Hurricane Michael became a Category 3 storm with 120 miles per hour maximum sustained winds on October 9-ahead of making landfall along the Florida Panhandle, according to the National Oceanic and Atmospheric Administration (NOAA).
Hurricane-force winds extended outward up to 45 miles from the center.
The University of Georgia's Marshall Shepherd, a former president of the American Meteorological Society, called it a "life-altering event", writing on Facebook that he watched the storm's growth on satellite images with a pit in his stomach.
- Storm riders: Roughly 375,000 people in Florida warned to evacuate; many refused, including 285 people in Mexico Beach where Michael made landfall.
Ken Graham, director of the Miami-based National Hurricane Center, said Michael is "unfortunately, a historical and incredibly risky and life-threatening situation". "There was a tree involved".
Tallahassee Mayor Andrew Gillum, Florida's Democratic nominee for governor, filled sandbags with residents and urged residents of the state capital city to finish up emergency preparations quickly.
North Carolina Governor Roy Cooper told residents: "I know people are tired from Florence, but don't let this storm catch you with your guard down".
"Don't think that you can ride this out if you're in a low-lying area", Mr Nelson said on CNN.
"Where that becomes a factor is with the evacuation of some of the local populations", he said.
Forecasters say Michael was upgraded to a Category 2 storm Tuesday morning, and it could make landfall as a Category 3.
As Michael moved over open water, energy companies halted almost one-fifth of Gulf of Mexico oil production and evacuated personnel from 10 platforms on Monday.
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Then, it will "move northeastward across the southeastern United States Wednesday night and Thursday, and move off the Mid-Atlantic coast away from the United States on Friday", the NHC said.
As it came ashore, Michael was just shy of a Category 5 - defined as a storm packing wind speeds of 157 miles per hour or above.
Evacuations spanned 22 counties from the Florida Panhandle into north-central Florida.
As a result of the strengthening, the National Weather Service in Tallahassee has issued the regions first ever extreme wind warning effective until 2:15 p.m. EDT.
"I haven't made up my mind yet, but I know some of my staff are going", said Dave Trepanier, owner of Firefly, a restaurant a block from the beach in Panama City, Bay County, an area where the hurricane is expected to come ashore.
"This situation has NEVER happened before", it said on Twitter.
Forecasters warned that Michael could ultimately dump a foot (30 centimeters) of rain in western Cuba, triggering flash floods and mudslides in mountain areas.
Trump issued an emergency declaration for Florida, freeing up federal funds for relief operations and providing the assistance of FEMA, which has more than 3,000 people on the ground.
Should the storm tear into Big Bend, high winds will likely take down the area's heavy oak and pine trees and cause widespread power outages, which could be a factor in Georgia and the Carolinas as well.
Previous year saw a string of catastrophic storms batter the western Atlantic - including Irma, Maria and Harvey, which caused a record-equaling $125 billion in damage when it flooded the Houston metropolitan area.
On its current track, it would make landfall somewhere along a coastline that includes the cities and towns of Fort Walton Beach, Panama City Beach, Port St. Joe, St. Teresa and the wildlife reserves bordering Apalachee Bay.