Doyle Rice, Alexi C. Cardona and John Bacon

USA TODAY NETWORK

NAPLES, Fla. — "Potentially catastrophic" 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, tied for the second-strongest Atlantic hurricane on record, could slam into Florida's coast over the weekend. A mandatory evacuation for visitors to the Florida Keys was set to begin at sunrise Wednesday, with residents required to evacuate later in the day.

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

Late Tuesday, the storm was almost upon islands like Antigua and Barbuda. Winds continued to gust at 185 mph. In its 11 p.m. ET advisory, the National Hurricane Center said the eye of the hurricane was 50 miles from the two islands.

The hurricane will blast the northern Caribbean with life-threatening flooding, 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.

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.

More:Hurricane Irma: What we know now

More:Hurricane Irma could be a $300 billion catastrophe if it hits South Florida

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."

Only Hurricane Allen in 1980, with winds of 190 mph, was stronger. Wilma in 2005, Gilbert in 1988, an unnamed 1935 hurricane and now Irma have hit 185 mph.

More:Hurricane Irma: How to prepare an emergency kit

When a Category 5 hurricane hits land, it can destroy a high percentage of framed homes, which suffer total roof failure and wall collapse, the hurricane center said. Fallen trees and power poles can isolate residential areas, and power outages in some areas can last for months.

Most of the area could be uninhabitable for weeks or months, the center said.

If Irma slams into the U.S. as a Category 5 hurricane, it wouldn't be the first. Hurricane Andrew roared into South Florida 25 years ago, flattening neighborhoods, tossing cars, boats and mobile homes like small toys and leaving millions without power. The storm destroyed more than 25,000 homes and damaged 100,000 others.

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

There is an increasing chance of seeing some impact 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.

Schools and government offices in the Keys will be closed Wednesday, and Monroe County's three hospitals "have begun plans to evacuate their patients," Senterfitt said.

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.

Authorities in Miami-Dade County ordered schools closed Thursday and Friday. Mayor Carlos Gimenez urged tourists to leave and said shelters will open Wednesday night.

In Naples, 100 miles west of Miami, Collier County Sheriff Kevin Rambosk took to Facebook, calling for residents to consider leaving town. "If you are planning to leave, please do it now," Rambosk said. "Do not wait."

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