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NAPLES, Fla. — "Extremely dangerous" Hurricane Irma strengthened Tuesday to a Category 5 storm with sustained winds of 185 mph, making it the strongest hurricane in the Atlantic since 2005.

Irma, the strongest hurricane ever recorded in the Atlantic Basin, outside of the Caribbean Sea and the Gulf of Mexico, could slam onto Florida's coast over the weekend. A mandatory evacuation of visitors to the Florida Keys was set to begin at sunrise Wednesday.

“If ever there was a storm to take seriously in the Keys, this is it,” Monroe County Emergency Management Director Martin Senterfitt said.

The hurricane will blast the northern Caribbean with life-threatening flooding rain, damaging winds and rough surf over the next few days, Accuweather said. A similar scenario could then play out somewhere along the Gulf or East coasts of the U.S. this weekend or next week, depending on where Irma tracks.

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Gov. Rick Scott warned Floridians to "prepare for the worst." He declared a statewide state of emergency and activated 100 Florida National Guard members to help with storm preparations. The state's full complement of 7,000 Guard members will report for duty Friday.

Scott urged residents to stock up on water and food and to learn the locations of local emergency shelters.

"We don't know what is in store, but we all have to be prepared," Scott said. "When there's an evacuation, listen. In the middle of a hurricane, no one can rescue you."

At 2 p.m. ET Tuesday, the center of Hurricane Irma was located 180 miles east of Antigua, and it was moving west at 14 mph.

When a Category 5 hurricane hits land, it destroyed a high percentage of framed homes, which suffer total roof failure and wall collapse, the National Hurricane Center said.

Fallen trees and power poles will isolate residential areas. Power outages will last for weeks, or possibly months. Most of the area will be uninhabitable for weeks or months, the center said.

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Hurricane warnings were in effect Tuesday for several Caribbean islands, including Puerto Rico and the U.S. Virgin Islands, where "preparations should be rushed to completion," according to the hurricane center.

There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend, the hurricane center warned. "While the track is still uncertain, the time to prepare is now," the Weather Channel warned.

Scott requested food, water and tarps from FEMA. The state has more than 300 truckloads of water and 1 million meals at the state Logistics Response Center in Orlando.

"Given the size of the population threatened by Hurricane Irma, however, the state will need additional emergency supplies," Scott wrote in a letter to the federal agency.

Retail traffic across Florida's Treasure Coast, north of Miami, indicated residents were preparing. By Monday afternoon, shelves of water had been emptied at Walmart in Vero Beach, and a water-filling station stayed busy at Peter's Hardware Center in Palm City.

American Airlines canceled flights into St. Kitts and St. Maarten for Tuesday and Wednesday, but added an extra flight Tuesday out of each destination to Miami to help those who want to clear out ahead the storm. The airline is also waiving change fees for passengers affected by Irma and traveling Tuesday through Friday.

Rice reports for USA TODAY from McLean, Va.; Cardona for the Naples (Florida) Daily News from Naples. Contributing: Jefferson Graham, USA TODAY; Andrew Atterbury, The Stuart (Fla.) News; Stacey Henson, The (Fort Myers, Fla.) News-Press; and Arek Sarkissian, Naples Daily News.