As of 5 am EDT (0900 GMT) on Wednesday, Idalia was packing maximum sustained winds of 215 kph (130 mph) as it churned toward shore 95 km (60 miles) west of Cedar Key, Florida.
Any storm reaching Category 3 or higher is considered a major hurricane.
Idalia is the fourth major hurricane to strike Florida in the past seven years, following Irma in 2017, Michael in 2018 and Ian, which peaked at Category 5, last September.
Here is what to know before Idalia makes landfall on Wednesday.
How are people preparing for Idalia?
Millions of people in the storm’s path tied down boats, boarded up windows, sandbagged their properties and headed for higher ground.
About 5,500 National Guard members have been mobilised, while 30,000 to 40,000 electricity workers were on standby. The state has set aside 1.1 million gallons of gasoline to address any interruptions to fuel supplies, Florida Governor Ron DeSantis said.
More than 40 school districts in Florida cancelled classes, he added, and Tampa International Airport suspended commercial operations on Tuesday.
Mandatory evacuation orders had been issued in at least 28 of Florida’s 67 counties as of Tuesday night.
“If you have not evacuated, you need to do that right now,” Florida emergency management chief Kevin Guthrie said during an evening news briefing. “You need to drop what you’re doing. You need to go to your room, pack up, pack your things and get to safety.”
Most of Florida’s 21 million residents, and many in the adjacent states of Georgia and South Carolina, were under hurricane warnings and other storm-related advisories. State emergency declarations were issued in all three.
Where is the hurricane expected to hit?
Idalia grew from a tropical storm into a hurricane early on Tuesday, a day after passing west of Cuba, where it damaged homes, knocked out power, flooded villages and prompted mass evacuations.
It attained “an extremely dangerous Category 4 intensity” on the five-step Saffir-Simpson wind scale on its way to make landfall in Florida on Wednesday morning, the National Hurricane Center (NHC) in Miami said.
The NHC said Idalia’s centre would most likely cross Florida’s shoreline somewhere in the Big Bend region, where the state’s northern Gulf Coast panhandle curves into the western side of the Florida Peninsula, roughly bounded by the inland cities of Gainesville and Tallahassee, the state capital.
Sparsely populated compared with the Tampa-St Petersburg area to the south, the Big Bend features a marshy coast, threaded with freshwater springs and rivers, and a cluster of small offshore islands forming Cedar Key, an historic fishing village demolished in 1896 by a hurricane’s storm surge.
Surge warnings were posted for hundreds of miles of shoreline, from Sarasota to the sport fishing haven of Indian Pass at the western end of Apalachicola Bay. In some areas, depending on tides, the surge of water could rise 12 to 16 feet (3.7 to 4.9 metres), the NHC said.
Officials said the storm’s most dangerous feature would be a powerful surge of wind-driven surf that is expected to flood barrier islands and other low-lying areas along the coast.
Where is Idalia expected to go next?
Idalia is forecast to cross the Florida peninsula and then drench southern Georgia and the Carolinas on Thursday with 4 to 8 inches (100 to 200mm) of rain.
Both Georgia Governor Brian Kemp and South Carolina Governor Henry McMaster announced states of emergency, freeing up state resources and personnel, including hundreds of National Guard troops.