Florida officials said Michael, packing winds of 155 miles per hour (250 kilometers per hour), was the most powerful storm to hit the state s northern Panhandle area in more than a century.
"We are in new territory", National Hurricane Center Meteorologist Dennis Feltgen wrote on Facebook.
Hurricane Michael claimed its first life after roaring ashore in Florida on Wednesday, flooding homes and streets and toppling trees and power lines in the Gulf of Mexico beachfront area where it made landfall as a raging Category 4 storm.
"Those who stick around to experience storm surge don t typically live to tell about it", said FEMA s Long.
While Florence wrung itself out for days and brought ruinous rains, fast-moving Michael is likely to be more about wind and storm surge.
"Radar data indicate that the eye of Michael is moving inland over portions of Bay and Calhoun counties in the Florida Panhandle".
As recently as Sunday morning, Michael was a badly organised system of rain clouds "meandering off the east coast of the Yucatan peninsula", according to the National Hurricane Centre. "There was a tree involved". Much of the affected area is rural and known for small tourist cities, beaches and wildlife reserves, as well as the state capital, Tallahassee.
North Carolina Governor Roy Cooper told residents: "I know people are worn out from Florence, but don't let this storm catch you with your guard down".
"Where that becomes a factor is with the evacuation of some of the local populations", he said.
This image captured by NASA shows Hurricane Michael, while still a Category 1 storm, as it moves through the Gulf. "We haven t seen as robust of an evacuation response from the civilian population that we have seen in other storms".
Then, it will "move northeastward across the southeastern United States Wednesday night and Thursday, and move off the Mid-Atlantic coast away from the United States on Friday", the NHC said.
Storm Callum: Amber weather alert issued for Wales
An amber warning for heavy rain has been issued for most of south Wales on Friday, as Storm Callum approaches. The Met Office is also warning of possible disruption to transport services.
Below are 15 images that personify the aftermath of the Category 4 storm, which blew ashore along the Panhandle packing 155 miles per hour winds.
Mandatory evacuations were issued as beach dwellers rushed to board up homes just ahead of what could be a devastating hit.
More than 300 miles of coastline are now under threat, the National Weather Service has said.
The storm appeared to be so powerful - with a central pressure dropping to 933 millibars - that it is expected to remain a hurricane as it moves over Georgia early Thursday.
"This situation has NEVER happened before", it said on Twitter.
The 11:30 a.m. advisory on what is forecast to be the largest hurricane on record to impact the panhandle and big bend area of Florida from the National Hurricane Center reports, Hurricane Michael has sustained winds of 150 miles per hour. The Carolinas are still recovering from Hurricane Florence last month.
The Miami-based center forecast as much as 1 foot (30 cm) of rain in parts of Florida, Georgia and SC.
A year ago saw a string of catastrophic storms batter the western Atlantic - including Irma, Maria and Harvey, which caused a record-equaling $125 billion in damage when it flooded the Houston metropolitan area.
Even before landfall, the hurricane disrupted energy operations in the Gulf, cutting crude oil production by more than 40 percent and natural gas output by almost one-third as offshore platforms were evacuated before the storm hit.
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