The storm had the lowest barometric reading of a hurricane to make landfall since 1969, making it the most intense storm to hit the continental US in half a century. Michael was also the most powerful hurricane to hit the Florida Panhandle.
The storm slammed ashore early afternoon near Mexico Beach as a Category 4 hurricane on the five-level Saffir-Simpson wind scale, uprooting trees and power lines, dumping rain and causing severe flooding.
"Michael saw our worst fears realized, of rapid intensification just before landfall on a part of a coastline that has never experienced a Category 4 hurricane," said University of Miami hurricane researcher Brian McNoldy.
Authorities said a man in the town of Greensboro was killed by a falling tree when it crashed through the roof of his house and a girl died when debris fell into a home in Georgia.
Some 375,000 people had been urged to leave their homes for stronger shelters in Florida, but many residents were trapped after they were caught surprised by the storm doubling in strength as it approached land.
By Wednesday night, more than 400,000 people in Florida, Georgia and Alabama were without power.
Emergency alerts for Alabama, GeorgiaThe storm's strength diminished to a Category 1 storm with 75-mph (120-kph) winds as it moved into Georgia late Wednesday. It was projected to cut through the state and move into the Carolinas as a tropical storm on Thursday.
The governors of North and South Carolina urged residents to prepare for heavy rain and winds, which come less than a month after Hurricane Florence battered the mid-Atlantic coast.
President Donald Trump said he had spoken with Florida Governor Rick Scott on Tuesday, and federal emergency services were coordinating with regional agencies in the areas likely to be impacted.
"It is imperative that you heed the directions of your State and Local Officials. Please be prepared, be careful and be SAFE!" the president tweeted to residents of Florida and Georgia.
Climate change bringing more destructive stormsIn the past year, several massive storms battered the US coasts, including Irma, Maria and Harvey. Houston's metropolitan area suffered a record-equaling $125 billion (€108 billion) in damage. North and South Carolina are still reeling from Hurricane Florence last month.
Climate scientists have long warned that the effects of global warming make storms more destructive and point to last year's string of hurricanes as visible evidence.
cw, es/jm (AP, Reuters)
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