The storm currently has sustained winds of around 120mph, a category 3 hurricane on the Saffir-Simpson Hurricane Wind Scale, but is expected to strengthen overnight.
When Michael makes landfall in the Florida Panhandle or the Florida Big Bend area it will be nearing the Category 4 mark.
It is expected to weaken after landfall, as it moves northeastward across the southeastern states on Wednesday night and Thursday.
A hurricane warning is in effect from the Alabama-Florida border to the Suwannee River.
Florida Governor Rick Scott said: “This storm is dangerous, and if you don’t follow warnings from officials, this storm could kill you.”
He added it may be the most destructive storm that the Florida Panhandle has seen in decades.
Hurricane-force winds extend outward up to 45 miles from the centre and tropical-storm-force winds extend outward of up to 175 miles.
Several US Gulf Coast counties have issued curfews ahead of Michael’s arrival.
A storm surge warning is also in effect from Florida’s Okaloosa/Walton County Line to the Anclote River.
Coastal residents have rushed to board up their homes and stock up on bottled water and other supplies.
Jason McDonald, a resident of Panama City said he and his wife were going to drive north into Alabama with their two young children.
He said: “We don’t know if it’s going to wipe out our house or not.”
Forecasters said the hurricane could also bring 3 to 6 inches of rain to Georgia, North and South Carolinas and Virginia, triggering flash flooding in areas still recovering from Florence.
North Carolina Governor Roy Cooper said: “I know people are fatigued from Florence, but don’t let this storm catch you with your guard down.
“A number of homes have rooftop tarps that could be damaged or blown away with this wind.”
Michael will be first major hurricane to strike the Florida Panhandle since Hurricane Dennis in 2005.
More than 20 million people are under hurricane watch or warning across five states (Florida, Alabama, Mississippi, Georgia and South Carolina).
There were mandatory evacuation orders for 120,000 people in Panama City Beach and other low-lying parts of the Gulf Coast.