„You need a perfect recipe to get a storm like Irma.“
Hurricane Irma is an absolute monster of a storm. Its powerful winds have kept it a Category 5 (the highest on the Saffir-Simpson wind scale) for more than two days, which makes it one of the most powerful and longest lasting Category 5 storms on record. And with two and a half days until a possible Florida landfall, it doesn’ t seem to be losing steam.
As of Thursday afternoon, Irma was blowing sustained 175 mph winds. “That’s similar to a tornado, except this tornado is 80 miles wide, ” says Jeff Weber, a meteorologist with the National Center for Atmospheric Research.
Much of South Florida is now under a hurricane and storm-surge watch. That means the National Hurricane Center believes dangerous winds, rains, and coastal flooding could arrive by Sunday. The greatest danger in a hurricane is usually flooding from storm surge. The hurricane center predicts 5 to 10 feet of surge will be from the Jupiter Inlet (just north of West Palm Beach) on the Atlantic Ocean, through the Florida Keys in the Gulf — an area that spans hundreds of miles. And impacts in Georgia and Carolina are also a possibility.
Irma is expected to maintain a Category 4 or 5 status for the remainder of its path toward Florida, through Turks and Caicos and the Bahamas.
We’ re witnessing a truly extreme event. But as Weber and Phil Klotzbach, an atmospheric scientist at Colorado State University, explain, this is the exact time of year you’ d expect the most powerful storms to form. It’s also not that surprising to see multiple tropical cyclones forming at once, as we’ re seeing with Hurricanes Jose and Katia, which could also make landfall this weekend.
But for a monster like Irma, all the conditions — that are actually quite common for this time of year — have lined up. “You need a perfect recipe to get a storm like Irma, ” Klotzbach says.
And we have it.
The strength of a hurricane is determined by three main factors: water temperature, wind shear, and moisture in the atmosphere. Warmer water and atmospheric moisture give the system energy. A low wind shear — i.e., sharp changes in wind directions as you go higher and higher in the atmosphere — keeps a hurricane from dissipating.
All these conditions have been ideal for Irma. As for the wind shear, Klotzbach says there’s “almost zero.” That’s just bad luck. “If you plot shear averages over the Atlantic over the past 30 years, there’s really no trend, ” he says.
The Atlantic, meanwhile, reaches its hottest temperatures for the year in September (that’s why the peak of hurricane season is September 10) . Klotzbach says the Atlantic is a degree or two Fahrenheit hotter than it usually is this time of year, which is providing Irma with some extra fuel. (The surface area of water in the Atlantic topping 82.4 degrees in September has been growing slightly since the 1970s, which also could contribute to Irma’s strength.)
Irma reached a top speed of 185 mph Tuesday, and maintained those winds for 37 hours. That’s “close to the maximum intensity you’ d ever get out of an Atlantic hurricane for that long, ” Klotzbach says.
What’s more, Irma only is going to encounter more hot water — approaching 90 degrees Fahrenheit — as it moves toward Florida. (The Gulf of Mexico, on the other hand, has been about 4 degrees above normal, which helped Harvey intensify so rapidly.) And, according to Klotzbach, “the atmosphere is more saturated than normal too.” So there’s plenty of energy and moisture to keep this system spinning quickly.
Also fortuitous for Irma: It hasn’ t made much direct contact with land. Yes, it devastated several islands in the Eastern Caribbean. But small islands “don’ t create dry air to entrain into the system to slow it down, ” Weber said. Irma’s eye has also traveled just North of islands like Puerto Rico and Hispaniola, which has allowed it to keep up its momentum. “It’s a perfect path [meteorologically speaking] through the islands, and riding north of Hispaniola and north of Cuba the whole way, ” he adds. Irma is following a runway of hot water all the way to Florida.
Within the past two days, two other storms — Katia and Jose — have achieved hurricane status in the Atlantic and Gulf. And what are we to make of the fact that three hurricanes are currently active at the same time?
This also isn’ t unprecedented. It happens on average once every 10 years, Klotzbach explains. “In the satellite era, since 1966, this has happened in: 1967,1980,1995,1998,2010 and now in 2017.” So it’s fairly common, especially near the peak of the season, like we are in now.
This is always a tricky question when it comes to hurricanes. These storms are powerful, but they’ re not necessarily much more powerful than storms we’ ve seen in the past. Also, meteorologists have only been tracking hurricanes by satellite since the 1970s. Hurricanes have ravaged the Atlantic coast for hundreds of thousands of years. Scientists are still figuring out what, exactly, is “normal.”
But climate change does likely play a role, albeit a subtle one.
Vox’s Dave Roberts explained it best in a piece about Hurricane Harvey:
Climate change science predicts that in the future, hurricanes will intensify. There will be simply more energy available for them to form, and more moisture in the air to sustain them. Both Klotzbach and Weber agree the current consensus on whether hurricanes today are more intense due to climate change is messy.
But we don’ t need the threat of climate change to know that hurricanes are a dangerous threat to South Florida and the surrounding area. We know this because hurricanes have ravaged Miami before. (If anything, it’s weird that the United States hasn’ t more hurricanes in the past 10 years, as the Washington Post explains .)
In the 1940s and into the’ 50s, “South Florida was hit by five Category 4 or 5 hurricanes in six years, ” Klotzbach says. “Just what’s happened in the past should be scary enough to realize this stuff can be bad.”
But yet the South Florida coastline has only grown denser, reaching a population of 6 million for the first time in 2016.
In 1926, Miami was hit with a storm that was likely a Category 4, and it devastated the young city, killing 375. If that same storm struck today, Brad Plumer explains at the New York Times, it “would inflict more than $200 billion in damage.
