KEY WEST, Fla. — Hurricane Irma barreled toward the U.S. mainland Wednesday, prompting counties in Florida to begin evacuations while the “potentially catastrophic Category 5” storm menaced Puerto Rico and battered a wide swath of the Caribbean.

Forecasters said Irma posed an increasing threat to South Florida, a sprawling and densely populated mass of cities and suburbs hugging the coastline. As dire warnings mounted, schools and offices across the state began to shut down, grocery store shelves were wiped clean and authorities ordered evacuations with more to follow.

The most powerful hurricane to threaten the Atlantic coast in more than a decade, Irma has swelled into a monster force with maximum sustained winds near 185 mph, according to the National Hurricane Center. The center said Wednesday afternoon that Irma’s “extremely dangerous core” was moving over the Virgin Islands and would “pass near or just north of Puerto Rico” later in the day.

Throughout the American territories and other Caribbean islands in Irma’s path, residents watched the storm with fear, wondering whether they would emerge with destroyed homes or no electricity for months. Irma’s eye passed over Barbuda at around 1:47 a.m. Wednesday, the National Weather Service said, while on the French Caribbean islands of St. Martin and St. Barthelemy, residents were ordered to remain indoors.

According to the Capital Weather Gang, Barbuda took a direct hit and the weather station there registered a wind gust of 155 mph before going offline, while the storm surge on the island — or the swell of water above normally dry land — reached at least eight feet. By Wednesday morning, the National Hurricane Center said Irma’s eye had passed over St. Martin and warned that the storm could bring dangers including life-threatening storm surges, destructive winds, flash floods and mud slides to Cuba, Haiti, the Dominican Republic, the Bahamas and beyond.

In San Juan, Puerto Rico, people in the Hato Rey neighborhood prepared for the storm under unusually calm, cool weather Wednesday morning that gave way to a light drizzle. People expect to lose power, which is nothing unusual for the neighborhood, where power often goes out for a few hours after a heavy rainstorm.

The power, in fact, went out at about 10:15 a.m. on Wednesday. Even though her house was boarded up, Rita Hernandez could hear the sing-song calls of “Yucca! Platanos!” from a pregonero — a roaming fruit and vegetable vendor — as he made what was likely his last run before the storm truly arrived.

Expectations that the storm would by this weekend make its way to Florida sent people in that state scrambling to stock up on food and water, and in many cases get out of the state. Monroe County, the southernmost county in Florida and home to the Keys, began mandatory evacuations of tourists and visitors on Wednesday. Mandatory evacuation orders were issued further north, in parts of populous Broward County. And in Miami-Dade, a county of 2.7 million, officials said they may order residents to leave coastal areas, including Miami Beach.

In Key West, hotels closed down ahead of the evacuation order, and the airport was scheduled to halt operations later Wednesday. Gas stations reported low fuel stocks and grocery stores ran out of bottled water. Residents and business owners boarded up windows and hauled boats out of the water, while tourists and residents had already begun crowding up the single highway that snakes through the 120-mile island chain and into the Florida mainland.

Many businesses on Key West’s famed Duval Street were shuttered Tuesday — with the exception of a few bars and restaurants — and many residents were streaming to the mainland by car on Route A1A.

“We’re emphatically telling people you must evacuate,” said the director of Monroe County’s Emergency Operations Center, Martin Senterfitt. “You cannot afford to stay on an island with a Category 5 hurricane coming at you.”

There are two nuclear sites in Florida — 45-year-old Turkey Point 25 miles south of Miami and 41-year-old St. Lucie further north along the coast. They belong to NextEra, a utility with about 5 million electricity customers in Florida. NextEra said that it will shut down its four nuclear reactors before Hurricane Irma arrives. That will reduce the heat in the reactors and the need for electricity.

NextEra also said that its reactors could weather a loss of electricity of the sort that caused a meltdown at Japan’s Fukushima Daiichi reactors after the tsunami there in 2011. A NextEra spokesman Peter Robbins said that the nuclear plants have diesel generators located 20 feet above sea level inside reinforced concrete structures.

Carolyn Boutte, 44, said she and her husband moved to a house in Key West four years ago from Gloucester, Mass., and they have never been through a hurricane threat like this. They searched for gas on Tuesday so they could escape, but the first three stations already had run out of fuel, and lines were long everywhere else. She finally ran into some luck — at a station where she had to wait 45 minutes for a fill.

“My husband and I are packing up the dog and our Harley Davidson,” said Boutte, a marine biologist. “Unless the hurricane changes paths, we are getting out of here in the next couple of days.”

Even as the full devastation from Hurricane Harvey was still being tallied in Texas and along the Gulf Coast, authorities have shifted their attention to Florida, with a particular unease in South Florida, home to 6 million people across Miami-Dade, Broward and Palm Beach counties — many of whom vividly remember Hurricane Andrew’s onslaught a quarter-century ago.

