Hurricane Michael strengthened into a Category 4 storm early on Wednesday before it was expected to plow into Florida's Gulf shore with towering waves and roof-shredding winds as 500,000 people were under evacuation orders and advisories.
Hurricane Michael was packing winds of up to 210km per hour, 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 4 metres, the National Hurricane Center (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.
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.
Michael gathered greater strength over warm Gulf of Mexico waters throughout the day on Tuesday as it jumped from a Category 2 to Category 3 on the five-step Saffir-Simpson wind scale.
The last NHC report said the fast-moving storm was about 325 km 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.
"THIS STORM COULD KILL YOU"
A hurricane warning was posted along more than 483km of the coast from the Florida-Alabama border south to the Suwannee River.
"If you don't follow warnings from officials this storm could kill you," said Scott, a Republican running for the US Senate in November's congressional elections.
While the swiftly moving storm is not expected to linger over Florida for long, widespread heavy downpours will likely track inland to flood-stricken areas of the Carolinas even as rain-gorged rivers there begin to recede, National Weather Service meteorologist Ken Widelski told the conference call.
About 2,500 National Guard troops were deployed to assist with evacuations and storm preparations, and more than 4,000 others were on standby. Some 17,000 utility restoration workers were also on call.
A passenger onboard a cruise ship bound for Cuba captured a view of Hurricane Michael on Monday evening (October 8).
The cruise ship, Royal Caribbean's Empress of the Seas, was sailing to Havana when strong winds and waves brought on by Michael forced the ship to stall off Cuba's southern coast as the storm cleared the Yucatan Channel, the passenger told Reuters.