- Hurricane Michael strengthened into a Category 4 storm early on Wednesday.
- The storm is 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.
- The very dangerous storm strengthened as it moved north over the Gulf of Mexico and by 8 a.m. ET it 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 center said.
Powerful Hurricane Michael was hours away from smashing into Florida's northwestern shore on Wednesday with a wall of water and roof-shredding winds, and the state's governor said it would be the worst hurricane to hit the region in a century.
Authorities told residents along the affected areas of Florida's Gulf of Mexico coast that 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 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.
The hurricane 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 in some areas, the center said. The storm could strengthen further before coming ashore, it said.
More than 2.1 million residents of at least 20 Florida counties were under mandatory or voluntary evacuation orders. Much of this part of Florida is rural areas known for small tourist cities, beaches, wildlife reserves and Tallahassee, the state capital.
"Hurricane Michael is forecast to be the most destructive hurricane to hit the Florida Panhandle in a century," Governor Rick Scott told reporters, speaking about seven hours before Michael was expected to make landfall.
Earlier, Scott said on Twitter that it was too late to evacuate the target zone and that people who had stayed should immediately seek refuge.
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 percent 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.
"Outer band of Hurricane Michael coming ashore here," Jon Ward in Panama City said on Twitter. "Light rain and thunder has 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.
He said the storm would still have hurricane-force winds as it pushed through Florida into Georgia and tropical storm-force winds when it reaches the Carolinas, which are still reeling from post-Florence flooding.
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 bed rolls of blankets and collapsed cardboard boxes at an elementary school serving as 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."
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.
Scott, a Republican running for the U.S. Senate in November's congressional elections, declared a state of emergency in 35 Florida counties.
Georgia Governor Nathan Deal declared a state of emergency on Tuesday for 92 counties in his state.
The last major hurricane, a storm of Category 3 or above, to hit the Panhandle was Dennis in 2005, according to hurricane center data.
Amtrak is adjusting service to Florida for the safety of its passengers and employees because of Hurricane Michael.
Silver Star trains traveling from New York to Miami will operate from Miami to Jacksonville, Florida, beginning Wednesday. No alternate transportation will be provided between Jacksonville to New York.
The Palmetto, which runs between New York and Savannah, Georgia, will operate between New York and Washington beginning Thursday. Amtrak says no alternate transportation will be provided between Washington to Savannah.
Amtrak is waiving fees for passengers who change their reservations.
—The Associated Press contributed to this report.