"Michael is likely to produce potentially catastrophic wind damage where the core of the hurricane moves onshore in the Florida Panhandle, and everyone in the hurricane warning area should prepare for life-threatening hurricane winds".
This will be the third storm to hit Northwest Florida this year.
Some 120,000 people have been warned to evacuate along Florida's coast, where schools and state offices are to remain shut this week.
"We just need to be prepared", McMaster said.
But Kottlowski noted that meteorologists are still analyzing Hurricane Michael, and it's possible that it will be retroactively upgraded to Category 5 status based on wind speeds.
Here are the key messages from this Major Hurricane.
A historic Hurricane Michael made landfall Wednesday near Mexico Beach, pulverizing homes, snapping trees and sending debris flying.
"Michael is forecast to be a major hurricane at landfall in Florida", the NHC says, adding that "weakening is expected after landfall".More news: European Leaders Demand Investigation Of Slain Bulgarian Journalist
Experts worry the most about Michael's storm surge - forecast at 10 to 12 feet (3 meters). The heavy rains could cause flash floods, the NHC said, and spawn tornados in northwestern Florida. On the current track, tropical storms force winds could extend as far west as Pensacola, with the greatest impact to the Panama City area.
The hurricane center's Graham said Michael represented a "textbook case" of a hurricane system growing stronger as it drew near shore, in contrast to Hurricane Florence, which struck North Carolina last month after weakening in a slow, halting approach.
1 A.M. As the outer rain bands of Hurricane Michael make their way onshore in the Panhandle, the eye of the storm has strengthened into a Category 4 hurricane.
Just before noon ET, Michael was 40 miles south of Panama City, Fla., moving north at 14 mph.
The earliest reasonable arrival times of tropical-storm-force winds from Hurricane Michael as of 5 a.m. ET on Wednesday.
About 3.7 million people are under hurricane warnings in Florida's Panhandle and Big Bend regions, along with parts of southeastern Alabama and southern Georgia.
Meanwhile, Hurricane Michael is now predicted to reach a level close to category 4 when it hits the USA state of Florida on Wednesday.
"I didn't expect all this", said Bill Manning, 63, a grocery clerk who left his camper van in Panama City, Florida, to move into a hotel where the power was already out.