Idalia has become a hurricane as it intensifies on a path toward Florida, bringing the risk of life-threatening storm surges and dangerous hurricane-force winds.
A day earlier, Florida residents loaded up on sandbags and evacuated from homes in low-lying areas along the Gulf Coast to prepare.
The National Hurricane Center projected the storm could have sustained winds of up to 193km/h.
That would make it a Category 3 hurricane – a potentially big blow to a state still dealing with lingering damage from last year’s Hurricane Ian.
The centre of Idalia is forecast to reach the Gulf Coast of Florida on Wednesday, and move close to the Carolina coastline on Thursday.
“Right now, the biggest hazards are storm surge,” Robbie Berg, a senior hurricane specialist at the National Hurricane Centre in Miami, said on Tuesday morning.
“We’re expecting a surge as much as 8 to 12 feet (about 2.5 metres to 3.5 metres) above normal tide levels in portions of the Big Bend area of Florida.”
Idalia thrashed Cuba with heavy rain, especially in the westernmost part of the island, where the tobacco-producing province of Pinar del Rio is still recovering from the devastation caused by Hurricane Ian.
Authorities in the province issued a state of alert, and residents were evacuated to friends’ and relatives’ homes.
As much as 10cm of rain fell in Cuba on Sunday, meteorological stations reported.
Idalia was expected to start affecting Florida with hurricane-force winds as soon as late Tuesday.
It is the first storm to hit Florida this hurricane season and authorities urged residents to wrap up storm preparation by Tuesday morning at the latest.
Idalia is also the latest in a summer of natural disasters, including wildfires in Hawaii, Canada and Greece; the first tropical storm to hit California in 84 years, and devastating flooding in Vermont.
Florida Governor Ron DeSantis declared a state of emergency in 46 counties, a broad swath that stretches across the northern half of the state from the Gulf Coast to the Atlantic Coast.
The state has mobilised 1100 National Guard members, who have 2400 high-water vehicles and 12 aircraft at their disposal for rescue and recovery efforts.
Mr DeSantis warned of a “major impact” to the state, noting the potential for Idalia to become a Category 3 hurricane.
“The property – we can rebuild someone’s home,” Mr DeSantis said during a news conference on Monday.
“You can’t unring the bell, though, if somebody stays in harm’s way and does battle with Mother Nature.”
Large parts of the western coast of Florida are at risk for storm surges and floods.
Evacuation notices have been issued in 21 counties with mandatory orders for some people in eight of those counties.
As Gulf Coast residents packed up their cars or hauled out generators in case of power outages, state officials warned about potential fuel contamination at dozens of petrol stations.
Southwest Florida is still recovering from Hurricane Ian, which was responsible last year for almost 150 deaths.
The Category 5 hurricane damaged 52,000 structures, nearly 20,000 of which were destroyed or severely damaged.
After moving across Florida, Idalia is forecast to blow through Georgia, South Carolina and North Carolina.
So far this year, the US east coast has been spared from cyclones. But in the west earlier this month, Tropical Storm Hilary caused widespread flooding, mudslides and road closures in Mexico, California, Nevada and points north.