Michael is growing into a potentially deadly Category 4 hurricane before it's due to smash into Florida's Gulf shore with towering waves and roof-shredding winds as 500,000 people are under evacuation orders.
Hurricane Michael was packing winds of up to 220km/h hours before it was set to make landfall on Florida's Panhandle or Florida's Big Bend where it potentially could unleash devastating waves as high as four metres, the National Hurricane Centre (NHC) warned.
"THIS IS YOUR LAST CHANCE to evacuate before conditions start deteriorating within the next few hours," said Florida Governor Rick Scott in a tweet early on Wednesday.
Some of the storm's most significant early impact was to offshore energy production. US producers in the Gulf cut oil production by about 40 per cent and natural gas output by 28 per cent on Tuesday, the Bureau of Safety and Environmental Enforcement said.
US President Donald Trump declared a state of emergency for the entire state of Florida, freeing up federal assistance to supplement state and local disaster responses.
The last NHC report said the fast-moving storm was about 230km from Panama City, Florida.
Winds as strong as Michael is producing can inflict substantial damage to roofs and walls of even well-constructed homes, according to the National Weather Service.
NHC Director Ken Graham said Michael represented a "textbook case" of a hurricane system growing stronger as it drew near shore, in contrast to Hurricane Florence, which struck North Carolina last month after weakening in a slow, halting approach.
The storm is likely to dump prodigious amounts of rain over Florida, Alabama and Georgia, as well as the Carolinas - still reeling from post-Florence flooding - and into Virginia. Up to 30cm of rainfall is forecast for some areas.
The region should brace for "major infrastructure damage," specifically to electricity distribution, wastewater treatment systems and transportation networks, Jeff Byard, associate administrator for the Federal Emergency Management Agency (FEMA), told reporters on a conference call.
Byard said an estimated 500,000 people were under evacuation orders and advisories in Florida, where residents and tourists were fleeing low-lying areas in at least 20 counties stretching along 322km of shore in the Panhandle and adjacent Big Bend region.
About 2,500 National Guard troops were deployed to assist with evacuations and storm preparations, and more than 4,000 others were on stand-by. Some 17,000 utility restoration workers were also on call.
