"Hurricane Michael is a monster storm and it keeps getting more dangerous," Florida Governor Rick Scott told a news conference on Tuesday. "The time to prepare is now."
The Republican governor, who is campaigning for a U.S. Senate seat in the November congressional elections, warned of the potential for a deadly storm surge that could be as much as 12 feet (3.7 meters) above normal sea water levels.
People in areas that could be affected should take no chances against such a powerful surge, Scott said, adding, "No one's going to survive," such a wall of water.
As Michael moved over open water, energy companies halted nearly one-fifth of Gulf of Mexico oil production and evacuated personnel from 10 platforms on Monday.
The Gulf of Mexico produces 17 percent of daily U.S. crude oil output and 5 percent of daily natural gas output, according to the U.S. Energy Information Administration. The partial shutdown ahead of Michael helped push oil prices slightly higher on Tuesday.
Scott declared a state of emergency in 35 counties along the Panhandle and Florida's Big Bend regions, mostly rural areas known for small tourist cities, beaches and wildlife reserves, and the state capital, Tallahassee.
A hurricane warning was in effect for a more than 300-mile (480-km) stretch of coastline from the Florida-Alabama border to the Suwannee River in Florida.
The Federal Emergency Management Agency had numerous teams deployed and ready to respond, said FEMA spokesman Jeff Byard. About 1,250 National Guard soldiers were assisting and more than 4,000 troops were on standby.
Trump, speaking to reporters at the White House, said the country was very well prepared for the hurricane, adding it was much bigger than had been expected.
CLOSURES AND LINES FOR GAS
In Panhandle counties, most state offices, schools and universities were closed for the rest of the week. Lines at gasoline stations grew as people left. Those who stayed emptied grocery store shelves of water and other supplies.
Gary Givens, owner of Gary's Oyster Shack in Panama City, Florida, said he was closing his restaurant around lunch time on Tuesday with the help of his crew ahead of the storm's arrival. Because he owns two businesses in the area, Givens, who lives inland, said he was not evacuating.
"I just got a crew that came in that are staying also and they're in here helping me get everything buttoned up, tightened down, getting the food secure," Givens said.
Some Panama City residents were seen on the beach enjoying the cool breeze ahead of the storm, while others were loading up cars with luggage and visitors were checking out of hotels.
The last major hurricane - Category 3 or above - to hit the Panhandle was Hurricane Dennis, which made landfall near Pensacola in 2005, according to hurricane center data.
Torrential downpours and flash flooding from the storm over the weekend caused 13 deaths in Central America.
At 4 p.m. ET (2000 GMT) on Tuesday, Michael's center was heading north at around 12 mph (19 kph), the NHC said.
On its current track, it would make landfall somewhere along a coastline that includes the cities and towns of Fort Walton Beach, Panama City Beach, Port St. Joe, St. Teresa and the wildlife reserves bordering Apalachee Bay. However, forecasters always note it is not possible to say where a hurricane will land until it is closer to the coast.
The storm was forecast to move through the southeastern United States on Wednesday and Thursday, passing through Georgia and the Carolinas, which are still recovering from Hurricane Florence last month. It would head off the Mid-Atlantic coast by Friday, the NHC said.
Georgia Governor Nathan Deal on Tuesday issued a state of emergency declaration for 92 counties in the state.
The Miami-based center forecast as much as 1 foot (30 cm) of rain in parts of Florida, Georgia and South Carolina.
CNBC's Falyn Page contributed to this report.