(CNN) -- Hurricane Michael strengthened into an "extremely dangerous" Category 4 storm as it moved toward the Florida Panhandle, where it could blow ashore as the strongest storm to hit the US this year.
Michael intensified from a Category 3 early Wednesday, hours before its expected landfall in the afternoon.
If it makes landfall as a Category 3 storm or higher, it'll be the first major hurricane to strike the Florida Panhandle since Hurricane Dennis in 2005 and the strongest storm to hit the US this year, CNN meteorologist Michael Guy said.
Only three major hurricanes Category 3 or higher have hit the Panhandle since 1950: Eloise in 1975, Opal in 1995 and Dennis in 2005.
The stormwill bring dangerous storm surges to low-lying areas, hurricane force winds and heavy rainfall along the northeastern Gulf Coast, the National Weather Service reported.
Once it comes ashore, it's likely to weaken as it moves across the southeastern United States. But its heavy rains and flooding effects will be felt far and wide.
Up to 12 inches of rain could fall in Florida's Panhandle and Big Bend, 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.
Governor to residents: 'Get out'
About 3.7 million people are under hurricane warnings in the Panhandle and Big Bend regions as Michael approaches,along with parts of southeastern Alabama and southern Georgia. Tropical storm warnings cover 8.5 million people in several states.
Florida Gov. Rick Scott described it as a "monstrous storm" and urged people to get out of the way. State officials issued mandatory or voluntary evacuation orders in at least 22 counties on the Florida Gulf Coast.
Scott extended a state of emergency to 35 counties and activated 2,500 National Guardsmen.
"Hurricane Michael is forecast to be the most destructive storm to hit the Florida Panhandle in decades," Scott said. "You cannot hide from storm surge, so get out if an evacuation is ordered."
Category 3 storms have sustained winds of 111 mph to 129 mph while a storm goes to Category 4 with sustained winds stronger than 130 mph.
President Donald Trump approved a pre-landfall emergency declaration to provide federal money and help in Florida.
Rain just one of several threats
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 13 feet expected between Mexico Beach and Keaton Beach.
"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.
'We need the residents to be leaving today'
A hurricane warning was in place from the Alabama-Florida border to the Suwannee River in Florida.
Meanwhile, tropical storm watches were in effect in some coastal areas of Mississippi, Florida, Georgia and South Carolina. Storm surge warnings were also in place along the Florida and Alabama coasts.
US Rep. Neal Dunn, whose district includes Panama City, urged people under evacuation orders to get out before tropical-storm-force winds arrived. He focused on islands off the coast whose bridges may close as the storm approaches.
"You haven't got anywhere to go (if the bridges close)," he said Tuesday. "And then you're riding it out in your car instead of something else. So, we need the residents to be leaving today because by this evening, those bridges are going to be in peril of being closed."
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.
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."
Her declaration activates the state's emergency operations plan.
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