The Category 4 storm was packing maximum sustained winds of 155 miles per hour as it made landfall near Mexico Beach.
Willis said that a team was still working in Crawford County to assess the damage and intensity of the tornado.
After slamming Florida and lashing Georgia, Michael is now barreling through the storm-weary Carolinas.
More than 1,000 search and rescue personnel were searching through rubble for survivors.
Storm damage is seen after Hurricane Michael October 10, 2018 in Panama City, Florida.
The sun will rise across the Florida Panhandle sometime around 7 on Thursday, and Hurricane Michael's trail of destruction will begin to come into clearer focus. The whole city looks like a nuke was dropped on it. "I would do anything to just know he's OK", she said.
Florida has had more hurricanes in October than in any other month.
Back in Mexico Beach, residents were picking up the pieces.
- Damage estimates: Boston-based Karen Clark & Company, an insurance company that produces models for catastrophes, is estimating Hurricane Michael caused about $8 billion in insured losses.
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.
Michael killed at least two people - a man who died when a tree toppled onto his house in Florida and a girl who died when debris fell into a home in Georgia, officials and local media said.
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Large parts of Georgia, the Carolinas and southeastern Virginia could still see deadly flash floods on Thursday and remain under a tropical storm warning, it said. Tens of thousands of Floridians had already lost power before landfall as the storm's winds lashed the coast.
The tropical storm is located about 30 miles (45 km) south-southwest of Macon, Georgia with maximum sustained winds of 70 miles per hour (115 km per hour), the NHC said in its 12 am EDT position update.
Across the storm-hit region in the USA, relief efforts were shaping up.
- High water: storm surge of 6 feet (2 meters) up to 14 feet (4 meters) forecast for Florida's Panhandle and Big Bend. SC set up 11 emergency shelters in anticipation of floods and tornadoes from Michael.
Sarah had the week off from school for fall break and she and her 12-year-old brother had been staying at their grandparents' house near a lake in Seminole County since Monday. When it landed, one of the legs tore through the roof, fatally striking the 11-year-old girl in the head.
Briefing President Donald Trump at the White House, Federal Emergency Management Agency (FEMA) chief Brock Long said Michael was the most intense hurricane to strike the Florida Panhandle since 1851.
Michael is shaking up the upcoming election in the battleground state of Florida. That warming ocean waters would lead to more frequent and stronger storms has been the consensus opinion of climate scientists for decades, yet no action has been taken by either the Republican or Democratic parties.
Television news footage showed homes submerged in floodwaters up to their roofs in Mexico Beach.
Police officers spent the night patrolling the area to prevent looting and responding to calls for assistance, making do with damaged patrol cars that eased across downed power lines and even around small buildings flipped into the middle of streets. "All of my furniture was floating", she said.
Reporter Peyton LoCicero went on Periscope, an app that allows people to live stream to a public audience from a cellphone, to give updates about the storm. A few stores were reopening, and cars lined up at the few open gas stations. "The more people that return, it's just going to get in the way".
One of the four victims from Florida has been identified as Steven Sweet, the Gadsden County sheriff's office said. A Jayhawk rescue helicopter crew pulled nine people from a bathroom of their Panama City, Florida, home after their roof collapsed, the Coast Guard said.