Millions of people are huddling in shelters in Florida as Hurricane Irma, one of the most powerful storms ever recorded in the Atlantic, hits the state with 210km/h winds and catastrophically high seas.
The centre of Hurricane Irma made landfall at Cudjoe Key in the lower Florida Keys at just after 1am (NZ time) and was moving north-northwest at 15km/h, the National Hurricane Center said on Sunday.
The NHC forecast potentially deadly storm surges - water driven ashore by the winds - of up to 4.6m along some parts of the coast as the storm hits Florida.
As the northern edge of the storm reached the Florida Keys archipelago off the tip of southern Florida, lashing rains and winds knocked out power to over 1 million homes and businesses on the mainland, Florida utilities said.
"Pray for us," Florida Governor Rick Scott said as his state braced for the massive storm, which left a trail of destruction through the Caribbean.
The new path forged by Hurricane Irma posed a severe threat to Florida's west coast and the Florida Keys, Brock Long, administrator of the Federal Emergency Management Agency, told Fox News, and the storm was bringing tornado watches and warnings around the state.
"Any time you're in that northeast quadrant as the storm is moving forward, that's where the maximum radius winds are that define the intensity of the storm," Mr Long said. "That's where the storm surge is most prevalent and the inland winds are going to be tough."
Irma, which prompted one of the largest evacuations in US history, is expected to cause billions of dollars in damage to the third-most-populous US state, a major tourism hub, with an economy comprising about five percent of US gross domestic product.
Irma was a Category 4 hurricane raging in the lower Florida Keys on Sunday morning, on a path that will take it up Florida's Gulf of Mexico coast near population centres including Tampa and St Petersburg, the NHC reported. Forecasters also warned tornadoes could form in large portions of the state.
Irma killed at least 22 people as it tore through Caribbean and has already claimed at least one life in Florida.
One woman in Miami's Little Haiti neighbourhood delivered her own baby because emergency responders were not able to reach her, the city of Miami tweeted. The two are now at the hospital.
Ahead of the storm, officials in Florida had ordered a total of 6.3 million people, or about a third of the state's population, to evacuate.
The NHC has put out a hurricane warning and a tropical storm warning stretching through almost all of Florida into Georgia and South Carolina, home to more than 20 million people.
Irma comes just days after Hurricane Harvey dumped record-setting rain in Texas, causing unprecedented flooding, killing at least 60 people and leaving an estimated US$180 billion in property damage in its wake. Almost three months remain in the Atlantic hurricane season, which runs through November.