FORT MYERS, Fla (Reuters) – Hurricane Irma gained strength as it was set to make landfall in Florida on Sunday with a double barrel threat of destructive winds and life-threatening storm surges, prompting one of the largest evacuations in U.S. history.

The storm, which went on a deadly march up Cuba’s northern coast a day earlier, was a Category 4 hurricane about 40 miles (65 kms) south-southeast of Key West, Florida, as of 5 a.m. EDT (0900 GMT) on Sunday with maximum sustained winds of 130 mph (210 kph), the National Hurricane Center said.

It was on a path that would take it along the state’s Gulf of Mexico coast near population centers including Tampa and St. Petersburg, it said.

Storm surges pushed by a high tide were forecast to be as high as 15 feet (4.6 meters) for low-lying area along the state’s southwest coast on Sunday, which could produce catastrophic flooding for thousands of homes.

“This is a life-threatening situation,” the NHC said.

Irma, which killed at least 22 people in the Caribbean, was likely to cause billions of dollars in damage to the third-most-populous U.S. state.

Wind gusts near hurricane force began to batter the Florida Keys late on Saturday, the NHC said, with Key West seeing gusts of more than 80 mph on Sunday morning and water levels about 2 feet (61 cms) above normal. The area was expected to feel the full brunt of Irma starting at around 7 a..m. to 8 a.m.

SEEKING SHELTER

Thousands of people spent the night in emergency shelters ahead of the storm expected to rip through Florida’s southwestern archipelago on Sunday morning as a Category 4 storm, the second-highest designation on the Saffir-Simpson scale. Irma should weaken as it moves inland over the Florida panhandle and southwestern Georgia on Monday afternoon, the NHC said.

The NHC has put out a hurricane warning and a tropical storm warning stretching through almost all of Florida into Georgia and South Carolina – an area where more than 20 million people live.

The city of Miami imposed a curfew until 7 a.m. on Sunday and more than 275,000 customers in Florida were without power, utilities reported.

Irma comes just days after Hurricane Harvey dumped record-setting rain in Texas, causing one of the costliest natural disasters in U.S. history.

Tracking models showed Irma would make landfall on the Keys and head along Florida’s west coast, slamming the state that is a major tourism hub, with an economy that comprises about 5 percent of U.S. gross domestic product.

More than 2,000 flights in and out of Florida were canceled on Saturday, according to tracking service FlightAware.com, and ground transport was scrambled by millions fleeing for safety.

Amid urgent warnings from state officials to evacuate before it was too late, downtown Miami was all but abandoned on Saturday.

Sheets of rain and wind gusts of more than 50 mph swept through the deserted city of 400,000 people several hours before expected landfall.

The wind sent a construction crane spinning on the roof of the Miami Worldcenter, a billion dollar mixed-use project near the home of the Miami Heat basketball team and the city’s performing arts center.

On Florida’s west coast, resident Charley Ball said he expected a storm surge to completely engulf the island of Sanibel where he lives.

“Just left the island and said goodbye to everything I own,” said Ball, 62.

CUBA HIT

Irma was a Category 5 storm, the highest ranking possible, when it crashed into Cuba on Saturday morning.

It gradually weakened to Category 3 as it bumped along the island’s northern coastline, flooding streets and sending waves crashing over sea walls. But it gained strength over warm waters as it headed toward Florida, the NHC said.

Irma was forecast to dump up to 20 inches (50 cm) of rain over Florida and southeast Georgia through Monday, less than the 50 inches Hurricane Harvey dropped on parts of Texas and Louisiana two weeks ago, killing at least 60 people and causing an estimated $180 billion in property damage.

Irma could cause insurance losses of between $15 billion and $50 billion in the United States, catastrophe modeling firm AIR Worldwide said.

But unlike with Harvey, dangerous winds will barely abate once Irma makes landfall on Sunday morning.

In Cuba, the destruction along the north central coast was similar to that suffered by other Caribbean islands over the last week as Irma plowed into Ciego de Avila province.

It was the first time the eye of a Category 5 storm had made landfall in Cuba since 1932, state media said, and the island’s Communist government ordered the evacuation of more than a million people from its path.

Officials in Florida have ordered a total of 6.3 million people, or about a third of the state’s population, to evacuate, creating massive traffic jams on highways and overcrowding shelters.

In Palm Beach, President Donald Trump’s waterfront Mar-a-Lago estate was under evacuation order.

“This is a storm of enormous destructive power, and I ask everyone in the storm’s path to heed ALL instructions from government officials,” Trump said on Twitter.

(For a graphic on how Irma compares to other major hurricanes, click tmsnrt.rs/2wP8csY)