Hurricane Michael unleashed a furious onslaught on Florida’s Panhandle on Wednesday, killing two people and tearing apart buildings before taking aim at Georgia and the Carolinas.

Michael’s 155mph winds at landfall were only 5mph short of category 5 status, making it the strongest storm to strike the United States since Hurricane Andrew ravaged southern Florida in 1992.

Governor Rick Scott, whose record on the environment has been slammed by critics, warned that a tempest of this strength had not hit the area in more than a century. Outside experts noted that human-induced climate change is increasing the regularity and intensity of monster storms.

“Communities are going to see unimaginable devastation,” Scott had warned.

Two deaths have been reported as a result of the hurricane. A man in Gadsden county, Florida, was killed after a tree crashed through the roof of his home and a child in Seminole county, Georgia, was believed to have been killed after something fell on a home they were visiting. Authorities said details about the child’s death were still unclear because they had not been able to reach the home yet.

No category 4 storm has ever made landfall in the Panhandle, and Michael’s vast size meant the effects would be felt along the coast from New Orleans to Tampa Bay, and inland as far as the Carolinas.

Late on Wednesday night, the National Hurricane Centre said the storm had weakened over land to a tropical storm.

Play Video 1:49 Hurricane Michael pounds Florida – video report

This was a very, very powerful storm,” said Louis Uccellini, director of the National Weather Service, told CNN. “When people get out tomorrow and start making their assessments they’re going to realise just how powerful a category 4 storm can be. You’re going to see a very extensive area of damaging winds all the way into central Georgia.”

Television pictures showed buildings under water in Mexico Beach, and several properties reduced to matchwood in Panama City Beach. More than 120,000 customers were without power by late afternoon, a number certain to rise.

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Michael’s official landfall happened just north of Mexico Beach, a small coastal town 25 miles east of Panama City and with a population of 1,000, at about 1.30pm local time. According to the National Hurricane Center (NHC), the storm blew in with sustained winds of 155mph and travelling at 14mph.

More than half a million people were either ordered or advised to evacuate as Michael, fuelled by the warm waters of the Gulf of Mexico, closed in on Panama City, bringing with it a life-threatening storm surge of up to 14ft. The storm has already been blamed for 13 deaths in Central America and the Caribbean.

Authorities in Florida warned that anybody who chose not to evacuate would be on their own, with conditions already too hazardous for emergency workers to operate.

“Nobody’s coming out to save your life today,” Scott said on CNN. “We’ve done everything we can to tell people to evacuate and some people have unfortunately chosen not to. I’m scared to death if there’s any kids who didn’t make that choice on their own.”

More than 20 million people in five states remained under either a hurricane or tropical storm warning, while areas in North and South Carolina flooded by Hurricane Florence last month braced for even more rainfall.

The fast-moving tropical cyclone’s outer bands were already causing torrential rain and flooding in Panama City Beach and Apalachicola by mid-morning on Wednesday, with the eye of the storm 50 miles away and travelling north at 14mph.

“This is a truly historic and very dangerous storm,” Ken Graham, the director of the NHC in Miami, said in a Wednesday morning briefing. “Even 185 miles away from the centre, tropical storm force winds are still occurring. It’s still a hurricane in Georgia, stretching over the Carolinas by the time it’s all done.”

Play Video 0:18 Hurricane Michael tears apart roof as it makes landfall in Florida - video

Greg Brudnicki, the mayor of Panama City, said on Wednesday that many residents had ignored warnings to evacuate.

“Two-thirds of the county, 120,000 people, were told would you please leave, and as of last night we probably had only about 25,000 leave,” he said.

“So the people that are staying, it’s all built on years and years of complacency because we said it’s gonna be bad, it’s gonna be bad for years, and it never turned out to be. [But] the odds are that eventually we would get a bad one, and we’ve got a bad one.”

Scott said at a morning press conference that he had activated 3,500 national guard soldiers, 54 shelters were open across the Panhandle and that 19,000 utility workers from around the country were positioned to move in after the storm’s passage to restore power.

“Communities are going to see unimaginable devastation. This is the worst storm in the Panhandle in more than a century,” Scott said. “The time to evacuate in coastal counties has come and gone. The worst thing you could do now is leave and put yourself and your family in danger.”

Meanwhile Brock Long, the administrator of the Federal Emergency Management Agency (Fema), said the storm’s effects would be far-reaching and long-lasting. “The citizens of Georgia need to wake up and pay attention,” he said. “The bottom line is it’s going to be the worst storm south-west and central Georgia has seen in many decades.

“They need to be prepared, not only in Florida but Georgia as well, to see the power off for multiple weeks.”

On Tuesday, Donald Trump approved a major disaster declaration request for Florida, freeing federal resources and money for recovery efforts. In Georgia, more than 90 counties remained under a state of emergency and the governor, Nathan Deal, activated 1,500 national guard personnel.

Earlier this week, Michael triggered flash floods and mudslides in mountain areas of western Cuba, while disaster agencies in El Salvador, Honduras and Nicaragua reported 13 deaths as roofs collapsed and residents were carried away by swollen rivers. Six people died in Honduras, four in Nicaragua and three in El Salvador.