"Unsafe hurricane-force winds will also extend well inland across portions of the Florida Panhandle, southern Georgia, and southeast Alabama as Michael moves inland", the National Hurricane Center said on Wednesday morning.
"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".
Communications outages and roads blocked by downed trees, strewn power lines and debris made it hard to get an overall assessment on Thursday of the damage wrought by Michael, but the initial picture was grim.
The devastation inflicted by Hurricane Michael came into focus Thursday with rows upon rows of homes found smashed to pieces and rescue crews struggling to enter stricken areas in hopes of accounting for hundreds of people who may have stayed behind. Homes were split open by fallen trees.
Long said several hospitals in the Panhandle were hit by the hurricane and patients had to be evacuated.
Although the full extent of the damage is not yet clear, one thing is certain: Hurricane Michael has been confirmed as the third-most powerful hurricane on record to hit the continental United States.More news: Pentagon orders temporary pause in F-35 flights after crash
Gulf Coast Regional Medical Center in hard-hit Panama City, 20 miles (32 km) northwest of Mexico Beach, was treating some of the injured. Those winds were tearing some buildings apart in Panama City Beach.
At the city's Jinks Middle School, the storm peeled back part of the gym roof and tore off a wall. A year ago the school welcomed students and families displaced by Hurricane Maria in Puerto Rico.
More than 900,000 homes and businesses in Florida, Alabama, Georgia and the Carolinas were without power.
"I have had employees going to the communities where our kids live, going door to door and checking", Principal Britt Smith by phone.
By 2 p.m. EDT (1600 GMT), the storm had pushed northeast to within 25 miles (40 km) of Greensboro, North Carolina, carrying 50-mph (85-kph) winds, the NHC said. He said people will "not be dragged out of their homes, " but anyone who stays behind will be on their own once the storm hits.More news: Africa's youngest billionaire Mohammed Dewji abducted
The number of people in emergency shelters was expected to swell to 20,000 across five states by Friday, said Brad Kieserman of the American Red Cross. By 8 a.m. Tuesday, top winds had reached 100 miles per hour (155 kph), and it was forecast to strengthen more, with winds topping 111 miles per hour (179 kph), capable of causing devastating damage.
State officials said 285 people in Mexico Beach had defied a mandatory evacuation order ahead of the storm.
Tornadoes, unsafe winds and more flooding are possible in numerous same areas still recovering from Hurricane Florence.
Hurricane Michael has killed at least one person as it continues to move inland over the Southeast.
The rescue came just hours after Hurricane Michael ripped through Florida's Panhandle, almost wiping out Panama City and Mexico Beach.More news: LOUTH WEATHER: Storm Callum has 'potential to cause damage'