Atlantic in over a decade, and roared toward islands in the northeast Caribbean Tuesday on a path that could eventually take it to the United States.
The U.S. National Hurricane Center said Irma was a "potentially catastrophic" storm with maximum sustained winds of 180 mph (285 kph) as it bore down on the twin-island nation of Antigua and Barbuda. It was centered about 225 miles (365 kilometers) east of Antigua in the late morning and moving west at 14 mph (22 kph).
The center said there was a growing possibility that the storm's effects could be felt in Florida later this week and over the weekend, though it was still too early to be sure of its future track.
If it stays on track and reaches the Florida Straits, the water there is warm enough that the already "intense" storm could become much worse with wind speeds potentially reaching 225 mph, warned Kerry Emanuel, an MIT meteorology professor.
"People who are living there (the Florida Keys) or have property there are very scared, and they should be," Emanuel said.
Irma's center was expected to move over portions of the northern Leeward Islands late Tuesday and early Wednesday, the hurricane center said, warning of "a life-threatening storm surge and large breaking waves" that could raise water levels up to 7 to 11 feet above normal.
The storm's eye was then expected to pass about 50 miles (80 kilometers) from Puerto Rico late Wednesday.
Irma is the strongest Atlantic hurricane since Rita in 2005, officials said.
"Puerto Rico has not seen a hurricane of this magnitude in almost 100 years," Carlos Anselmi, a National Weather Service meteorologist in San Juan, told The Associated Press.
Authorities warned that the storm could dump up to 12 inches (31 centimeters) of rain, cause landslides and flash floods and generate waves of up to 23 feet (7 meters). Government officials began evacuations and urged people to finalize all preparations as shelves emptied out across islands including Puerto Rico.
"The decisions that we make in the next couple of hours can make the difference between life and death," Puerto Rico Gov. Ricardo Rossello said. "This is an extremely dangerous storm."
Hurricane warnings were issued for 12 Caribbean island groups including Antigua, where buzzing chain saws and pounding hammers could be heard widely on Tuesday. Crews delivered water to neighboring Barbuda, one of the islands closest to the hurricane's path.
Prime Minister Gaston Browne told the AP he was confident Barbuda would weather the storm because its shelter was built with reinforced concrete and equipped with a backup generator.
"I am satisfied that at a governmental level that we have done everything that is humanly possible to mitigate against the effects or the potential effects of this storm," he said. "What is really required now is for Antiguans and Barbudans ... to follow the warnings and to act appropriately so that we do not end up with any serious casualties or any fatalities."
Antigua's airport announced it was closing with an ominous statement advising visitors and residents to protect themselves from the "onslaught" of the storm: "May God protect us all."
Puerto Ricans braced for blackouts after the director of the island's power company told reporters that storm damage could leave some areas without electricity for about a week and other, unspecified areas for four to six months.
The utility's infrastructure has deteriorated greatly during a decade-long recession, and Puerto Ricans experienced an island-wide outage last year.
Both Puerto Rico and the U.S. Virgin Islands expected 4 inches to 10 inches (10-25 centimeters) of rain and winds of 40-50 mph with gusts of up to 75 mph.
"This is not an opportunity to go outside and try to have fun with a hurricane," U.S. Virgin Islands Gov. Kenneth Mapp warned. "It's not time to get on a surfboard."
A hurricane warning was posted for Antigua and Barbuda, Anguilla, Montserrat, St. Kitts and Nevis, St. Martin, Saba, St. Eustatius, St. Maarten and St. Barts, Puerto Rico, and the U.S. and British Virgin islands.
Hurricane watches were in effect for the Turks and Caicos, Guadeloupe and parts of the Bahamas, Haiti and the Dominican Republic.
In Florida, residents took advantage of the Labor Day holiday to empty many store shelves of drinking water and other supplies in advance of the storm.
Gov. Rick Scott activated 100 members of the Florida National Guard to be deployed across the state, and 7,000 National Guard members were to report to duty Friday when the storm could be bearing down on the area. On Monday, Scott declared a state of emergency in all of Florida's 67 counties.
A new tropical storm also formed in the Atlantic on Tuesday, to the east of Irma. The hurricane center said Tropical Storm Jose was about 1,505 miles (2,420 kilometers) east of the Lesser Antilles with maximum sustained winds of 40 mph (65 kph). It was moving west-northwest at 13 mph (20 kph) and was expected to become a hurricane by Friday.