Irma, on the other hand, expanded to its massive size in the tropical Atlantic, east of the Caribbean and the Gulf of Mexico. “To get something east of the islands—at least from the historical record, it hasn’t happened before,” said Klotzbach. “When people in the [Leeward] Islands were saying, ‘We’ve never seen a storm this strong,’ that’s true. They haven’t.”

In its sustained 185 mile-per-hour winds, and its record-breaking low pressure (914 millibars), it is the strongest storm ever measured in the open Atlantic Ocean.

That record-breaking cyclone has now come in for its horrific finale. On Sunday, Irma made landfall in Florida twice—first in the Keys, then on the mainland—as a Category 4 storm. Wind speeds maxed out at 142 miles per hour in Naples, near where the storm came ashore. Irma is the most ferocious storm seen in the Sunshine State since Hurricane Wilma cut across the peninsula in 2005.

Irma weakened to a Category 2 storm late Sunday evening. Almost 3 million Floridians were left without power, and more than 6.5 million people had been ordered to leave their homes, the largest evacuation in state history. Schools were already closed on Monday as far away as Atlanta.

The storm’s death toll stood at 27.

The storm appeared to devastate the Florida Keys, where it made its first landfall. “Everything is underwater, I mean everything,” said Larry Kahn, an editor of FlKeysNews, describing the city of Marathon. Some effects of Irma’s storm surge seemed to set in late that afternoon: The National Weather Service’s forecast office in Key West stayed up through the worst of the storm’s winds, but it lost contact with the outside world that afternoon.

The Miami Herald reported that people might remain in shelters in the Keys for several days.

The storm, now slowed, was expected to continue moving up the coast of Florida through Sunday and Monday. The National Hurricane Center warned that storm surge could remain dangerous for another day, and it said flash flooding and high rainfall totals would follow the storm into the continental United States. Irma is expected to weaken to a tropical depression near the Kentucky-Tennessee border Wednesday afternoon.

As tropical-storm-force winds began to batter Miami on Saturday, Rick Scott, the governor of Florida, spoke in a press conference of 15-foot storm surge, enough to submerge a one-story house. “Do not think the storm is over when the wind slows down,” he said. “The storm surge will rush in and it could kill you.”

“This is a storm of absolutely historic destructive potential. I ask everyone in the storm’s path to be vigilant and to heed all recommendations from government officials and law enforcement,” said President Donald Trump on Saturday.

“Irma has me sick to my stomach,” said Eric Blake, a scientist with the National Hurricane Center, on his personal Twitter account on Thursday evening. “This hurricane is as serious as any I have seen. No hype, just the hard facts. Take every lifesaving precaution you can.”