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(CNN) Hurricane Michael intensified as it aimed for the Florida Panhandle, where it could make landfall as the strongest storm in terms of wind speed in the country this year.

Michael churned in the Gulf of Mexico on Tuesday as a Category 3 storm, poised to grow stronger and slam Florida's Panhandle.

If the storm's core comes ashore as a Category 2 or higher, it would be the strongest storm in terms of wind speed to make landfall in the country this year.

Florida Gov. Rick Scott called Michael a "monstrous storm" that could bring "total devastation," and urged people to get out of the way.

"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."

The storm had sustained maximum winds of 120 mph Tuesday night, with gusts to 150 mph. Category 3 storms have sustained winds of 111 mph to 129 mph, according to the hurricane center's scale . A storm goes to Category 4 with sustained winds stronger than 130 mph.

Winds of tropical storm force will be felt in the area starting early Wednesday, and mandatory or voluntary evacuation orders already have been issued in at least 22 counties on the Florida Gulf Coast head of the storm.

About 3.7 million people were under hurricane warnings in the Panhandle and Big Bend regions, as well as parts of southeastern Alabama and southern Georgia. Tropical storm warnings cover 8.5 million people in four states.

Michael was moving north at 12 mph, about 220 miles southwest of Panama City, Florida, the National Hurricane Center said Tuesday night.

The main threats

The storm's center and where it makes landfall with its destructive winds represent just one of several concerns. Among them:

• 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.

• Heavy rain and flooding are expected not just for Florida but also for other parts of the Southeast. Up to 12 inches of rain could fall in Florida's Panhandle and Big Bend, southeastern Alabama and southern Georgia, while parts of the Carolinas -- recently deluged by Hurricane Florence -- and southern Virginia eventually could see up to 6 inches, the hurricane center said.

• Tornadoes could spawn in the Southeast Wednesday into Thursday, forecasters said.

• Damaging winds are expected to rake not just Florida but also southeastern Alabama and southern Georgia.

#HurricaneMichael isn't heading to any one town...

There are warnings for more than 300 miles of coastline. It's forecast to be a large and dangerous hurricane at landfall.

✔️Life-threatening storm surge

✔️Damaging winds

✔️Life-threatening flash floodinghttps://t.co/VyWINDk3xP pic.twitter.com/nsHYkBjy2r — NWS (@NWS) October 9, 2018

'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 warnings extended from Florida's Chassahowitzka River to the Mississippi-Alabama border. 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.

Krystal Day, of Homosassa, Fla., leads a sandbag assembly line at the Old Port Cove restaurant on Tuesday in Ozello, Fla.

"This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions," the Hurricane Center said

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 morning. "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."

Workers boarded the windows of Marco's Pizza in Panama City Beach , Florida as Hurricane Michael approached Tuesday.

Locally-ordered evacuations have started ahead of Hurricane Michael in some FL counties. Visit https://t.co/OnxOrhg593 to find your evacuation zone, & follow alerts from your local emergency management center. If you are directed to evacuate, leave, DO NOT WAIT. — Rick Scott (@FLGovScott) October 8, 2018

Scott extended a state of emergency to 35 counties and activated 2,500 National Guardsmen. The governor said President Donald Trump approved a federal pre-landfall emergency declaration. It will provide federal money and help.

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 #HurricaneMichael."

Her declaration activates the state's emergency operations plan, according to Ivey's office.

Boarded up businesses, a flooded deck, an empty harbor

In Pass Christian, Mississippi, high tide started to flood the front deck of a coastal restaurant, Sea Level, on Tuesday morning.

In Florida, almost three dozen shelters opened or were prepared to open as the storm neared.

"Y'all, I just don't know what to say anymore. We may need to change the name to Below Sea Level. Closed until further notice," co-owner Dana Barrett wrote on the restaurant's page.

In St. Petersburg, water seeped into the street. "Ahead of #HurricaneMichael, #rain has started and many streets are already #flooded in the #Tampa Bay area of #Florida. Please be safe everyone!" Ali Sages wrote on Twitter.

In Panama City Beach, many businesses were closed and homes boarded up.

Hurricane Michael preparations in full effect in Panama City Beach. pic.twitter.com/VvpmclSbeE — Taylor (@rollbluetide) October 9, 2018

Cars were in long lines at one of the town's gas stations Tuesday.

"Since 6 a.m. it's been backed up. We're just now running out of regular (gas)," worker Danny Hess told CNN affiliate WJHG

In Destin, a harbor was empty and quiet in the peace before the storm.

"Never like to see the fishing docks on #DestinHarbor emptied of boats. #notgood #hurricanemichael2018 #prestorm #eeriness #OCSOonPatrol," the Okaloosa County Sheriff's Office posted on Twitter.