A "potentially catastrophic" Hurricane Irma is roaring toward northeastern Caribbean islands as a Category 5 storm, the National Hurricane Center said, threatening to slam into Antigua, the Virgin Islands and Puerto Rico by Wednesday before possibly taking aim at the US mainland.
Irma was churning Tuesday morning in the Atlantic about 225 miles east of Antigua and Barbuda, heading west with maximum sustained winds of 180 mph -- well above the 157 mph threshold for a Category 5 -- the hurricane center said.
The storm's forecast track currently has it near or over Antigua and Barbuda, St. Kitts and Nevis, and Anguilla by late Tuesday or early Wednesday, and the British and US Virgin Islands and Puerto Rico on Wednesday afternoon.
Preparations to protect life and property in those areas "should be rushed to completion," the hurricane center said in an 11 a.m. ET advisory.
"We could see storm surges of 7 to 11 feet -- that's certainly life-threatening -- and very, very heavy flooding rainfall" in the far northeastern Caribbean islands as well winds that could cause catastrophic damage near the eye wall, the hurricane center's Michael Brennan said.
Computer models show the system eventually heading near the Dominican Republic, Haiti and Cuba on Friday into Saturday, and possibly turning north toward Florida by the weekend.
While Irma's exact path is uncertain, Florida -- where storm-wary shoppers were standing in long lines outside some stores Tuesday -- is bracing for the storm.
And the state's Monroe County, which includes the Florida Keys, says it will order visitors to evacuate by sunrise Wednesday, and that it expects to announce an evacuation for residents soon.
After declaring a state of emergency across Florida, Gov. Rick Scott said President Donald Trump had "offered the full resources of the federal government as Floridians prepare for Hurricane Irma."
Scott also ordered 7,000 National Guard troops to report for duty by Friday morning. Of those, 100 were activated Tuesday to begin helping with preparations, he said.
"We do not know the exact path of this storm, but weather can change in an instant and while we hope for the best, we must prepare for the worst," Scott said in a statement.
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Antigua, Barbuda, Anguilla and the Virgin Islands
Forecasters' most immediate concerns are for the people of the northeastern Caribbean, Brennan said.
"Anguilla, all the way toward (Antigua and) Barbuda, all the way up even toward the British Virgin Islands (are) in grave danger of an eye wall hit at (at least) 150 mph -- that devastates the island, no matter what island it is," CNN meteorologist Chad Myers said Tuesday.
Those islands are under hurricane warnings, as are Puerto Rico, the US Virgin Islands, Montserrat, St. Kitts and Nevis, Saba, St. Eustatius, St. Martin/St. Maarten, and St. Barts.
Hurricane warnings are issued to areas that are expected to experience hurricane-force winds (at least 74 mph).
Read: Hurricane Irma could be next weather disaster
On Antigua, home to roughly 80,000 people, fishermen used machines to lift their boats onto docks and other residents flocked to stores to stock up on food and other supplies ahead of the storm, video broadcast by ABS TV Antigua and distributed by Reuters shows.
The US Virgin Islands, with about 100,000 people, declared a state of emergency Tuesday and ordered the National Guard into active service.
Hundreds of people rushed to the stores, emptying shelves of food and drinking water just as Puerto Rico Gov. Ricardo Rosselló declared a state of emergency and activated the National Guard on Monday.
For hours, people also lined up outside hardware stores hoping to get plywood, batteries and power generators. If Irma knocks out power, Puerto Ricans said they are worried it would take weeks or months before the power is restored.
"It (power) is something absolutely necessary, especially due to Puerto Rico's weather. We need to have the A/C or a fan on all night," a woman told CNN affiliate WAPA.
Last month, the director of Puerto Rico's power utility, Ricardo Ramos Rodríguez, said several factors have made the island's electric system "vulnerable and fragile," WAPA reported.
One of those is the shortage of employees. Many workers recently retired or left their jobs for better prospects on the US mainland, Ramos Rodríguez said.
Public schools and officials at the University of Puerto Rico campuses have canceled classes, and many businesses are closed.
"Make a U-turn and die in the ocean, Irma. The Caribbean islands don't need more problems!" Twitter user mujertropical wrote about the storm.
Florida: Stocking up on water, food
In Florida, people were standing in lines at stores to buy water and other goods Tuesday, though landfall there was uncertain.
In Miami, supermarkets are already selling out on water and nonperishable food. People are trying to beat the rush in case Irma makes landfall.
"I've been through hurricanes and they're like 'Oh it's going to hit right here' and then it hits 30, 40 miles up the coast and it kind of changes the way everything goes so better safe than sorry," Greg Andrews told CNN affiliate WPLG.
In Davie, a Publix store appeared to be sold out of bottled water Tuesday morning, Jen Panditaratne said. She posted a picture of empty shelves on Twitter.
"Sorry!! No water at this time! Waiting for deliveries. Estimated time of arrival is unknown," a sign in the picture reads.
In Clearwater, along Florida's Gulf Coast, a Publix store still was selling six-packs of water but had run out of larger cases, Carrie Hart said. Hart told CNN that workers there were trying to calm shoppers.
On Twitter, she described a "mad run" on the remaining six-packs.
"@publix is out of cases. Expecting more tonight. Store 1300. They are doing their best people. Calm down," she tweeted.
Irma could head to Florida by the weekend, Myers said Tuesday.
"Just tremendous damage if we get this storm, this big, over parts of Florida," Myers said.
Why Irma could be especially intense
Irma is a classic "Cape Verde hurricane," meaning it formed in the far eastern Atlantic, near the Cape Verde Islands (now known as the Cabo Verde Islands), before tracking all the way across the Atlantic, CNN meteorologist Brandon Miller said.
And Cape Verde storms frequently become some of the largest and most intense hurricanes. Examples include Hurricane Hugo, Hurricane Floyd and Hurricane Ivan.