There were scenes of devastation in Mexico Beach, a community of about 1,000 people where Michael made landfall around 1pm Eastern time - with houses floating in flooded streets, some ripped from their foundations and missing roofs.

Roads were filled with piles of floating debris.

After being battered for nearly three hours by strong winds and heavy rains, roads in Panama City were virtually impassable and trees, satellite dishes and traffic lights lay in the streets.

“Hurricane Michael is forecast to be the most destructive storm to hit the Florida Panhandle in a century,” Governor Rick Scott said.

Briefing US President Donald Trump at the White House, Federal Emergency Management Agency (FEMA) chief Brock Long said Michael was the most intense hurricane to strike the Florida Panhandle since 1851.

“Along our coast, communities are going to see unimaginable devastation,” Scott said, with storm surge posing the greatest danger.

“Water will come miles in shore and could easily rise over the roofs of houses,” he said.

“Those who stick around to experience storm surge don’t typically live to tell about it,” said FEMA’s Long.

Hundreds of thousands of people were ordered to evacuate their homes and the governor told residents who had not done so to “hunker down and be careful.”

Ken Graham, director of the Miami-based National Hurricane Centre, said Michael is “unfortunately, a historical and incredibly dangerous and life-threatening situation.”

Smith, in Gadsden County, said the situation was dangerous even for emergency personnel.

“We’ve been very cautious with sending our first responders out right now,” she said.

Olivia Smith, public information officer for the Gadsden County Board of County Commissioners, said there was “one hurricane-related fatality”, adding that the incident was “debris-related. There was a tree involved.”

Smith could not provide details on the victim.

The death, in an area west of state capital Tallahassee, was the first confirmed since Michael made landfall.

General Terrence O’Shaughnessy, commander of North American Aerospace Defence Command, said some Florida residents may have been surprised by the rapid growth of the storm.

“It really started as a tropical storm, and then it went to Category 1, then it was Category 2 and before you know it, it was Category 4,” O’Shaughnessy said.

Several hours after the hurricane made landfall, the eye of the storm had moved out of the Panhandle and was just west of Albany, Georgia, the NHC said.

“Michael should weaken as it crosses the southeastern United States through Thursday,” the NHC said.

Long, the head of FEMA, said many Florida buildings were not built to withstand a storm above the strength of a Category 3 hurricane on the five-level Saffir-Simpson Hurricane Wind Scale.

As it came ashore, Michael was just shy of a Category 5 – defined as a storm packing wind speeds of 250km/h or above.

An estimated 375,000 people in more than 20 counties were ordered or advised to evacuate.

The National Weather Service office in Tallahassee issued a dramatic appeal for people to comply with evacuation orders.

“Hurricane Michael is an unprecedented event and cannot be compared to any of our previous events. Do not risk your life, leave NOW if you were told to do so,” it said.

The NWS said it had found no record of any previous Category 4 hurricanes that made landfall in the panhandle or the “Big Bend” coastal region.

“This situation has NEVER happened before,” it said on Twitter.

Trump issued an emergency declaration for Florida, freeing up federal funds for relief operations and providing the assistance of FEMA, which has more than 3,000 people on the ground.

State officials issued disaster declarations in Alabama and Georgia and the storm is also expected to bring heavy rainfall to North and South Carolina.

The Carolinas are still recovering from Hurricane Florence, which left dozens dead and is estimated to have caused billions of dollars in damage last month.

It made landfall on the coast as a Category 1 hurricane on September 14 and drenched some parts of the state with 101cm (40 inches) of rain.

Last year saw a string of catastrophic storms batter the western Atlantic – including Irma, Maria and Harvey, which caused a record-equalling US$125 billion in damage when it flooded the Houston metropolitan area.

Scientists have long warned that global warming will make storms more destructive, and some say the evidence for this may already be visible.