Powerful Hurricane Michael was hours away from smashing into Florida's northwestern shore with a wall of water and roof-shredding winds on Wednesday and authorities told residents along the coast they had run out of time to evacuate and should hunker down.
A very dangerous Category 4 storm on five-step Saffir-Simpson scale, Michael had strengthened as it moved north over the Gulf of Mexico and by 8 a.m. ET (1200 GMT) was carrying top winds of 145 miles per hour (230 km per hour), the National Hurricane Center said.
It was set to make landfall later on Wednesday on Florida's Panhandle and could drive sea water levels as high as 14 feet (4.3 meters) above normal, the centre said. The storm could strengthen further before coming ashore, it said.
"The time for evacuating along the coast has come and gone. First responders will not be able to come out in the middle of the storm," said Florida Governor Rick Scott in a tweet early Wednesday. "If you chose to stay in an evacuation zone, you must SEEK REFUGE IMMEDIATELY."
The head of the Federal Emergency Management Agency (FEMA), Brock Long, echoed the governor's warning to coastal residents that there might be no time left to evacuate.
"Unfortunately, this is a hurricane of the worst kinda rapidly intensifying storm, which gives people very little time to prepare. It also gives them a shorter window to evacuate."
Residents and tourists were told to flee low-lying areas in at least 20 counties along the shore in Florida's Panhandle and adjacent Big Bend region.
Some of the storm's most significant early impact was to offshore energy production. U.S. producers in the Gulf cut oil production by about 40 per cent and natural gas output by 28 percent on Tuesday, the Bureau of Safety and Environmental Enforcement said.
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.
Apalachicola Mayor Van Johnson said the city, which could suffer some of the worst of the storm surge, was under mandatory evacuation orders.
"My greatest concern is that some people are just now starting to take this storm seriously and are evacuating," he told CNN. "And I just hope the others that have not made that decision get out while the roads are still passable and before the bridges close."
The storm was heading north at 13 mph (20 kph) and was about 90 miles (145 km) southwest of Panama City, Florida, the NHC said in its latest report.
"Outer band of Hurricane Michael coming ashore here," Jon Ward in Panama City said on Twitter. "Light rain and thunder have just begun. Winds should be picking up in the next couple of hours.
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 was forecast to dump torrential rain over Florida, Alabama and Georgia, as well as the Carolinas - still reeling from post-Florence flooding - and into Virginia. Up to a foot (30 cm) of rainfall was 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 FEMA, told reporters on a conference call.
Among people who had fled their homes was Betty Early, 75, a retiree who joined about 300 fellow evacuees huddled on makeshift bedrolls of blankets and collapsed cardboard boxes at an elementary school converted into an American Red Cross shelter in Panama City, near the storm's expected landfall.
She was unsure how well her old, wood-framed apartment block would hold up. "I'm blessed to have a place to come," she told Reuters. "My greatest concern is not having electricity, and living on a fixed income, losing my food."
Scott, a Republican running for the U.S. Senate in November's congressional elections, declared a state of emergency in 35 Florida counties, mostly encompassing rural areas known for small tourist cities, beaches, wildlife reserves and Tallahassee, the state capital.
Georgia Governor Nathan Deal declared a state of emergency on Tuesday for 92 counties in his state.
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.
The last major hurricane, a storm of Category 3 or above, to hit the Panhandle was Hurricane Dennis in 2005, according to hurricane centre data.
(With inputs from agencies.)