The hurricane was expected to gradually weaken as it passed over the southeastern United States, the hurricane center said.
Michael will be the strongest hurricane to hit the Panhandle in recorded history.
The storm is projected to cause roughly $30 billion in damage and lost economic productivity, the service said in a news release.
Members of the Florida National Guard chemical, biological, radiological and nuclear defense (CBRN) Enhanced Response Force Package prepare to help citizens in affected areas prior to landfall of Hurricane Michael at Camp Blanding Joint Training Center near Starke, Fla., on October 9, 2018. Forecasters said it will unleash damaging winds and rain all the way into the Carolinas, which are still recovering from hurricane Florence's epic flooding.
In Florida, Scott declared a state of emergency for 35 counties, from the Panhandle to Tampa Bay, activated hundreds of Florida National Guard members and waived tolls to encourage evacuations. Rick Scott posted a series of tweets warning residents to prepare and evacuate.
Hurricane Michael was upgraded to a Category 4 storm Wednesday ahead of its landfall and could cause "potentially catastrophic" damage to the state of Florida's panhandle, according to the National Hurricane Center (NHC). The hurricane's winds and waves were so strong, their rumblings were detected on seismometers - equipment created to measure earthquakes.
The outer bands of the massive storm are beginning to reach the Gulf Coast.
"As always, we ask that the public take extra care when making repairs following a storm", Campbell said.
The highest winds will be near the coast, with sustained winds over 40 miles per hour and gusts that may exceed 50 miles per hour. Storm surge warnings were also in place along the Florida and Alabama coasts.More news: Category 4 Hurricane Michael making landfall today
The 72-hour rainfall forecast from Hurricane Michael.
Tyndall is home to the 325th Fighter Wing.
Category four is the second highest level on the Saffir-Simpson hurricane wind scale. Tropical-storm force winds extending 185 miles (295 kilometers) from the center were already lashing the coast.
The Miami-based National Hurricane Center said the storm, which local forecasters are calling an "unprecedented" weather event for the area, is expected to slam ashore later in the day with "life-threatening" storm surges. It also was about 170 miles (273 kilometers) southwest of Apalachicola, Florida.
Experts worry the most about Michael's storm surge - forecast at 10 to 12 feet (3 meters).
Along with the mandatory evacuation orders come a notably tighter security.
Six airports in the Florida Panhandle have closed in anticipation of the storm's impacts.
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. According to The Weather Channel, no Category 4 hurricane in historical records dating back to 1851 has ever made landfall in the region.