"I'm terrified," Antigua resident Carol Joseph said as she made a final trip to the supermarket to stock up on extra batteries, with no one knowing how long power will be out.
Donald Trump declared emergencies in Florida, Puerto Rico and the US Virgin Islands, and authorities in the Bahamas said they would evacuate the residents of six islands at the southern end of the island chain.
The northern Leeward Islands are expected to see waves as high as 3.4 metres, while the Turks and Caicos Islands and southeastern Bahamas could see towering six-metre waves later in the week, forecasters said.
Shoppers are raiding supermarkets and hardware shops as they brace for landfall, with long lines forming at petrol stations as the storm surges closer.
"Seeing the devastation in Texas is a sad reminder that you have to take the events very seriously," Parker Eastin, a 43-year-old lawyer, told the Associated Press.
At least 63 people were killed when Harvey hit Corpus Christi late last month, before tearing a path of destruction all the way to Houston.
"Even as the aisles are emptying, we are trying to replenish as quick as possible," said Maria Brous, spokeswoman of grocery chain Publix.
Hurricane Irma is currently 365km east of Antigua, above South America, and is expected to devastate the Dominican Republic, Haiti and Cuba in coming days.
Authorities warn it could dump up to 45cm of rain in some areas, causing landslides and flash floods, while the storm swell could be as high as seven metres.
The US National Hurricane Centre, based in Miami, warned conditions are "ideal for further intensification" and the storm was "potentially catastrophic".
Once it reaches warmer waters, wind gusts could reach up to 362km/h.
"People who are living there (the Florida Keys) or have property there are very scared, and they should be," US meteorology expert Kerry Emmanuel told AP.
The storm is expected to slowly turn right as it batters the Caribbean, but it's not yet clear whether it will directly strike Florida or dissipate into the Atlantic.
Irma's eye is expected to pass about 80km from Puerto Rico tomorrow morning, and forecasters say the storm will retain its power for at least the next five days.
Florida has a long history of weathering hurricanes.
The last major storm to hit Florida was Hurricane Wilma in 2005, which packed maximum winds of 193km/h and killed five people.
Resident Janet Roberts, who is evacuating the Florida Keys, said she remembers how much damage Hurricane Andrew caused when it struck in 1992.
"We didn't hit the eye and I had nothing left," she told AP.
"This has Andrew beat. This is really bad - really, really, really bad."
However, while the American state has lots of resources and plenty of time to prepare, there are grave fears for the other Caribbean nations.
At least 580 were killed and more than 35,000 left homeless when Hurricane Matthew struck Haiti last year.
The developing nation was also hit by a powerful earthquake and a cholera epidemic in 2010, which killed a more than 233,000 people.
- With AP
