Portions of Cuba were expected to experience a “life-threatening storm surge and hurricane-force winds” beginning on Monday, a graphic from the National Hurricane Centre showed. The storm is expected to be at major hurricane strength when it reaches the island.
“Please treat this storm seriously. It’s the real deal. This is not a drill,” Hillsborough County Emergency Management Director Timothy Dudley said at a news conference.
As of 11am ET on Monday, Ian was moving north-west at 20 kilometres per hour about 385 kilometres south-east of the western tip of Cuba, with top winds increasing from 120 kph earlier in the day to 130 kph.
Ian is forecast to emerge over the south-eastern Gulf of Mexico on Tuesday, passing the western part of the Florida Keys late on Tuesday and then approaching Florida's west coast on Wednesday, the National Hurricane Centre said.
The centre urged residents living in the path of the storm to rapidly undertake safety precautions.
As of Monday, the cities of Tampa and St. Petersburg in Florida appeared to be likely targets of the hurricane.
As many as 300,000 residents could be evacuated in Hillsborough County alone, where Tampa is located, said county administrator Bonnie Wise. Some of those evacuations were already under way on Monday.
And President Joe Biden postponed his September 27 trip to the state because of the storm. Nasa also rolled back the Artemis 1 rocket from its launch pad at the Kennedy Space Centre in Cap Canaveral.
Tropical storm and hurricane warnings have been issued for parts of western Florida.
The centre said that, regardless of Ian's track and intensity, Florida should expect a life-threatening storm surge, hurricane-force winds and heavy rainfall during the middle of the week.
Florida Governor Ron DeSantis has declared a state of emergency throughout the state and urged residents to prepare for the storm.
“We’re going to keep monitoring the track of this storm. But it really is important to stress the degree of uncertainty that still exists,” Mr DeSantis said at a news conference on Sunday.
He added that even if residents are not in the path of the storm, there will be “pretty broad impacts throughout the state”.
Mr Biden has authorised emergency aid to Florida through the Federal Emergency Management Agency to assist in response efforts.
The Associated Press contributed to this report