Hurricane Irma expanded into a 180mph “superstorm” on Tuesday as officials warned of life-threatening winds, storm surge and rainfall in several Caribbean islands and emergency planners in Florida escalated preparations for a possible weekend strike.

The first effects of what is now the most powerful Atlantic basin hurricane of the 2017 season were expected to be felt in the Leeward Islands later on Tuesday and Puerto Rico and the Virgin Islands by early Wednesday.

In a late-morning advisory Tuesday, the National Hurricane Center in Miami said Irma was “a potentially catastrophic category 5 hurricane”, already the strongest in its records outside the Caribbean Sea or Gulf of Mexico.

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Fears of devastation and potentially vast loss of life across the Caribbean have been growing since Irma formed into a hurricane near the Cabo Verde islands off the west coast of Africa last week. Forecasters confidently predicted that the storm would pick up strength as it marched westward across the warm waters of the Atlantic, and believed it would maintain that intensity on a path that now takes it dangerously close to Florida’s southern coastline by Sunday morning.

Play Video 0:15 Florida residents stock up ahead of Hurricane Irma – video

Despite some expected fluctuations in strength in the coming days, Irma is expected to be at least as strong at any US landfall as the category 4 Hurricane Harvey that slammed into the Texas coastline and led to devastating floods in Houston last month.

“The chance of direct impacts from Irma later this week and this weekend is increasing in the Florida Keys and portions of the Florida peninsula,” the NHC senior hurricane specialist Dan Brown said in his advisory.

“Everyone in hurricane-prone areas should ensure that they have their hurricane plan in place.”

In Florida, the governor, Rick Scott, declared a state of emergency for all 67 counties on Monday, and repeated the state’s often-heard hurricane mantra of hoping for the best but preparing for the worst.

“Hurricane Irma is a major and life-threatening storm and Florida must be prepared,” Scott said. He added that he had spoken to Donald Trump, who had promised “the full resources of the federal government as Floridians prepare for Hurricane Irma”.

Scott spent Tuesday morning meeting with state emergency planners, and a progressive evacuation order for the low lying Florida Keys island chain was announced later in the day, along with mobilisation of national guard troops.

With continuing uncertainty over exactly where, or if, Irma would strike, residents across Florida used the Labor Day holiday to stock up on essential supplies. Supermarkets and home improvements stores reported shortages of water and plywood, and supplies of emergency generators were also quickly depleted.

Any US landfall from Irma would stretch the resources of the Federal Emergency Management Agency (Fema), which is still dealing with the aftermath of the disaster in Texas caused by Hurricane Harvey. The agency’s administrator, Brock Long, however, said the agency was prepared.

Fema tweeted a photograph of a fully stocked distribution centre in San Juan, Puerto Rico, on Tuesday and Long had an extended phone conversation with the island’s governor, Ricardo Rossello, about storm preparations.

Some of the worst effects were expected in the US territory of Puerto Rico, where Ricardo Ramos, director of the island’s electric power authority (Prepa), warned that some areas could be without electricity for up to six months, although he added others would be back online in less than a week.

“It’s no secret that the infrastructure of the Puerto Rico power authority is deteriorated,” Ramos said, referring to the decade-long recession that has stripped his agency of all but essential funding and seen many of his employees leave for better-paid jobs on the US mainland.

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Schools closed in Puerto Rico on Tuesday ahead of Irma’s arrival and officials said more than 450 shelters were set to open across the island to accommodate 62,000 people. Rossello declared a state of emergency on Monday, activating the national guard and urging islanders to stock up on supplies.

Elsewhere in the Caribbean, hurricane warnings were in place Tuesday for Antigua and Barbuda, Anguilla, Montserrat, St Kitts and Nevis, St Maarten and St Barts. The NHC also posted a hurricane watch for the entire north coast of the island of Hispaniola, comprising Haiti and the Dominican Republic, and the southern islands of the Bahamas.

In the US Virgin Islands, the governor, Kenneth Mapp, warned that the rough seas generated by Irma would be deadly. “This is not an opportunity to go outside and try to have fun with a hurricane. It’s not time to get on a surfboard,” he said.

In Antigua, the island’s meteorological office noted that Irma was now part of “an exclusive club” with only five other Atlantic hurricanes on record having recorded winds of 180 mph or higher. “Irma is now gone off the scale,” it said in a tweet. “May the lord bless our souls.”

The Associated Press contributed to this report