Thousands of people evacuated their homes on the state's Gulf Coast and crowded into places like local high schools to shelter from the storm.
With winds of up to 145 mph (233 kph), storm surges of between 9 and 14 feet (2.7 to 4.2 meters) have been forecast for coastal areas.
Florida Governor Rick Scott said Michael could be the worst storm in decades to hit the state's northwest, also known as the Florida Panhandle.
As many as 180,000 residents were ordered to evacuate their homes. Most evacuations were ordered from Bay County in the panhandle, a low-lying area made up mostly of resorts and retirement communities.
US President Donald Trump issued an emergency declaration for Florida and announced on Twitter that the government was freeing up federal funds for relief operations and providing assistance for the Federal Emergency Management Agency (FEMA).
"It is imperative that you heed the directions of your State and Local Officials. Please be prepared, be careful and be SAFE!" the president tweeted.
Tracking Michael's route
National Hurricane Center spokesman Dennis Feltgen said "we are in new territory with now Hurricane Michael and its 130 mph sustained winds," adding that Bay County was the likely "ground zero" for the hurricane on Wednesday afternoon.
The outer bands of the storm were beginning to reach the Gulf Coast at 7am local time, with some of the worst storm surge expected to hit Florida's Tyndall Air Force Base.
According to the NHC, some parts of Florida could see storm surges of up to 13 feet (4 meters) and up to a foot of rain.
Read more: Texas recovers under the long shadow of Hurricane Harvey
The storm is expected to weaken as it moves up into the southeastern United States.
Nevertheless, state officials have also issued a disaster declaration for Alabama and Georgia. The Carolinas, meanwhile, are still reeling from Hurricane Florence, which killed dozens and caused billions of dollars in damage last month.
However, when Florence barreled towards the Carolinas, residents had five days' notice from the time it had turned into a hurricane and the moment it hit. Michael's increasing strength effectively gave locals in Florida just two days' notice.
Satellite images of Hurricane Michael over the Caribbean
Read more: Climate change and extreme weather: Science is proving the link
Last year saw an array of devastating storms batter the USA's western Atlantic seaboard, including Irma, Maria and Harvey. Houston's metropolitan area suffered a record-equaling $125 billion (€108 billion) in damage.
Climate scientists have long warned that the effects of global warming make storms more destructive and point to last year's string of hurricanes as visible evidence.
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2017: Devastating effects of climate change
Sweltering heat
Unprecedented heat waves swept across the globe in 2017, leading to droughts, wildfires and even deaths. Australia started the year with temperatures near 50 degrees Celsius (122 Fahrenheit), the "Lucifer" heat wave brought the mercury above 40 degrees Celsius throughout Southern Europe in July and August and scorching heat hit India's most vulnerable people. Get ready for next summer...
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2017: Devastating effects of climate change
Disappearing wonder
Earlier this year, scientists realized that coral bleaching in Australia's Great Barrier Reef was worse than first thought. In some parts of the UNESCO World Heritage site, up to 70 percent of the coral has already been killed. By 2050, scientists have warned 90 percent of the reef could disappear. Rising sea temperatures and ocean acidification are the main culprits.
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2017: Devastating effects of climate change
Deadly combination
Armed conflicts are pushing millions of people to leave their homes or live in terribly precarious situations — and climate change is making it worse. A lack of natural resources increases the risk of conflict and makes life even harder for refugees. South Sudanese families, for instance, are escaping to neighboring countries like Uganda and Kenya — countries already suffering from drought.
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2017: Devastating effects of climate change
World on fire
From New Zealand to Spain, from California to even Greenland: the world has seen a nonstop year of wildfires. Global warming has been blamed for the increased fire risk, and in some countries that risk has turned into reality. Wildfires engulfed large areas of Europe's Iberian Peninsula, causing death and destruction, while firefighters in California have had no rest for more than six months.
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2017: Devastating effects of climate change
Record-shattering storms
Hurricanes Maria and Irma, which hit the Caribbean region in August and September, were two of the year's most damaging weather events. The list of deadly storms also included Ophelia in Ireland, Harvey and Nate in Central America and the US, and Xavier and Sebastian in Germany. Warming of the ocean surface has led to more evaporation, and that water may help fuel thunderstorms and hurricanes.
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2017: Devastating effects of climate change
Melting Antarctica
In July, one of the largest icebergs ever recorded separated from the Larsen C ice shelf — one of Antarctica's biggest — reducing its area by more than 12 percent. While calving icebergs in the Antarctic are part of a natural cycle, scientists have linked the retreat of several Antarctic ice shelves to global warming and are closely monitoring potential long-term effects.
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2017: Devastating effects of climate change
Struggle to breathe
Deteriorating air quality causes thousands of deaths around the world every year. India's capital, New Delhi, is one of the world's most polluted cities. In November, large parts of northern India and Pakistan were engulfed by a blanket of thick smog carrying harmful particulate matter. Schools were forced to close, and hospitals were full of people with respiratory problems.
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2017: Devastating effects of climate change
Oceans at risk
The high levels of carbon dioxide in the atmosphere represent a major threat for our oceans, already in danger due to plastic pollution, overfishing and warming waters. Ocean acidification could make these waters — covering more than two-thirds of our planet's surface — a hostile environment for sea creatures. And without marine animals, entire ocean ecosystems are at risk.
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2017: Devastating effects of climate change
Fierce floods and mudslides
Superstorms often trigger flash floods and mudslides. In late December, more than 230 people were killed when a storm hit the Philippines' second-largest island of Mindanao, a tragedy exacerbated by years of deforestation. In 2017, severe floods also hit countries such as Vietnam, Peru and Sierra Leone. European countries, including Greece and Germany, also felt the damaging effects of heavy rain.
Author: Irene Banos Ruiz
dm,es/jm (AP, AFP, Reuters)
