The storm took its first victim when a tree fell on a house, killing a man. With roaring winds, Michael blew ashore near Mexico Beach, a tourist town about midway along the Panhandle, a 320-km stretch of white-sand beach resorts.
Hurricane Michael claimed its first life after roaring ashore in Florida on Wednesday, flooding homes and streets and toppling trees and power lines in the Gulf of Mexico beachfront area where it made landfall as a raging Category 4 storm.
Florida officials said Michael, packing winds of 155 miles per hour (250 kilometres per hour), was the most powerful storm to hit the state's northern Panhandle area in more than a century.
Michael had weakened to a Category 1, with maximum winds of 90 mph as of 8:00 pm Eastern time (0000 GMT), but that still left it an extremely dangerous storm.
Pictures and video from Mexico Beach – a community of about 1,000 people where Michael made landfall around 1:00 pm Eastern time (1700 GMT) – showed scenes of devastation, 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.
Jay Gray reports from Panama City Beach, Florida.
More than 375,000 people up and down the Gulf Coast were urged to evacuate. But emergency authorities lamented that many people ignored the warnings and seemed to think they could ride it out.
US President Donald Trump expressed support to the residents of Florida in a tweet, saying "We are with you Florida!"
Briefing 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.
Scientists say global warming is responsible for more intense and more frequent extreme weather, such as storms, droughts, floods and fires.
But without extensive study, they cannot directly link a single weather event to the changing climate.
With Election Day less than a month away, the crisis was seen as a test of leadership for Rick Scott, a Republican running for the Senate, and Tallahassee Mayor Andrew Gillum, the Democratic nominee for governor.