Hurricane Harvey, which killed 60 people and may end up costing $150 billion, parked over Houston and dumped four feet of rain. The water overwhelmed the sprawling city’s flood control systems. Meteorologists and atmospheric scientists used up their superlatives describing the storm’s size and impact.

They should have saved some.

Hurricane Irma has become the most powerful Atlantic hurricane on record, category 5 on the Saffir-Simpson scale—over 800 miles wide, roughly the size of Texas, sustained winds of over 185 miles per hour for more than 24 hours, gusts over 200 mph—and it has made landfall in the Caribbean. Irma’s storm track, the predicted line of its travel, projects its eye gliding north of the islands of Hispaniola and Cuba starting Thursday, zooming up Florida to Miami late Sunday, and then reaching Georgia and South Carolina the next day.

As Irma grew and developed, it brushed up against its theoretical maximum intensity.

All hurricanes have a theoretical maximum intensity, a thermodynamic limit on how fast their winds can blow given ocean temperature and atmospheric temperature. Few hurricanes ever actually reach that limit. But as Irma grew and developed, it came very, very close. If Harvey was a perfect storm, Irma is an almost impossible one. “Irma is anomalous,” says Jim Kossin, an atmospheric scientist with the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information. “This is a record-breaker. Unprecedented. Catastrophic.”

How did Irma get so powerful? Well.

“Irma had everything going for it,” says Kerry Emanuel, an atmospheric scientist at MIT who developed the theory behind that theoretical maximum. “The water was warm, the layer of warm water was deep, and there was almost no wind shear, which tends to be very destructive to hurricanes. It can live up to its potential, if you will.”

The most efficient hurricanes stretch from the ocean up to to the bottom of the stratosphere, between 50,000 and 60,000 feet in the tropics. That vertical column lowers the air pressure and the storm gets more powerful. Wind shear knocks down the column, but so far Irma hasn’t run into much.

Will it? That’s tough to predict. Average conditions, as Kossin has written, would predict higher wind shear as Irma approached Florida. But right now the water is warm, and surface temperature doesn’t vary quickly; it’s safe to say Irma will keep that fuel at its feet for some time. “That thermodynamic speed limit in the straits of Florida right now is ridiculously high, a frightening prospect,” Kossin says—maybe more than 200 mph by some calculations. “If a storm spins through there, in the absence of shear it can get really strong.”

Other possibilities could rein Irma in. Direct hits on the variegated topography of Hispaniola and Cuba, for example, might be disastrous for the islands but could mellow the storm. Irma might even behave in a way that lessens its own impact. “If it moves slowly it could churn up the water, actually cooling the water beneath itself, so it has a self-regulating feature,” Kossin says. “We don’t necessarily expect that to happen.”

Irma's consequences could be enormous. Already the small island of Barbuda had at least one death and lost 90 percent of its built structures. Other islands have had at least eight more deaths. Puerto Rico’s electrical power authority is predicting total loss of power for up to six months.

And Florida? Miami has been trying to fight back rising sea level—no storm necessary—for years. Like Houston, it’s a sprawling coastal city with lots of development and lots of people up against the water. Here’s where all the superlatives describing Irma may fall short: The problem for Miami might well be the high winds that the Saffir-Simpson scale measures, because buildings there are meant to withstand 185-mph gusts but not the possible 200-mph blowouts meteorologists are worried about. (No, that wouldn’t make Irma a “category 6” hurricane, because there is no such thing.)