Millions of Florida residents secured boats and properties and fled to higher ground before Hurricane Idalia made landfall this afternoon.
Evacuations were ordered in 28 of Florida's 67 counties, as of yesterday.
Most of the state and parts of Georgia and South Carolina are under storm warnings or advisories.
Here are the most important things to know about Idalia's projected impact.
What is Hurricane Idalia's path?
Idalia grew from a tropical storm into a Category 4 hurricane on its way to landfall in Florida early today, before making landfall as a Category 3 system, having caused floods in western Cuba that forced residents of coastal towns to seek higher ground.
As of 5am EDT (10am Irish time), Idalia was packing maximum sustained winds of 215 km/h as it churned toward shore, 95km west of Cedar Key, Florida.
Three major hurricanes have struck Florida in the past seven years: Irma in 2017, Michael in 2018 and Ian last September.
Idalia's centre will cross Florida's coastline somewhere in the Big Bend region, where the state's northern panhandle meets the Gulf side of the Florida peninsula, the National Hurricane Center said.
What are 'storm surges' and 'king tides'?
The deadliest threat Idalia poses is a surging wall of seawater 3 to 4.6 metres high that could flood low-lying areas along Florida's coast, according to authorities.
"Storm surges" occur when high winds and atmospheric pressure from an oncoming hurricane force ocean water onto land. The resulting floodwaters may take a couple of days to subside.
A "king tide" - the highest type of high tide - caused by the extra gravitational pull that occurs when the sun and moon align with Earth is also expected today, which will likely exacerbate the surges from Idalia.
Hundreds of miles of Florida shoreline are under storm surge warnings, from Apalachicola Bay in the panhandle through Tampa, the state's third-largest city, and down to Sarasota in the south.
Evacuation zones
Authorities are scrambling to move thousands of Florida residents out of danger before nightfall.
The Florida Emergency Management Agency listed 28 counties with evacuation orders.
Sixteen counties issued mandatory evacuation orders for certain residents, especially those living in coastal and flood-prone areas or in mobile homes, recreational vehicles, or structurally unsound housing.
Local authorities opened dozens of shelters for evacuees and urged residents to take the evacuation orders seriously.
"Please do not become complacent - take action now if you have not done so already," wrote the sheriff's office of Citrus County, north of Tampa, in a Facebook post.
What disruption could be caused?
Even Floridians not under evacuation orders should expect disruption as a result of Idalia, from power outages to shuttered schools and businesses.
Florida Governor Ron DeSantis has declared a state of emergency for 46 Florida counties.
About 5,500 National Guard members were mobilised, while 30,000 to 40,000 electricity workers were on standby to help restore power quickly after the hurricane passes.
School districts around the region cancelled classes starting on Monday afternoon.
Commercial operations were suspended at Tampa International Airport and are not expected to resume until tomorrow.
