After Harvey and Irma—and with Maria on the way—we look at why this hurricane season has been so active. This report by National Geographic.

Published 7 years ago, updated 5 years ago

PUBLISHED SEPTEMBER 19, 2017

By National Geographic

Just as Hurricane Harvey wrapped up its devastation of Houston, Irma got into line behind it and quickly built into the strongest Atlantic hurricane in recorded history. Now, the Category 5 storm Maria leaves behind widespread destruction on the Caribbean island of Dominica, and it’s now taking aim at Puerto Rico and the U.S. Virgin Islands. (Learn more about how hurricanes work.)

It’s hard to avoid comparisons to the last time two such powerful storms threatened U.S. landfall in the catastrophic 2005 hurricane season, 12 years ago.

As in 2005, when Katrina and Rita devastated the Gulf Coast in rapid succession, the country is staring down the barrel of multiple hurricanes making landfall. In the face of multiple major storms, a reasonable person might wonder why this season seems worse for U.S. cities, and why the last dozen years brought fewer large hurricanes to U.S. shores.

If you have a question about this hurricane season compared with recent years, we’ve got you covered:

HOW ACTIVE WAS 2017’S HURRICANE SEASON FORECAST TO BE?

Above average. The U.S. National Oceanic and Atmospheric Administration (NOAA), Colorado State University, and the Weather Channel all estimated that this year, we’d likely see more hurricanes than usual spawning in the Atlantic Ocean. NOAA’s Climate Prediction Center forecasted that we’d see between 14 to 19 named storms and five to nine hurricanes this season.

In comparison, the average hurricane season from 1981 to 2010 featured 12 named storms and six hurricanes.

WHY IS THIS SEASON SO ACTIVE?

In short: atmospheric conditions were hurricane-friendly, and surface sea temperatures were warmer than usual. The Climate Prediction Center says that multiple conditions, such as a strong west African monsoon, have aligned to make the Caribbean Sea and part of the tropical Atlantic—a storm-spawning area called the “Main Development Region”—particularly well-suited to hurricanes.

Kerry Emanuel, an atmospheric scientist at MIT who studies hurricanes, says that two factors stand out. For one, there’s currently little difference in wind speeds near the surface and those roughly 10 miles up, which ensures that miles-tall hurricanes can form and remain stable. What’s more, the tropical Atlantic is exhibiting high “thermal potential,” meaning that water can rapidly evaporate into the atmosphere.

“[Thermal potential] is a thermodynamic speed limit on hurricanes,” Emanuel says. “The greater the speed limit, the more favorable conditions are for hurricanes to form, and the more powerful they can get.”

What’s more, El Niño is stuck in neutral this year, improving Atlantic hurricanes’ prospects. When this warming of the equatorial Pacific is active, there tends to be more wind shear and less thermal potential over the Atlantic, hurting hurricanes’ chances of survival. (See the latest pictures of Hurricane Irma as the historic storm makes landfall.)

IT FEELS LIKE IT HAS BEEN A LONG TIME SINCE WE’VE HAD A YEAR LIKE THIS. WAS THERE REALLY A “DROUGHT” OF HURRICANES FOR MORE THAN A DECADE?

The phrase “landfall drought” refers to the fact that before Harvey, it had been nearly 12 years since a hurricane rated Category 3 or above-made landfall in the U.S., going back to Hurricane Wilma in 2005.

Depending on how you define a Category 3 hurricane, this stretch had been the longest since at least 1900.

WHAT CAUSED THE DROUGHT?

Largely, it’s an artifact of how we measure hurricanes. As Hart and colleagues demonstrated in a 2016 study, if you slightly tweak the definitions of hurricane categories, the “drought” mostly vanishes.

It’s worth noting that the category system, which bins hurricanes by their maximum wind speeds, is just one way to measure hurricane intensity. By loss of life or economic toll, there’s been less of a drought to speak of.

“Tell the folks who survived [2007’s Category 2] Hurricane Ike that that wasn’t a major hurricane—it destroyed a large part of the Texas coastline,” says Emanuel. “Tell folks that Sandy wasn’t a major event… and it wasn’t even a hurricane.”

ALL THAT SAID, IS THIS SEASON UNUSUAL?

The longer it goes, the more severe it seems to get.

