25 October 2016

"Major Hurricane" Amnesia in the U.S.

October 24th marked the 11-year anniversary of the last "major" hurricane to make landfall on the United States.  On the morning of October 24, 2005, Hurricane Wilma hit the southwest Florida peninsula as a Category 3 storm, and since then, no other hurricane Category 3 or stronger has made landfall on the U.S. -- a span that is unprecedented in the historical records.

Hurricanes are rated solely by the strongest wind speed found in the storm, not by size, rainfall, storm surge, fatalities, cost, or whether or not they make landfall. A major hurricane is conventionally defined to refer to a Category 3, 4, or 5 on the Saffir-Simpson Hurricane Wind Scale... those storms which from an engineering perspective will create "devastating" or "catastrophic" damage to buildings, trees, and infrastructure.  The term "major" is not a socio-economic label, it applies to hurricanes anywhere based on their peak winds.

Since Wilma, there have been 29 major hurricanes observed in the Atlantic, 11 of which made landfall as major hurricanes on other countries, including Mexico, Cuba, Bahamas, Haiti, Nicaragua, and Bermuda.  The U.S. has just gotten lucky to the point where 11 hurricane seasons have gone by and none of those 29 major hurricanes have crossed our shores.

Tracks of 27 of the 29 major hurricanes that have been observed in the Atlantic since Hurricane Wilma.  Hurricanes Gaston and Matthew from 2016 are not shown.
There have been close calls, most notably Ike in 2008, which made landfall near Galveston with peak wind speeds just shy of the Category 3 threshold.  Sandy in 2012 hit Cuba as a major hurricane, but by the time it made landfall on New Jersey, it was not technically a hurricane anymore, and even if it had been, it would have been a Category 1 storm. Recently, Matthew was a very close call... it hit Haiti and Cuba at Category 4 intensity, and was a Category 4 hurricane just off the Florida coast, but the eye and eyewall remained offshore, sparing coastal cities the worst of the storm's fury and avoiding an official landfall.  By the time it made landfall in South Carolina, it was a Category 1 storm.

A hurricane of any intensity is going to be destructive if it makes landfall somewhere.  By the time a tropical cyclone is organized enough to be classified as a hurricane, it is a significant storm capable of producing inundating storm surges and tremendous rainfall if it is near land.  It does not take a "major" hurricane to do those things -- Ike, Irene, Sandy, and Matthew prove that point well.  In fact, over 75% of all hurricane-related fatalities in the U.S. are caused by storm surge and rainfall, while just 8% are linked to wind.  This is why it is never a good idea to shrug off a hurricane because it's "only" a Category 1.  There's more to the story than the category! 
Fraction of tropical cyclone related fatalities in the U.S. caused by various factors from 1963-2012. (Rappaport 2014)
So why make a distinction between Category 1-2 hurricanes and Category 3-4-5 hurricanes?  Once sustained wind speeds reach over 110 mph, the damage caused by wind increases notably and dramatically.  Then, in addition to the water-based problems cause by storm surge and flash flooding, you also have widespread power loss, roof removal/damage, uprooted trees, etc.

If you have experienced the devastation caused by just the water-based facet of Category 1-2 hurricanes such as Ike, Irene, Sandy, and Matthew in the U.S., you can appreciate that a hurricane of any intensity means business.  But the stakes are increased even higher when the destructive wind speeds are added from a Category 3+ hurricane.

The 2016 season ends in five weeks, and something could still happen to end the U.S.'s major hurricane "drought", but the odds are historically very slim in November... so the record span will likely reach well into 2017.

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