Accuweather predicted Hurricane Michael's total damage and economic impact will reach close to $30 billion, which would make it one of the 10 costliest hurricanes in US history.
It is the third-most-powerful storm on record to hit the USA, and the worst in almost 50 years, since Hurricane Camille in 1969.
The storm made landfall on the Florida Panhandle Wednesday afternoon as one of the most powerful storms to ever hit the U.S. The intense Category 4 hurricane was packing maximum sustained winds of 155 miles per hour when it crashed ashore near Mexico Beach, a lightly populated tourist town about midway along the Panhandle.
Michael's barometric pressure at landfall was 919 millibars, by that measurement the third-strongest hurricane to hit the US on record, trailing on the 1935 Labor Day Hurricane and Hurricane Camille in 1969.
Fast-moving Michael, a Category 4 storm on the five-step Saffir-Simpson hurricane scale when it came ashore, was about 20 miles (35 km) northwest of Raleigh, North Carolina, at 5 pm EDT (1900 GMT) and set to speed up as it headed for the Atlantic coast, the NHC said.
"Michael should weaken as it crosses the southeastern United States through Thursday", the NHC said.
"This is a nightmare hurricane for the Big Bend", said Ryan Truchelut, chief meteorologist at WeatherTiger.
Florida Sen. Marco Rubio tweeted he was hearing "stunning" reports of damage, including that "Mexico Beach is gone" and the "damage in Panama City is catastrophic".
Flooding is expected in parts of the state that were previously hit by deadly Hurricane Florence in September, he said. Local authorities fear power outages and major tree damage from Michael.
- The human cost: A man in Gadsden County, Florida, killed by a falling tree, and an 11-year-old girl in Seminole County, Georgia, killed when a carport blew through the roof of her home.
In Roberta, Georgia, a reported tornado devastated the small town.
The sky cleared on Thursday.
Michael could also have lingering impact on agriculture in state, officials warned.
Vance Beu, 29, was staying with his mother at her home at Spring Gate Apartments, a complex of single-story wood-frame buildings. But it moved so fast and intensified so quickly that people didn't have much time to prepare, and emergency authorities lamented that many ignored the warnings.
As the storm's fierce winds pushed ocean water onto the Panhandle's Mexico Beach, ABC News' Ginger Zee said she saw an "entire home, a well-built home, rolling down the street".
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"If you decide to stay in your home and a tree falls on your house or the storm surge catches you and you're now calling for help, there's no one that can respond to help you, " Morgan said at a news conference.
The governor pleaded with people in Florida not to go home yet.
The Florida Panhandle is the wide strip in the northwestern corner of the state bordered by the Gulf of Mexico to the south and Alabama and Georgia to the north. The soil in many areas is saturated. Its largest city is Pensacola, with a population approaching half a million.
Tallahassee, a city of 200,000, sits 100 miles east of Panama City.
Where has Michael hit and where is it now? .
- Staying safe: almost 6,700 people took refuge in 54 shelters in Florida. "We've had a number of them come into the Gulf, and either come to the left or the right of us. It was too late", Swab said. Rows and rows of other homes were rendered piles of splintered lumber.
"The worst thing you can do now is act foolishly" by putting yourself in danger or keeping law enforcement from saving lives, he said at a news conference.
"During disasters, Floridians take care of each other", Scott said.
More than 830,000 homes and businesses were without power in Florida, Alabama and Georgia early Thursday.
Betty Wexler, 86, lost a beach house to a storm more than 20 years ago.
State officials estimate about 375,000 Floridians are under either mandatory or voluntary evacuation orders.
Federal officials were anxious that the fast-moving nature of the storm didn't give residents enough time to get out.
"This storm has the potential to be a historic storm, please take heed, " the sheriff's office said in the post.
Hundreds of thousands of people were ordered to evacuate their homes and the governor told residents who had not done so to "hunker down and be careful".