Alabama, Florida and Georgia have declared states of emergency in all or parts of the states.
At least 13 people have reportedly died in Central America due to the storm.
US President Donald Trump told reporters on Tuesday: "We are very well prepared for the incoming hurricane."
Governor Scott called for coastal residents to evacuate, saying: "It could be the difference between life and death."
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.
"Strengthening is expected overnight and on Wednesday, and Michael is forecast to be near category four strength when it makes landfall in the Florida Panhandle or the Florida Big Bend area," the NHC said in its latest bulletin.
"Weakening is expected after landfall as Michael moves across the south-eastern US," it added.
The Florida governor called Michael "a monstrous storm" and urged residents to listen to officials.
It is expected to quickly travel up the US East Coast after making landfall on the Gulf Coast.
Heavy rains are forecast for the Carolinas, which were drenched by Hurricane Florence last month.
North Carolina Governor Roy Cooper told residents: "I know people are fatigued from Florence, but don't let this storm catch you with your guard down."
The agency warned residents in Florida and Alabama of possible storm surges, high winds and flash flooding.
He added that it is predicted to be "the most destructive storm to hit the Florida panhandle in decades".
Around 120,000 people have been warned to evacuate along Florida's coast, where schools and state offices are to remain shut this week.
Scott warned that he may order more evacuations due to the size of the potential storm surge.
(With inputs from agencies.)