By Jason Hanna, Susannah Cullinane and Faith Karimi, CNN

(CNN) -- A terrifyingly powerful Category 4 Hurricane Michael was poised to become the strongest hurricane to hit the Florida Panhandle in recorded history Wednesday, its rapid strengthening catching some by surprise and leaving anxious officials telling those who didn't evacuate: It's time to hunker down.

"This is the worst storm that our Florida Panhandle has seen in a century," Gov. Rick Scott said. "Hurricane Michael is upon us, and now is the time to seek refuge."

The extremely dangerous Michael is expected to make landfall in the Panhandle, perhaps near Panama City and Laguna Beach, in the afternoon, CNN meteorologist Chad Myers said.

Among the concerns: Flash-flooding with heavy rain; life-threatening storm surges up to 14 feet high; and devastating winds, not just in the Panhandle, but southern Alabama and Georgia.

The storm's center had maximum sustained winds of 150 mph late Wednesday morning.

"There will be hundreds of thousands, if not millions, without power for a very long time," Myers said.

Key developments

• As of 11:30 a.m. ET, Michael's center had maximum sustained winds of 150 mph, and was about 50 miles south-southwest of Panama City.

• About 3.8 million people were under hurricane warnings in Florida's Panhandle and Big Bend regions, along with parts of southeastern Alabama and southern Georgia. Tropical storm warnings cover 15.9 million people in several states.

• Bridges connecting oceanfront communities to inland areas, such as the Hathaway Bridge linking Panama City Beach to Panama City, were closed Wednesday morning because of deteriorating conditions.

• At 11 a.m. ET, the storm surge in coastal Apalachicola was bringing water levels more than 6 feet above the normal tide level. Up to 14 feet of storm surge is expected in Apalachicola, likely this afternoon, near the peak of the storm.

Governor: 'It's too late to get on the road'

Gov. Scott on Monday and Tuesday urged people to get out of the way as Michael strengthened rapidly over the Gulf of Mexico after lashing Central America and western Cuba. Officials issued mandatory or voluntary evacuation orders in at least 22 counties on the Florida Gulf Coast.

On Wednesday morning, he said on Twitter that "the time for evacuating along the coast has come and gone."

"If you chose not to evacuate ... you're not going to be able to get out. This thing is coming now. ... It's too late to get on the road," he told CNN.

Scott extended a state of emergency to 35 counties and activated 2,500 National Guardsmen; he said more than 1,000 search-and-rescue personnel will be deployed once the storm passes.

President Donald Trump approved a pre-landfall emergency declaration to provide federal money and help in Florida.

If it makes landfall as a Category 4, Michael would not only be the strongest hurricane to hit the Panhandle in recorded history, but it also would be the strongest storm in terms of wind speed to make landfall in the country this year.

Only three major hurricanes Category 3 or higher have struck the Panhandle since 1950: Eloise in 1975, Opal in 1995 and Dennis in 2005.

'I'm definitely getting a little bit more scared'

Michael's rapid intensification -- it was a tropical storm in the Gulf on Sunday and a Category 1 hurricane midday Monday -- may have caught some coastal residents by surprise, despite forecasters' warnings of strengthening.

Newlyweds Jessica Ayers and Don Hogg told CNN they and some relatives were staying put in Panama City on Wednesday morning, having decided against leaving because they weren't in an evacuation zone.

Michael's intensification was unwelcome news.

"I'm definitely getting a little bit more scared, I have to say," Ayers said.

They have a generator, so they hope to have power, should regular service stop. They've identified an interior bathroom as a place to take cover if winds get extreme.

Janelle Frost and Tracy Dunn told CNN they were staying put in nearby Panama City Beach. They said they wanted to stay to help those who couldn't afford to leave, such as retirees.

"There's so many people that live around where we're at, and we wanted to make sure they're OK," Frost said. "We made the decision to stay to try and help them."

In Tallahassee, Kaitlyn Mae Christensen Sacco said she was taking refuge in her home. She has a generator and a camp stove, and she parked her car at a nearby church lot bare of any trees that might come down.

"We have our bathroom set up with blankets, a battery-powered fan, water, snacks and the tub set up for our dogs with pee pads," she said.

Even before landfall, Michael was sending ocean water onto the Panhandle's shores. Water was creeping into the southern Wakulla County town of Panacea, a picture from the National Weather Service showed.

In Pensacola Beach, well west of where Michael's eyewall was heading, huge waves were crashing ashore on Wednesday morning, video from Joe Durant showed.

Rain just one of several threats

A hurricane warning was in place from the Alabama-Florida border to the Suwannee River in Florida.

Meanwhile, tropical storm warnings were in effect for parts of Alabama, Florida, Georgia, South Carolina and North Carolina. Storm surge warnings were in place along the Florida and Alabama coasts.

While it's likely to weaken as it moves across the southeastern United States, its heavy rains and flooding effects will spread far and wide.

Up to 12 inches of rain could fall in Florida's Panhandle and Big Bend areas, as well as southeastern Alabama and southern Georgia. Some parts of the Carolinas -- recently deluged by Hurricane Florence -- and southern Virginia could see up to 6 inches, the hurricane center said.

Florence made landfall last month as a Category 1 storm, killing dozens in the Carolinas and Virginia.

But the storm's center and where it makes landfall with its destructive winds represent just one of several concerns.

Life-threatening storm surges could slam the Florida Gulf Coast, with the deadliest of possibly 9 to 14 feet expected near the eyewall and to the east -- perhaps between Tyndall Air Force Base and the Aucilla River.

"That means the water will come miles in shore and could easily be over the roofs of houses," Scott said.

Damaging winds are expected in Florida, southeastern Alabama and southern Georgia. Tornadoes could spawn in the Southeast Wednesday into Thursday, forecasters said.

Georgia and Alabama declare emergencies

Tolls were being suspended in the state's northwest region to ease the evacuation process, and the Florida Highway Patrol is sending nearly 350 state troopers to the Panhandle and Big Bend areas, Scott said.

Six airports in the Florida Panhandle closed in anticipation of the storm's impacts. Tallahassee International Airport, Pensacola International Airport, Destin-Fort Walton Beach Airport, Destin Executive Airport, Bob Sikes Airport and Northwest Florida Beaches International Airport each issued statements saying they were closed Wednesday morning.

Georgia Gov. Nathan Deal declared an emergency for 92 counties.

Alabama Gov. Kay Ivey issued a statewide state of emergency, saying on Twitter it was "in anticipation of wide-spread power outages, wind damage and debris produced by high winds & heavy rain associated with Hurricane Michael."

Effect of climate change

Michael's strength may reflect the effect of climate change on storms. The planet has warmed significantly over the past several decades, causing changes in the environment.

Human-caused greenhouse gases in the atmosphere create an energy imbalance, with more than 90% of remaining heat trapped by the gases going into the oceans, according to the National Oceanographic and Atmospheric Association. There's evidence of higher sea surface temperature and atmospheric moisture, experts say.

While we might not get more storms in a warmer climate, a majority of studies show that those that do form will get stronger and produce more rain. Storm surge is worse now than it was 100 years ago, thanks to sea level rise.

According to Climate Central, a scientific research organization, the coming decades are expected to bring hurricanes that intensify more rapidly, should there be no change in the rate of greenhouse gas emissions.

"Rapid intensification" took Michael from a tropical storm with sustained winds of 40 mph at mid-day Sunday to a Category 1 hurricane with sustained winds of 75 mph by mid-day Monday. It experienced a second bout of intensification on Tuesday, going from a 100 mph Category 2 to a dangerous Category 4 storm with 145 mph winds by Wednesday morning.