahassee resident who serves as the commodore of the Shell Point Sailboard Club, on the Gulf Coast about 48 kilometres south of the state capital.

In Tallahassee, where stations had started to run out of gas, 19-year-old Florida A&M student Kameron Benjamin filled sandbags with his roommate to protect their apartment before evacuating. Both their school and Florida State University shut down ahead of the storm.

"This hurricane is heading straight to Tallahassee, so I really don't know what to expect," Benjamin said. 

Hurricane's centre could make landfall Thursday

Helene, which formed Tuesday in the Caribbean, is expected to move over deep, warm waters, fuelling its intensification. The hurricane was about 810 kilometres southwest of Tampa, Fla., and had top sustained winds of 130 km/h, according to the hurricane centre.

Forecasters said it is expected to become a major hurricane with its centre making landfall in the Big Bend area of Florida's northwestern coast as soon as late Thursday.

Mara Lezama, the governor of the coastal Mexican state of Quintana Roo, shared photos of rain-swept streets. In Cancun, heavy waves threatened to worsen the resort city's problem with beach erosion.

In western Cuba, authorities moved cattle to higher ground and medical brigades were dispatched to communities usually cut off by storms. The government preventively shut off power in some communities as waves as high as five meters slammed into Cortes Bay.

The Cayman Islands began recovering after heavy rains and big waves lashed them Tuesday. Schools remained closed Wednesday as flood warnings continued and residents pumped water out of their flooded homes.

Tropical storm warnings were in effect for the upper Florida Keys, southern Florida and the state's northeast coast and were extended northward Wednesday morning to Altamaha Sound, Georgia. A tropical storm watch was in effect for the South Carolina coast north of the South Santee River to Little River Inlet.

Hurricane watches, a step down from warnings, were also in effect for parts of western Cuba and Florida, including the Tampa Bay area, the hurricane centre said.

Biden declares emergency in Florida

U.S. President Joe Biden declared an emergency in Florida, and federal authorities positioned generators, food and water, along with search-and-rescue and power restoration teams. 

Florida Gov. Ron DeSantis, who already issued an emergency for most counties, warned residents Wednesday they needed to heed evacuation orders. A dozen health-care facilities including hospitals and nursing homes had evacuated preemptively, DeSantis said. 

"There's clearly a pathway for this to rapidly intensify prior to making landfall," DeSantis said at a news conference in Tampa.

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Georgia Gov. Brian Kemp also declared an emergency in his state. The hurricane warning area included Valdosta, a city of 55,000. Helene approached barely a year after Hurricane Idalia inflicted more than $6 million US in damage to 1,000 homes and other property.

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.