Forecasters said it could bring three to six inches of rain to Georgia, the Carolinas and Virginia, triggering flash flooding in a corner of the country still recovering from Florence.
Almost a Category 5 storm with maximum sustained winds of 155 miles per hour when it hit near Mexico Beach in Florida, Michael traveled northeast, decreasing to a tropical storm with 60 miles per hour winds in central Georgia by 2 a.m. EDT Thursday.
In large areas of Mexico Beach, homes were obliterated or reduced to stacks of rubble, aerial video from CNN showed on Thursday.
Some Panama City residents were seen on the beach enjoying the cool breeze ahead of the storm, while others were loading up cars with luggage and visitors were checking out of hotels.
Bobby Smith boarded up windows at Jani's Ceramics in Panama City, Fla.
"The roof fell in but we lived through it", he said.
Florida Governor Rick Scott declared a state of emergency in 35 counties along Florida's Panhandle and Big Bend regions.More news: The FSF Wants Microsoft To Do More To Help Fight Software Patents
It was not yet known what had happened to about 280 residents of Mexico Beach who authorities said had ignored evacuation orders as the storm approached the state's northwest.
Tallahassee Mayor Andrew Gillum, Florida's Democratic nominee for governor, filled sandbags with residents and urged residents of the state capital city to finish up emergency preparations quickly. National Hurricane Center, the storm made landfall before 2 p.m. ET.
Forecasters say that 4 to 8 inches of rain is expected along the path of the storm, and 12 inches could fall in some parts of the Panhandle.
While Florence took five days between the time it turned into a hurricane and the moment it rolled into the Carolinas, Michael gave Florida what amounted to two days' notice.
"It really started as a tropical storm, and then it went to Category 1, then it was Category 2 and before you know it, it was Category 4", O Shaughnessy said. Alabama Gov. Kay Ivey said she fears widespread power outages and other problems would follow.
A hurricane warning was in effect for the Alabama-Florida border to the Suwannee River, with a hurricane watch in place from the Alabama-Florida border to the Mississippi-Alabama border. Gulf County in the Panhandle of Florida.More news: Samsung launches A9, world's first smartphone with four rear cameras
"The wind that came through here was surreal".
Some 17,000 utility restoration workers were also on call.
Thousands of people hunkered down in shelters overnight after fleeing their homes ahead of the storm. The system has now graduated to Category 2 hurricane status and is expected to reach Category 3 (with winds in excess of 110 mph) when it makes its US mainland landfall on Wednesday afternoon. Wind damage was also evident.
Because of the low-lying land, many people living inland could see their homes flooded, too.
"Hurricane Michael is a monster storm and it keeps getting more risky", Florida Governor Rick Scott told a news conference on Tuesday.
President Donald Trump, speaking to reporters at the White House, said the country was very well prepared for the hurricane, adding it was much bigger than had been expected. About 1,250 National Guard soldiers were assisting and more than 4,000 troops were on standby.
On its current track, it would make landfall somewhere along a coastline that includes the cities and towns of Fort Walton Beach, Panama City Beach, Port St. Joe, St. Teresa and the wildlife reserves bordering Apalachee Bay.More news: USA arrests alleged Chinese national for targeting major aviation companies
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