Sustained winds have been measured at up to 155 miles per hour, which is at the extreme upper limit of what weather experts expect from a Category 4 storm.

Now a tropical storm, Michael is the most powerful storm ever to hit Florida's Panhandle; senior correspondent Rick Leventhal reports from Panama City Beach.

"It is extremely emotional", she said, fighting tears.

Multiple airports were closed on Wednesday, including Tallahassee International Airport, Northwest Florida Beaches International Airport, Pensacola International Airport and Destin-Fort Walton Beach Airport.

At the Panama City news station WJHG/WECP, reporter Tyler Allender tweeted that his colleagues were taking shelter in a hallway in the middle of the building because "this wind is SERIOUS". But the fast-moving, fast-strengthening storm didn't give people much time to prepare, and emergency authorities lamented that many ignored the warnings and seemed to think they could ride it out.

Half a million people were advised to evacuate, but officials said Wednesday morning that it was too late to flee and residents who haven't already left should hunker down.

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Michael was downgraded to a tropical storm as it raced across Georgia late Wednesday night.

Camille struck the Mississippi Gulf Coast as a Category 5 hurricane, bringing with it devastating storm tides and strong winds that demolished buildings and destroyed orchards.

"Going back through records to 1851 we can't find another Cat 4 in this area, so this is unfortunately a historical and incredibly risky and life-threatening situation", he said. Beachfront structures could be seen collapsing and metal roofing materials were blown away amid the heavy rain.

At Tyndall Air Force Base south of Panama City leaders declared HURCON (Hurricane Condition) 1 status, ordering out all but essential personnel, after some 600 military families appeared squarely targeted for the worst of the storm's fury.

"It's going to be frightful", Thomas told CNN.

The National Weather Service office in the state capital Tallahassee issued a dramatic appeal for people to comply with evacuation orders.

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There have been mandatory evacuations of several coastal counties in Florida, so the full extent of the damage will not be clear until later today.

The Federal Emergency Management Agency (FEMA) said on Tuesday an estimated 500,000 Florida residents had been ordered or urged to seek higher ground before the storm in 20 counties spanning a 200-mile (320-km) stretch of shoreline.

"This situation has NEVER happened before", it said on Twitter.

It made landfall on the coast as a Category 1 hurricane on September 14 and drenched some parts of the state with 101 centimeters of rain.

The Carolinas are still recovering from Hurricane Florence, which left dozens dead and is estimated to have caused billions of dollars in damage last month.

As the hurricane moves out of Florida, Scott warned that flash floods and tornadoes are still possibilities and said "the weather is still extremely risky". The most likely cause for such sudden changes?

Tropical Storm Michael looms over Gulf of Mexico, could become hurricane
Tropical Storm force winds are very possible along a large portion of the viewing area, with hurricane force winds also possible. Sunday when it was 190 miles south-southwest of the western tip of Cuba, according to the National Hurricane Center .

In all, more than 400 people died in Florida. More than 40,000 people lost power across the state. However, even at a lower category, the storm will pose a major threat.