With winds of up to 145 mph (233 kph), storm surges of between 9 and 14 feet (2.7 to 4.2 meters) have been forecast for coastal areas.
Florida Governor Rick Scott said Michael could be the worst storm in decades to hit the state's northwest, also known as the Florida Panhandle.
As many as 180,000 residents were ordered to evacuate their homes. Most evacuations were ordered from Bay County in the panhandle, a low-lying area made up mostly of resorts and retirement communities.
US President Donald Trump issued an emergency declaration for Florida and announced on Twitter that the government was freeing up federal funds for relief operations and providing assistance for the Federal Emergency Management Agency (FEMA).
"It is imperative that you heed the directions of your State and Local Officials. Please be prepared, be careful and be SAFE!" the president tweeted.
Tracking Michael's routeNational Hurricane Center spokesman Dennis Feltgen said "we are in new territory with now Hurricane Michael and its 130 mph sustained winds," adding that Bay County was the likely "ground zero" for the hurricane on Wednesday afternoon.
The outer bands of the storm were beginning to reach the Gulf Coast at 7am local time, with some of the worst storm surge expected to hit Florida's Tyndall Air Force Base.
According to the NHC, some parts of Florida could see storm surges of up to 13 feet (4 meters) and up to a foot of rain.
Read more: Texas recovers under the long shadow of Hurricane Harvey
The storm is expected to weaken as it moves up into the southeastern United States.
Nevertheless, state officials have also issued a disaster declaration for Alabama and Georgia. The Carolinas, meanwhile, are still reeling from Hurricane Florence, which killed dozens and caused billions of dollars in damage last month.
However, when Florence barreled towards the Carolinas, residents had five days' notice from the time it had turned into a hurricane and the moment it hit. Michael's increasing strength effectively gave locals in Florida just two days' notice.
Read more: Climate change and extreme weather: Science is proving the link
Last year saw an array of devastating storms batter the USA's western Atlantic seaboard, including Irma, Maria and Harvey. Houston's metropolitan area suffered a record-equaling $125 billion (€108 billion) in damage.
Climate scientists have long warned that the effects of global warming make storms more destructive and point to last year's string of hurricanes as visible evidence.
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dm,es/jm (AP, AFP, Reuters)