Hundreds of thousands were told to evacuate but many have not fled.
At least 13 people reportedly died in Central America over the weekend as a result of storm rains and floods.
According to the Associated Press, six people were killed in Honduras, four in Nicaragua and three in El Salvador.
In their latest advisory, the National Hurricane Center warned coastal residents “not to venture out into the relative calm of the eye, as hazardous winds will increase very quickly as the eye passes”.
More than 370,000 people in Florida have been ordered to evacuate and move to higher ground, but officials estimate that far fewer have actually left.
“Do not leave your house,” Florida Governor Rick Scott said on Wednesday, hours before landfall.
“The worst thing you can do now is leave,” he said, adding that those who do “put yourself and your family in danger”.
Federal Emergency Management Agency Director Brock Long told the president at the White House that he was especially concerned about buildings that were built before 2001, and are not able to withstand category three winds.
“We just hope those the structures can hold up. And if not, that they’re not in those structures,” Mr Trump responded.
Florida has declared a state of emergency, as have Alabama and Georgia.
Images on social media show scenes of flooding and wind damage.
— OkaloosaSheriff (@OCSOALERTS) October 10, 2018
Gulf waters covering most of beach this am #HurricaneMichael #PCB @wmbbjustin @Whitleyweather @TylerAllender @smithwjhg @SamLuceyWX @NWSTallahassee @weatherchannel @The_News_Herald pic.twitter.com/3hdPSQrKZw
— Sandman (@sandman_pcb) October 10, 2018
What are latest developments?
And “life-threatening” flash floods may occur as a result of up to 12in (30cm) of rain.
On the Saffir-Simpson hurricane scale, category four includes winds of up to 156mph with possible severe damage to even well-built homes and trees being felled.
Schools and state offices in the area are to remain shut this week.
On Wednesday, Gov Scott said he had activated 3,500 Florida National Guard troops.
Heavy rains are forecast for the Carolinas, which were drenched by Hurricane Florence last month.
North Carolina Governor Roy Cooper told residents: “I know people are fatigued from Florence, but don’t let this storm catch you with your guard down.”
More than 300 miles of coastline are currently under threat, the National Weather Service has said.
Forecasters in Alabama have warned of possible tornados.
Not everyone has fled
Driving through the streets of Panama City Beach this morning, there were almost no other cars on the road. Here and there the odd neon sign still glowed, but many houses and businesses were boarded up after an emergency evacuation order.
Not everyone though had heeded the instruction.
His house, he says, is built to withstand winds of 150mph and it’s just high enough above sea level that the predicted surge may well fall short.
Just along the road, Stacey and Michael Buckner sit in their car, staring out to sea. Their house is two blocks from the water, and they’ve chosen not to leave because of Michael’s elderly parents.
For the coming hours, Dave, Michael and Stacey are on their own – the emergency services are staying out of the storm and nature is about to take its course.
What cities will be hit hardest?
Utility lines are expected to be damaged due to falling trees, leading to power outages.
Southern Georgia will be hit next, with residents there warned to brace for possible flash floods.
Winds will die down as Michael pushes north into the soggy Carolinas with cities Columbia and Charlotte each forecasted to see up to 5in of rain.
Local officials warn people in the regions affected by Hurricane Florence last month – even those living far from waterways – to watch for landslides or flash flooding.