Michael is forecast to move off the Mid-Atlantic coast by early Friday, the NHC said.
By early Thursday, the ferocious storm's winds had slowed to 50 miles per hour as it crossed Georgia.
Michael is continuing to strengthen as its minimum central pressure has dropped to 919 millibars.
"We are in new territory", National Hurricane Centre Meteorologist Dennis Feltgen wrote on Facebook.
"We are catching some hell", said Timothy Thomas, who rode out the storm with his wife in their second-floor apartment in Panama City Beach. Classified as a Category 5 - the highest ranking on the Saffir-Simpson scale - the greatest number of fatalities was in Honduras where severe rainfall led to mud slides and flooding. Once that happens, wind speeds increase. That's just 2 miles per hour away from officially being a Category 5 hurricane.
Two to three inches of rain is expected for the area during the storm as well.
The hurricane strengthened due to the water temperature in the Gulf of Mexico.
People seek safety in a shelter as Hurricane Michael approaches, October 10, 2018, in Panama City, Fla.
Brock Long, the head of the Federal Emergency Management Agency (FEMA), said many Florida buildings were not built to withstand a storm above the strength of a Category 3 hurricane. "This is going to have structure-damaging winds along the coast and hurricane force winds inland".
Among the smaller utility providers, Talquin Electric -- which provides electricity in Gadsden, Liberty, Leon, and Wakulla counties -- reported 35,314 of its 52,013 customers were affected by the storm.
It is truly hard to imagine what the conditions are like right now down in parts of the Florida panhandle.
Meanwhile, Michael continues its track across southern Georgia toward the Carolinas.
Michael was forecast to bring a storm surge up to 14 feet (4.3 metres) in some areas.
After landfall as a Category 4 storm, Michael will march northeast through the southeast US and remain at least a Category 1 storm well into Georgia. With the Tropical Storm Warning in effect across southeast Georgia the Sidney Lanier will be closed at 6pm Wednesday.
A Storm Surge Watch has been issued for the coast of North Carolina from Ocracoke Inlet to Duck. The storm had top sustained winds of 112 km/h and was moving to the northeast at 27 km/h.
The governors of North and SC urged residents to brace for more heavy rain and storm-force winds as Michael plows northward up the Atlantic seaboard. Expected landfall is mid-afternoon.
The lower the air pressure, the less air molecules there are. "The historical record, going back to 1851, finds no Category 4 hurricane ever hitting the Florida Panhandle". Even with all of those warnings, more than 15 million Floridians lost power during that hurricane.
In terms of damage the most costly year was 2017, when besides the Caribbean several U.S. coastal states were hit by powerful hurricanes that moved slowly and dumped record rainful for days.