The hurricane's sustained winds are expected to hit up to 120 miles per hour by late Tuesday, making it a major hurricane - the designation for Category 3 storms and above, on the Saffir-Simpson wind scale.
Now ranked as a Category 1 storm on the five-step Saffir-Simpson wind scale, Michael could grow into a Category 3, becoming the second major hurricane in the Atlantic basin this year following Florence last month, forecasters said.
"The center of Michael is expected to move inland over the Florida Panhandle or Florida Big Bend area on Wednesday and then move northeastward across the southeastern United States Wednesday night and Thursday", the agency's advisory said.
Florida Governor Rick Scott declared a state of emergency for 35 counties, activating 500 National Guard troops in preparation for the storm. Alabama Gov. Kay Ivey said she feared widespread power outages and other problems would follow. It may reach category 3 strength by landfall on Wednesday.
Tallahassee Mayor Andrew Gillum, who is also the Democratic gubernatorial candidate in the November 6 election, said on Sunday he was cancelling his campaign events and returning to Tallahassee, the state capital, to focus on storm preparations.
Michael's large size, strong winds and heavy rains could produce hazardous flooding along a stretch of Florida's Gulf coast with many rivers and estuaries where seawater pushed ashore by a hurricane could get trapped, said Hurricane Center Director Ken Graham. Based on the latest analysis from the NHC, surge inundation flooding of 9 to 13 feet is possible in vulnerable areas between Mexico Beach and Keaton Beach. That's when Maryland could see the first remnants of the storm.
The category-1 hurricane left dozens dead in the Carolinas and is estimated to have caused billions of dollars in damage.
Florida has already declared an emergency in 26 counties.
"This will not be just a coastal event, with unsafe winds and flooding rains spreading far inland over the southeastern U.S", said Rick Knabb, The Weather Channel's hurricane expert.
It made landfall on the coast as a Category 1 hurricane on September 14 and drenched some parts of the state with 40 inches of rain. It will weaken into a tropical storm near the Carolinas and Virginia on its way back to the Atlantic Ocean, forecasters said.
In anticipation of travel impacts, Southwest has implemented a flexible rebooking policy for customers scheduled to fly to or from several cities in the storm's path this week, including New Orleans, Panama City, Florida and Pensacola, Florida Tuesday through Thursday. He added, "Today it is about life and safety". King tides also could factor in to how much water the area will see, but, he said, those also are being monitored. The storm will also unleash flooding rain and destructive winds starting Tuesday night and continuing through Wednesday.