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Hurricane Michael has strengthened to a category three storm, with winds near 120mph (193km/h) as it churns towards the Florida coast, the National Hurricane Center (NHC) says.
The storm is expected to make landfall on Wednesday. Florida Governor Rick Scott has warned residents to evacuate.
Alabama, Florida and Georgia have declared states of emergency in all or parts of the states.
At least 13 people have reportedly died in Central America due to the storm.
Governor Scott called for coastal residents to evacuate, saying: “It could be the difference between life and death.”
Alabama is under a state-wide state of emergency ahead of the hurricane, along with 92 counties in southern Georgia and 35 counties in Florida.
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Some Floridians are storing their boats in dry docks ahead of the storm
Forecasters say some regions of the US may see 12in (30cm) of rain, and storm surges of up to 12ft (3.6m).
“Strengthening is expected overnight and on Wednesday, and Michael is forecast to be near category four strength when it makes landfall in the Florida Panhandle or the Florida Big Bend area,” the NHC said in its latest bulletin.
“Weakening is expected after landfall as Michael moves across the south-eastern United States,” it added.
The Florida governor called Michael “a monstrous storm” and urged residents to listen to officials.
It is expected to quickly travel up the US East Coast after making landfall on the Gulf Coast.
Heavy rains are forecast for the Carolinas, which were drenched by Hurricane Florence last month.
North Carolina Governor Roy Cooper told residents: “I know people are fatigued from Florence, but don’t let this storm catch you with your guard down.”
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Residents put plywood up in Port St Joe, Florida
More than 300 miles of coastline are currently under threat, the National Weather Service has said.
The agency warned residents in Florida and Alabama of possible storm surges, high winds and flash flooding.
Forecasters in Alabama warned of possible tornados.
President Donald Trump told reporters on Tuesday: “We are very well prepared for the incoming hurricane.”
Governor Scott warned in a news conference that Hurricane Michael is a “massive storm that could bring total devastation to parts of our state, especially in the panhandle”.
He added that it is predicted to be “the most destructive storm to hit the Florida panhandle in decades”.
Some 120,000 people have been warned to evacuate along Florida’s coast, where schools and state offices are to remain shut this week.
Gov Scott warned that he may order more evacuations due to the size of the potential storm surge.
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“No one’s going to survive” such a wall of water, he said.
On Tuesday, Gov Scott said he activated 2,500 Florida National Guard troops in preparation for the storm.
The neighbouring states of Alabama and Georgia have also declared a state of emergency.
Where has Hurricane Michael hit so far?
The storm caused widespread destruction in Central America over the weekend, where at least 13 people have been reported dead.
According to the Associated Press, six people were killed in Honduras, four in Nicaragua and three in El Salvador.
Images on social media showed evacuating families wading through water to get to safety.
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EPA
Western Cuba, including Havana, was struck by Hurricane Michael on Monday
Parts of western Cuba, which was hit by the storm on Monday, were forecast to receive up to a foot of rain.
Offshore oil platforms in the Gulf of Mexico have evacuated workers, halting nearly a fifth of daily production.
Five drilling rigs have been moved out of the storm’s path, according to the US Bureau of Safety and Environmental Enforcement.
Hurricanes
A guide to the world’s deadliest storms
Hurricanes are violent storms that can bring devastation to coastal areas, threatening lives, homes and businesses.
Hurricanes develop from thunderstorms, fuelled by warm, moist air as they cross sub-tropical waters.
Warm air rises into the storm.
Air swirls in to fill the low pressure in the storm, sucking air in and upwards, reinforcing the low pressure.
The storm rotates due to the spin of the earth and energy from the warm ocean increases wind speeds as it builds.
When winds reach 119km/h (74mph), it is known as a hurricane – in the Atlantic and Eastern Pacific – or a typhoon in the Western Pacific.
“Everybody has a plan until they get punched in the face. Well, we’re about to get punched in the face.”
Florida Mayor Bob Buckhorn, ahead of Hurricane Irma (2017)
The central eye of calmer weather is surrounded by a wall of rainstorms.
This eyewall has the fastest winds below it and violent currents of air rising through it.
A mound of water piles up below the eye which is unleashed as the storm reaches land.
These storm surges can cause more damage from flooding than the winds.
“Urgent warning about the rapid rise of water on the SW FL coast with the passage of #Irma’s eye. MOVE AWAY FROM THE WATER!”
Tweet from the National Hurricane Center
The size of hurricanes is mainly measured by the Saffir-Simpson scale – other scales are used in Asia Pacific and Australia.
Winds 119-153km/h
Some minor flooding, little structural damage.
Storm surge +1.2m-1.5m
Winds 154-177km/h
Roofs and trees could be damaged.
Storm surge +1.8m-2.4m
Winds 178-208km/h
Houses suffer damage, severe flooding
Storm surge +2.7m-3.7m
Hurricane Sandy (2012) caused $71bn damage in the Caribbean and New York
Winds 209-251km/h
Some roofs destroyed and major structural damage to houses.
Storm surge +4m-5.5m
Hurricane Ike (2008) hit Caribbean islands and Louisiana and was blamed for at least 195 deaths
Winds 252km/h+
Serious damage to buildings, severe flooding further inland.
Storm surge +5.5m
Hurricane Irma (2017) caused devastation in Caribbean islands, leaving thousands homeless
“For everyone thinking they can ride this storm out, I have news for you: that will be one of the biggest mistakes you can make in your life.”
Mayor of New Orleans Ray Nagin ahead of Hurricane Gustav, 2008