Hundreds of thousands of people scrambled to get out of South Florida as the storm loomed, while airlines began canceling flights and issuing fee waivers to travelers.

Broward County Mayor Barbara Sharief said mandatory evacuations would begin Thursday at noon for people in the eastern portion of the county that runs alongside the Atlantic Ocean. The National Weather Service said Wednesday that the threat to South Florida “continues to increase,” with concerns about what could happen between Friday night and Monday.

“This storm is bigger, faster and stronger than Hurricane Andrew,” Florida Gov. Rick Scott (R) said Wednesday of Irma.

Scott stressed that even with Irma’s uncertain trajectory, officials were preparing for a direct impact from the hurricane.

“Do not sit and wait for the storm to come,” he said. “It is extremely dangerous and deadly and will cause devastation. Get prepared right now.”

President Trump on Tuesday evening declared an emergency in Florida as well as the U.S. Virgin Islands and Puerto Rico, and Scott activated 100 members of the Florida National Guard and said he has directed all 7,000 members to report for duty on Friday.

ABS TV Radio Antigua & Barbuda broadcasted live on Sept. 5 as Hurricane Irma and its 180 mph winds made landfall in Antigua. (Monica Akhtar/The Washington Post)

The Pentagon approved the use of two ships — originally deployed for Hurricane Harvey relief — to assist in Florida as needed. William “Brock” Long, administrator of the Federal Emergency Management Agency, said that incident management assistance personnel already are on the ground in vulnerable areas.

“Just like in Texas, the response to Irma is going to take all levels of government and the whole community,” Long said in a statement. “This has the potential to be a catastrophic storm.”

Officials across Florida responded to the dire forecasts by slowly shutting down the contours of daily life. Monroe County canceled classes for the rest of the week, while school districts in Miami-Dade, Broward and Palm Beach — three of the country’s largest, with a combined enrollment north of 800,000 students — said they were canceling classes on Thursday and Friday. On the western edge of the state, officials in Pinellas County, which includes St. Petersburg, also canceled classes on Thursday and Friday.

The NFL said Wednesday that the Miami Dolphins’ season opener scheduled for Sunday afternoon against Tampa Bay would be postponed to November. The University of Central Florida in Orlando, which could face punishing weather if Irma crawls up the coastline, moved a football game to Friday night. And the University of Miami announced it was canceling its football game set for this Saturday in Arkansas so the team doesn’t have to travel.

Scott, who earlier this week declared a statewide emergency, has warned that Irma could require large-scale evacuations and may severely impact areas battered last year by Hurricane Matthew, which sent punishing flooding into parts of the state.

“If you’re told to evacuate, get out quickly,” Scott said at a news briefing Wednesday. He added: “I cannot stress this enough: Do not ignore evacuation orders. Remember, we can rebuild your home but we cannot rebuild your life.”

But it was still not clear Wednesday how much of the state could be imperiled by Irma. The uncertainty of Irma’s track and the geography of the Florida peninsula combined to create an unusually broad, essentially statewide sense of emergency in a place where most of the population lives along the coasts. Irma could potentially ride up either side of Florida or track farther west into the Gulf of Mexico and endanger the state’s panhandle.

In Estero, on the state’s Gulf Coast, residents were either hunkering down or starting to flee. Stocks of water and flashlights at grocery stores were wiped out, and gas was becoming scarce. A sign on the door of a Speedway gas station warned customers: “No gas, no propane, no water, sorry.”

“We’ve never been this worried in our entire lives,” said Jose Torres, 25, who plans to evacuate to Georgia on Wednesday.

The main routes out of South Florida are Interstate 95 and the Florida Turnpike, which can be prone to heavy traffic on the best of days. In an effort to smooth travel, Scott on Tuesday ordered that no tolls be collected.

The main route west, meanwhile, is Interstate 75 — the so-called “Alligator Alley” — but that would only funnel South Florida’s residents to the imperiled southwest coast of the state. If the storm does track up the state’s spine, as Scott noted, that could make evacuations extremely complex.

The last major hurricane — registering as a Category 3 storm or stronger — to make landfall in Florida was Hurricane Wilma in October 2005. Wilma also was the last major hurricane to make landfall in the United States until Harvey struck Texas late last month.

Berman reported from Washington. Daniel Cassady in San Juan, Puerto Rico; Janine Zeitlin in Estero, Fla.; and Sandhya Somashekhar, Brian Murphy, Andrew deGrandpre, Dan Lamothe, Joel Achenbach, Angela Fritz, Lindsey Bever, Steven Mufson and Jason Samenow in Washington contributed to this story, which will be updated throughout the day. 

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