When Will The Next Big Hurricane Hit The U.S.?

I remember the coverage of Hurricane Katrina very clearly. I woke up and immediately went to watch the news, and Katrina was just coming in. We knew it would cause a lot of damage, but I don’t think anyone knew that it would cause quite as much as it did. In the weeks afterward, the costliest natural disaster in American history dominated the headlines.

I have a special reminder of Katrina every year: The hurricane made landfall in New Orleans on my birthday. This connection makes me think of the power of hurricanes annually- and they always fascinate me.

We like to think that we’re above the power of nature. We’ve climbed to the top of the world, and explored the depths of the sea. We’ve built buildings that are ridiculously high- it’s almost like we’re mocking nature, pretending to be invulnerable. And then, something like Katrina happens, be it a hurricane, earthquake, tsunami, anything; and it sends us crashing- sometimes literally- back to earth.

This year, I had another reason to think of hurricanes. On my birthday this year, another hurricane- Hurricane Isaac- made landfall in New Orleans. Thankfully, the damage caused by Isaac was nowhere close to the damage caused by Katrina; however, it was a reminder: at any time, there could be another hurricane just as strong as Katrina. So is there a way to predict when the next “big one” will hit?

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The attribute that makes natural disasters so powerful is their unpredictability. Volcanoes may give a few weeks’ at max. Hurricanes form and make landfall within a couple of days. We’ve vastly improved our tornado warning system; now we have nearly fifteen minutes of notice! And earthquakes- well, good luck. The 2005 hurricane season was especially unpredictable- we’ll get to that in a little bit. But the variability of hurricanes is what makes predicting the next powerful one especially difficult.

I could use all Atlantic hurricanes as my data set for this project, but I have neither the time nor the patience for that. Instead, I’m looking at only hurricanes who have been powerful enough to have their names retired:

There have been 77 retired names since 1954. One, Gracie (1959), is listed as retired by some sources but not others. Another, Allison (2001), was retired despite never actually becoming a hurricane.

There are a lot of extremely strong hurricanes in this set, but there’s still a very large number (there’s even a tropical storm!).  I’m going to narrow it down even further, to hurricanes that caused most of their damage to the United States. This eliminates hurricanes like Mitch in 1998, but I’m investigating when the next big hurricane will hit the U.S., not the rest of the Atlantic.

42 storms are represented above. I assigned a score to each hurricane based on each of the statistics on the right (scores not shown); Katrina (2005) was easily the most extreme hurricane ever to hit the United States.

Now we can begin looking for trends. Hurricanes overall appear to be getting stronger and it looks like there are more that have been retired in recent years, but…

R² =0.07818

That’s not a very conclusive regression line, and logarithmic or exponential lines don’t really fit either. While the numbers from 2003-2005 are eye-catching (12 hurricane names retired in three years!), 2006 and 2009 did not have any hurricanes retired, and neither hurricane on that graph from 2010 made landfall in the U.S..

On the other hand, there has been a small increase in total named storms over the years:

http://upload.wikimedia.org/wikipedia/en/timeline/61b5be0856ccb449ab4978b2909ae8d7.png

There might be a bit of a cycle going on- in recent years, a two to four year cycle of powerful hurricane seasons appears. 2008 had several strong hurricanes three years after 2005, and 2009 had none after the weak 2006 season. But as we look back (and forward- just Irene in 2011), we can see that this theory doesn’t hold.

How about ENSO? We covered this phenomenon in depth a few months back, and it would make sense. El Nino supposedly represses hurricane growth, and our data supports that. Unusually strong El Nino effects match up with unusually weak hurricane season.  The reverse of El Nino, La Nina, would then spur hurricane growth, right? Not so fast- the data on hurricane seasons that correspond with La Nina doesn’t provide a strong correlation either way.

The key to this problem would appear to be the 2005 hurricane season. It was unusual in many ways. By all measures, it was the strongest hurricane season ever. Hurricane Emily was the earliest category 5 storm ever. Vince formed father northeast (into cooler waters) than any other storm on record. Wilma strengthened ridiculously fast after its formation. Hurricane Epsilon was the latest hurricanes ever, lasting deep into December. Tropical Storm Zeta went all the way into January. And yes; those are Greek letters, meaning that the 21 letters from the English alphabet were exhausted- the only time this has ever happened. In all, the 2005 season produced 31 tropical depressions, 27 named storms, and 15 hurricanes (7 of category 3 or above). It accounted for 3,913 deaths and over $150 billion of damage.

I can find just two unusual aspects concerning the climate in 2005. The first is that El Nino was expected to develop, but didn’t. The second is that the years leading up to it- 2002-2005- were four of the five warmest years on record (at the time). Fourteen hurricane names were retired for these four years, and the one other year in the top five was 1998, which saw the wraths of Hurricanes Mitch and Georges. Did the heat finally reach a climax in 2005, causing the extreme hurricane season? Maybe, but more likely not. In the Pacific, 2005 was an average or below average year. (It should be noted that 2006 was the most active Pacific season since 2000, however.) Additionally, many of the years since 2005 have been warmer, and yet none of them had hurricane seasons quite as strong.

Overall, I just can’t find a trend for powerful storm season. Hurricanes are simply so unpredictable that a storm season predicted to be “slightly above-average” can produce 28 named storms; the opposite can be true as well. We don’t know when, specifically, in a season the next big hurricane will hit, or even what year it will be.* We can make guesses, but nature will laugh at us and prove us wrong anyway. The best we can do is prepare for the worst and try to minimize the damage and loss of life.

*In 2011, Colorado State University, one of the leading hurricane prediction centers, announced that it would no longer be releasing quantitative forecasts six months prior to each season, as  "...forecasts of the last 20 years have not shown real-time forecast skill." Hurricanes are unpredictable even for the professionals from a long distance away.