MIAMI (AP) — From Puerto Rico to Florida, people are keeping a close eye on Hurricane Irma, a monster storm that threatens to plow through the Caribbean and perhaps deliver a devastating blow to the U.S.

People are preparing for the strongest Atlantic hurricane since Rita in 2005, and one that comes on the heels of Harvey ravaging Texas less than two weeks ago.

Where is Irma headed?

Hurricane Irma grew into a dangerous Category 5 storm on Tuesday and roared toward islands in the northeast Caribbean.

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.

The center said there was a growing possibility that the storm's effects could be felt in Florida later this week, though it was still too early to be sure of its future track: "Everyone in hurricane-prone areas should ensure that they have their hurricane plan in place."

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Powerful Irma

Irma had the most powerful winds ever recorded for a storm in the Atlantic Ocean.

Four other storms have had winds that strong in the overall Atlantic region but they were in the Caribbean Sea or the Gulf of Mexico, which are usually home to warmer waters that fuel cyclones.

What makes Irma so strong is unusually warm waters for that part of the Atlantic.

MIT meteorology professor Kerry Emanuel said Irma is in an area where all the weather factors are ripe for further strengthening.

He said the storm "is big and it's intense," and urged people to "please, please pay attention to what the emergency people tell you."

"If they tell you to get out, get out," he said.

In the Florida Keys, that is exactly what officials told both tourists and residents.

Monroe County spokeswoman Cammy Clark said in a news release that a mandatory evacuation of tourists would begin at sunrise Wednesday. A plan for evacuation of residents is also underway but a timetable hasn't been determined.

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Florida gets ready

People in Florida are buying drinking water and other staples as they prepare for Irma, which could start impacting the state by the weekend.

There are empty grocery store shelves across South Florida. Florida Gov. Rick Scott declared a state of emergency to give local governments "ample time, resources and flexibility" to prepare for the storm. He urged residents to stay vigilant and monitor weather conditions.

The last major hurricane to hit Florida was Wilma in 2005. A major hurricane has winds of 111 mph (180 kph) or higher.