For starters, Irma made landfall in the Florida Keys as a Category 4 hurricane—the second Category 4 storm to make landfall on the continental U.S. this year. Such a vicious one-two punch hasn’t hit the U.S. in over a century, though 1954 came close, says Florida State University meteorologist Robert Hart. That year, the Category 4 Hurricane Hazel devastated the Carolinas, and two Category 3 hurricanes just missed landfall.

But focusing on only the continental U.S. obscures the utter devastation that this hurricane season has brought to the Caribbean. Irma effectively wiped out civilization on the island of Barbuda, which had been continually inhabited for 300 years. The U.S. and British Virgin Islands have suffered horrific damage. Puerto Rico faced massive power outages in the wake of Irma.

Now, in less than a day, Maria has intensified from Category 1 to Category 5, battering Dominica at full strength, with Puerto Rico still in its sights. According to meteorologist Eric Holthaus, no Category 5 hurricane has struck Dominica since at least 1851—and no Category 5 storm has made landfall in Puerto Rico since 1928.

“There are no words left to describe this season,” Holthaus said on Twitter hours before Maria slammed into Dominica.

DOES A MORE ACTIVE SEASON MEAN THAT MORE HURRICANES WILL HIT LAND?

Not quite. Forecasters caution that within a single year, there’s no solid relationship between the number of storms in a hurricane season and the number of landfalls. That’s because local weather conditions govern hurricanes’ specific paths to landfall—and forecasters can estimate those only a few days in advance.

That said, Emanuel notes that on 100-year timescales, the number of hurricanes and number of landfalls correlates. However, he emphasizes the year-to-year role of chance.

“Andrew, which occurred in 1992, was at the time the most expensive hurricane ever to hit the U.S., [and] that occurred in one of the quietest years we’ve seen the Atlantic, as a whole,” he says.

SO DO SEASONAL HURRICANE FORECASTS REALLY MEAN ANYTHING?

Yes, but they’re more useful to forecasters than the public, says Emanuel. He laments that people sometimes base key decisions, such as whether to purchase insurance, on forecasts calling for “quiet” seasons—despite the fact that even seasons with few hurricanes can yield highly destructive storms, such as 1992’s Hurricane Andrew. “The seasonal forecast is so widely misinterpreted that it’s actually counterproductive,” he says.

“While the seasonal forecasts are useful across a broad range of demographics, one can’t simply and solely determine their own personal preparation based on those seasonal forecasts,” adds Hart, who also points to Andrew’s example.

Forecasts predicting landfall for a current hurricane, however, are another matter.

“People who are potentially in the path of a hurricane really need to pay attention and absolutely need to follow direction of emergency managers,” Emanuel says. “If you’re told to get out, get out—don’t mess around.”

HOW DOES CLIMATE CHANGE FIGURE INTO THE PICTURE?

It’s complicated, but there’s a reason to think that a changing climate will have at least some impact on hurricane season activity. (Discover how climate change likely strengthened Hurricane Harvey.)

“When it comes to next year’s hurricane activity seasonal prediction, anthropogenic impacts are not a primary concern, as the change due to anthropogenic effects from this year to next year is obviously small,” says Hart. “However, in the decades and century to come, it could easily become the primary concern for driving hurricane activity.”

The sweeping U.S. Climate Science Special Report prepared ahead of the U.S. government’s 2018 National Climate Assessment, says that detecting climate change’s fingerprints on hurricane behavior is challenging. Because hurricanes are rare events, there aren’t many data points for scientists to examine for a trend.

That said, in coming decades, predictions based on warming suggest that average-intensity tropical cyclones—Atlantic hurricanes included—will likely get more intense. Emanuel adds that there’s “pretty good consensus” that high-intensity (Category 3, 4, or 5) hurricanes will also become more common in coming decades.

It’s unclear whether the total number of hurricanes will increase or decrease. More than 70 percent of tropical cyclones worldwide are Category 1 or 2 storms, and these weaker storms may or may not become less common in future decades.

Emanuel and the report both say that on average, individual hurricanes will drop more precipitation in the future, since warmer air can hold more water vapor. We are likely beginning to see this act out today: Every scientist contacted by National Geographic for a previous story agreed that Hurricane Harvey’s record-breaking rain was almost certainly shaped by rising temperatures from human activity.

Future storm surges may also worsen, says Emanuel—partly because the intense hurricanes that cause them will be more numerous, and partly because of sea level rise.