In October of 2005, Hurricane Wilma – at one point the strongest cyclone ever recorded in the Atlantic basin (see image below) – barreled across southern Florida with winds of 120 mph. Wilma sounded the last roar of one of the most active Atlantic hurricane seasons ever, one that included Hurricanes Rita and Katrina devastating the Central Gulf Coast within weeks of each other. Overall the 2005 season saw hurricanes amass a record $159 billion worth of damage, not to mention cost almost 4,000 lives. But we haven’t witnessed a storm as strong as Wilma make landfall in the US since.
It’s a lull of 3,700 days, more than a decade, that’s unprecedented in the historical record. President Obama has one more season to sweat out but he’s currently the first president since Benjamin Harrison not to have to deal with a major hurricane landfall. He only has to look back to his immediate predecessor to see how consequential a president’s response to a hurricane disaster can be to his legacy. Statistically speaking, we’re overdue for another major hurricane strike, but what does that really mean?
The average number of years between the recurrence of an event of a given magnitude is known as the “return period”. A prominent scientific paper by Francis Parisi and Robert Lund delved into the return periods of landfalling hurricanes in the US and came out with some fascinating results. Major hurricanes, cyclones rated category three or higher on the Saffir-Simpson scale, have a return period of about 2 years. It’s been more than 10 years since Wilma. Does this mean that the US is likely to be hit with a major hurricane next season? Not necessarily. Statistically, we are well overdue, but cold stats like that aren’t as predictive in the real world as are actual meteorological conditions. Every year, shifting climate cycles and weather patterns reliably predictable only a few weeks or days in advance combine to determine whether the Atlantic basin is capable of producing strong cyclones, and the steering of cyclones after they form. In the statistical sense, we’re actually no more likely to see a major hurricane make landfall next year than we were this year, or five years ago, or in 2005. This season was unusually quiet, as expected given the current El Nino, the strongest ever measured. However, the El Nino is expected to weaken steadily through 2016, perhaps even flipping to a La Nina by the Fall, a pattern much more favorable for an active Atlantic hurricane season. That fact much more than some abstract statistical measure like return period should be used to forecast the likelihood of our next major hurricane landfall.
Having such a long break between major hurricane landfalls is great for residents of hurricane prone areas, but it’s important to recognize the inherent dangers of such a lull. Many in the weather and emergency management communities are extremely worried about populations suffering from hurricane amnesia – that is, forgetting that they live in areas exposed to hurricanes. Coastal areas like Miami-Dade County are in a constant state of growth and development. Complacency can be as much of an enemy as a storm itself, so forgetting how vulnerable a city or state is, and not appreciating the historical danger, could have dire consequences when the next big storm strikes. Coastal residents can counteract this amnesia by making some basic preparations each hurricane season, for instance by knowing where to go in the event of an evacuation, and by having a disaster supply kit on hand.