(PANAMA CITY, Fla.) -- Hurricane Michael, a potentially catastrophic Category 4 storm with 155 mph winds, made landfall in the Florida Panhandle Wednesday afternoon, becoming the strongest storm to hit the U.S. since Hurricane Camille in 1969 and the third most powerful on record ever for the country.
A broad swath of the Southeast is affected, with about 20 million people under either a warning or a watch for hurricane, flooding or tornadoes, said ABC News contributor Tom Bossert, former Homeland Security Adviser to President Donald Trump.
For the Florida Panhandle where the storm came ashore, Michael is the worst hurricane since the mid-1800s, the director of FEMA said.
Life-threatening winds, dangerous storm surge
As the winds pushed the ocean water onto the coast, ABC News chief meteorologist Ginger Zee said she saw an "entire home, a well-built home, rolling down the street. ... it makes you shake."
Unlike last month’s Hurricane Florence that brought massive flooding to the Carolinas, one of the biggest threats from Michael is the wind.
Michael -- described by Florida Gov. Rick Scott as "monstrous" -- made landfall with the almost the highest wind speed possible for a Category 4 -- 155 mph. When a hurricane reaches 157 mph, it is in the highest category, a Category 5.
As Michael approached the coast its pressure dropped to about 919 millibars (mb). The lower the pressure, the more intense the storm. Hurricane Katrina in 2005 made landfall with a pressure of 920 millibars.
The hurricane also was forecast to bring a storm surge of up to 14 feet high, prompting warnings from officials.
"Anybody that doesn't evacuate that experiences storm surge doesn't typically live to tell about that story," Brock Long, administrator of the Federal Emergency Management Agency, told "Good Morning America" on Wednesday.
Residents seek shelter, hunker down
Thousands fled the Florida coastline before the heavy rain -- which may reach up to 12 inches -- began.
But by Wednesday morning, it was too late to evacuate. Those who remained were urged to shelter in place.
"This was a shock waking up knowing it was a [Category] 4," said Panama City Beach resident Julie Gordon. "Thinking it was a [Category] 2 was a very different story."
All bridges from Panama City Beach to further inland have closed, so Gordon said she is riding out Michael at home, "hoping and praying that the storm will continue to drift to the northeast ... [an area] where it's not quite as populated."
The Panhandle is the wide strip of northwest Florida with the Gulf of Mexico to the south and Alabama and Georgia to the north. Popular with tourists for its beaches, the area also has many year-round residents -- its largest city is Pensacola, with a population approaching half a million.
"First responders will not be able to come out in the middle of the storm," the governor tweeted. "If you chose to stay in an evacuation zone, you must SEEK REFUGE IMMEDIATELY."
About 4,000 people have entered 70 evacuation shelters, FEMA officials said.
Michael may bring weekslong power outages, officials added.
Mobile homes are especially a concern since they aren't built to withstand hurricane-force winds, so in one county in south Georgia, mobile home residents were invited to shelter at a local church, reported ABC affiliate WALB in Albany, Georgia.
"The size of this thing is growing," said Reggie Rachals, sheriff of Lee County, Georgia. "It will tear up mobile home parks real bad."
Bossert said he's concerned not enough people evacuated and many rescues may be needed after the brunt of the storm passes.
"I am very, very worried" about the recovery, Bossert added. "People are going to really struggling after this one."
States of emergency across the South
The last Category 4 hurricanes to strike the U.S. mainland were both in 2017 -- Irma, which slammed into Florida, and Harvey, which hit Texas.
Scott declared a state of emergency in 35 counties in Florida. Trump approved an emergency declaration for Florida, permitting the federal government to provide resources and aid during the dangerous storm.
"This is a small storm in an area that they never thought that it would be, and they said it grew into a monster," Trump said Wednesday from the Oval Office.
"We're very, very prepared," he said. "We have massive amounts of food and water that gets brought in immediately."
Trump said "it is not so easy" for some residents to evacuate.
"Some of the areas are very poor. Not easy for a person without the necessary money to leave," he said.
Despite the storm, Trump said he plans to still attend his Wednesday night rally in Pennsylvania, telling reporters, "You have so many people already there and it’s sort of unfair to them."
Trump added that he will likely visit Florida on Sunday or Monday.
After tearing through Florida, Michael will take aim on Georgia, which will be hit by damaging winds and downed power lines.
Georgia Gov. Nathan Deal on Tuesday declared a state of emergency.
"What you're going to see is a storm moving very rapidly through Georgia, and it will maintain hurricane strength through southwest Georgia and central Georgia as it passes through later today and early tomorrow," FEMA's Long told "GMA."
In Alabama, where residents may see massive power outages, high winds and heavy rain, Gov. Kay Ivey issued a state of emergency statewide.
North Carolina and South Carolina will likely see heavy rainfall, which could cause flooding in areas already damaged and rain-soaked by last month’s Hurricane Florence.
A state of emergency was declared Wednesday in North Carolina, said Gov. Roy Cooper, as he warned that winds will be strong enough to down trees.
The hardest rain is expected there Thursday to Thursday night, he said.
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