Hurricane Michael closed in on Florida's Gulf Coast on Wednesday as an "extremely dangerous" category four storm packing powerful winds and a huge sea surge, US forecasters said.
The Miami-based National Hurricane Center said the storm, which local forecasters are calling an "unprecedented" weather event for the area, is expected to slam ashore later in the day with "life-threatening" storm surges.
Top winds were swirling at 230 kilometres per hour, with the NHC warning the storm could grow stronger still, as outer rain bands from the storm lashed the coast.
A storm surge of up to 14 feet and up to a foot of rain was expected in some areas.
"Water is going to come up, it's going to cover your roof. You do not have a chance to survive that, so I'm very concerned," Governor Rick Scott told Fox News.
"It's too late to evacuate now on the coast... Hunker down, and be careful, listen to locals. Don't go out in the middle of this. You are not going to survive it. It's deadly." An estimated 375,000 people in more than 20 counties were ordered or advised to evacuate.
The National Weather Service office in the state capital Tallahassee issued a dramatic appeal for people to comply with evacuation orders.
"Hurricane Michael is an unprecedented event and cannot be compared to any of our previous events. Do not risk your life, leave NOW if you were told to do so," it said.
The storm was forecast to make landfall somewhere along the Florida Panhandle -- a finger-shaped strip of land in the Gulf of Mexico.
It was expected to bring hurricane force winds and heavy rainfall, the NHC said.
It will then move across the southeastern US for another day or so as it heads toward the Atlantic.
The NWS office in Tallahassee said it had found no record of any previous category four hurricanes that made landfall in the panhandle or the "Big Bend" coastal region.
"This situation has NEVER happened before," it said on Twitter.
Category four is the second highest level on the Saffir-Simpson hurricane wind scale.
President Donald Trump issued an emergency declaration for the state, freeing up federal funds for relief operations and providing the assistance of the Federal Emergency Management Agency.
State officials issued disaster declarations in Alabama and Georgia, both of which are also expected to feel the impact from the storm.
As of 1200 GMT, Michael was about 90 miles southwest of Panama City and moving north at 13 miles per hour.
The heavy rains could cause flash floods, the NHC said, and spawn tornados in northwestern Florida.
About 120,000 residents were under mandatory evacuation orders in Bay County in the Panhandle, a low-lying area of beachfront resorts and retirement communities.
In other areas, residents of mobile homes were urged to leave.
Michael was forecast to have the power to uproot trees, block roads and knock out power for days when it hits Florida. It is expected to weaken as it moves up into the southeastern United States.
Drivers waited in long lines at gas stations and residents hurried to fill sandbags.
Tolls were suspended on some roads to aid movement ahead of the storm's landfall.
"Since 6:00 am it's been backed up. We're just now running out of regular (gasoline)," Danny Hess, an employee at a gas station in Panama City, told local WJHG television.
The Carolinas are still recovering from Hurricane Florence, which left dozens dead and is estimated to have caused billions of dollars in damage last month.
It made landfall on the coast as a Category 1 hurricane on September 14 and drenched some parts of the state with 40 inches (101 centimetres) of rain.
Last year saw a string of catastrophic storms batter the western Atlantic -- including Irma, Maria and Harvey, which caused a record-equaling USD 125 billion in damage when it flooded the Houston metropolitan area.
Scientists have long warned that global warming will make storms more destructive, and some say the evidence for this may already be visible.
(With inputs from agencies.)