With more than 375,000 people up and down the Gulf Coast warned to clear out, the hurricane's leading edge began lashing the white-sand shoreline with tropical storm-force winds, rain and rising seas before daybreak, hours before Michael's center was expected to blow ashore.
The hurricane is expected to make landfall Wednesday, with population centers like Panama City and Tallahassee sitting in its path. "There are warnings for more than 300 miles (480km) of coastline", the National Weather Service tweeted, predicting that Michael would become a "large and unsafe hurricane".
"The time for evacuating along the coast has come and gone". Up to a foot of rainfall (30 cm) is forecast for some areas.
On the exposed coast of Florida's Big Bend, most of the waterfront homes in Keaton Beach stood vacant amid fears of a life-threatening storm surge in an area that hadn't seen a potentially catastrophic major hurricane in decades.
In addition to the imminent threat to human life, authorities are predicting sustained power outages, and "major infrastructure damage", according to FEMA administrator Jeff Byard.
Among them was Betty Early, 75, a retiree who joined about 300 fellow evacuees huddled on makeshift bedrolls of blankets and collapsed cardboard boxes at an elementary school converted into an American Red Cross shelter in Panama City, near the storm's expected landfall. She was unsure how well her old, wood-framed apartment block would hold up. "My greatest concern is not having electricity, and living on a fixed income, losing my food".
The National Hurricane Center called Michael "potentially catastrophic" and "extremely unsafe". Florida Governor Rick Scott called for residents along the coast to seek refuge immediately. About 2,500 National Guard troops were deployed to assist with evacuations and storm preparations, and more than 4,000 others were on standby. Some 17,000 utility restoration workers were also on call.
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.
The storm caused widespread destruction in Central America over the weekend, where at least 13 people have been reported dead.