Michael, whose rapid intensification as it churned north over the Gulf of Mexico caught many by surprise, made landfall on Wednesday afternoon (local time) near Mexico Beach, about 32km southeast of Panama City in Florida's Panhandle region, with top sustained winds reaching 249kmh.
The storm came ashore as a Category 4 hurricane on the five-step Saffir-Simpson wind scale, the biggest storm on record to strike the Florida Panhandle. Its sustained winds were just 3.2kmh shy of an extremely rare Category 5.
The storm's intensity waned steadily as it pushed inland and curled northeasterly into Georgia after dark. It was downgraded to a Category 1 storm, with top sustained winds diminishing to 120kmh, about nine hours after it made landfall.
The governors of North and South Carolina urged residents to brace for more heavy rain and storm-force winds as Michael ploughs northward up the Atlantic seaboard. The Carolinas are still reeling from severe flooding in the aftermath of Hurricane Florence less than a month ago.
The National Hurricane Center said Michael would pass through the Carolinas as a tropical storm on Thursday, dumping as much as 20cm of rain in some areas. Up to 30cm of rain was forecast in Florida.
Gadsden County sheriff's spokeswoman Anglie Hightower said a "male subject" was killed by a tree toppling on to his house in Greensboro, Florida, near the state capital, Tallahassee, in the first report of a fatality from the hurricane.
Severe flooding, heavily damaged buildings, uprooted trees and downed power lines appeared widespread in coastal areas near the storm's landfall.
Television news footage during the day showed many homes submerged in floodwaters up to their roofs in Mexico Beach, where the fate of about 280 residents who authorities said defied evacuation orders was unknown.
Numerous buildings in Panama City were demolished, partially collapsed or without roofs amid deserted streets littered with debris, twisted, fallen tree trunks and dangling wires.
Authorities said the full extent of devastation would not be known until after daybreak on Thursday. In the meantime, curfews were imposed across much of the region.
Bill Manning, a 63-year-old grocery clerk, fled his camper van in Panama City for safer quarters in a hotel only to see the electricity there go out.
"My God, it’s scary. I didn’t expect all this," he said.
Without power, the city was plunged into darkness at nightfall and its flooded streets were mostly silent and devoid of people or traffic.
By Wednesday night, more than 403,000 homes and businesses were without electricity in Florida, Georgia and Alabama, utility companies said.
Twenty miles south of Mexico Beach, floodwaters were more than 2.3m deep near Apalachicola, a town of about 2,300 residents, hurricane center chief Ken Graham said. Wind damage was also evident.
"There are so many downed power lines and trees that it's almost impossible to get through the city," Apalachicola Mayor Van Johnson said.
The Federal Emergency Management Agency (FEMA) said on Tuesday an estimated 500,000 Florida residents had been ordered or urged to seek higher ground before the storm in 20 counties spanning a 320km stretch of shoreline.
But Brad Kieserman of the American Red Cross said on Wednesday as many as 320,000 people on Florida's Gulf Coast had disregarded evacuation notices.
An estimated 6,000 evacuees took cover in emergency shelters, most of them in Florida, and that number was expected to swell to 20,000 across five states by week's end, Kieserman said.
Bo Patterson, the mayor of Port St. Joe, just south of Mexico Beach, rode out the storm in his house seven blocks from the beach, describing the scene outside as "very, very scary."
He was one of about 2,500 of the town's 3,500 residents who ended up staying put, many of them caught off guard by the storm's rapid escalation as it approached. "This happened so quickly," he said.
FEMA head Brock Long acknowledged that early evacuation efforts in the area were slow in comparison to how quickly the hurricane intensified. Michael grew from a tropical storm into a Category 4 hurricane over the course of about 40 hours.
"Satellite images of Michael's evolution on Tuesday night were, in a word, jaw-dropping," wrote Bob Henson, a meteorologist with weather site Weather Underground.
With a low barometric pressure recorded at 919 millibars, the measure of a hurricane's force, Michael ranked as the third-strongest storm on record to make landfall in the continental United States. Only Hurricane Camille on the Mississippi Gulf Coast in 1969 and the so-called Labor Day hurricane of 1935 in the Florida Keys were more intense.
President Donald Trump declared a state of emergency for all of Florida, freeing up federal assistance to supplement state and local disaster responses.
About 3,500 Florida National Guard troops were deployed to assist with evacuations and storm recovery, along with more than 1,000 search-and-rescue personnel, Governor Rick Scott said.
The Pentagon said it had pre-positioned more than 2,200 active-duty military personnel, along with helicopters, high-water vehicles and swift-water boats for deployment as needed.
Even before landfall, the hurricane disrupted energy operations in the Gulf, cutting crude oil production by more than 40 percent and natural gas output by nearly a third as offshore platforms were evacuated ahead of the storm's arrival.
