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01 Dec 15 15:09

November 30 officially marked the close of the 2015 Atlantic hurricane season, and for the 10th year in a row, the US escaped without a major hurricane visiting its shores.  Thanks to the on-going El Nino, which suppresses hurricane formation in the Atlantic by increasing wind shear, this year's hurricane season was relatively quiet and benign. Hurricane Joaquin, the strongest hurricane of the season and the strongest observed in the Atlantic since Hurricane Igor in 2010, provided a little jolt of concern when the Global Forecast System (GFS), the American weather model, consistently showed Joaquin taking a Sandy-like left turn into the mid-Atlantic region for several consecutive model runs. Fortunately, Joaquin took a turn out into the open Atlantic, after causing significant impacts in the less populated areas of the Bahamas.  The final statistics for the season, including Joaquin, are 11 named storms, 4 hurricanes and 2 major hurricanes, which is slightly below the normal activity levels expected during a year with warmer than average water in the Atlantic.

The US has never before gone for such a long stretch without a major hurricane landfall. The last major hurricane to make landfall in the US was Hurricane Wilma in October 2005, which struck Florida as a category 3 hurricane (it's worth noting that this is also the 10th year where Florida hasn't been hit by a hurricane; this is unprecedented in the historical record). According to the official historical hurricane record, HURDAT, the next longest gap between major hurricane landfalls occurred between 1861 and 1868.  Clearly, this statistic is suspect, since the US was embroiled in the Civil War, and the most hurricane-exposed states had seceded from the Union and joined the Confederacy. 

Between 1900 and 2005, there are 49 years where at least one major hurricane has struck the US.  Therefore, using historical statistics alone, there is a 46% probability of a major hurricane making landfall somewhere along the US coast in any given year. During warm years of the Atlantic Multidecadal Oscillation (AMO), when the conditions in the Atlantic are more favorable for major hurricane development, as they have been since 1995, this probability jumps to 53%. The probability of having no major hurricane landfalls in an AMO warm year is then 100% - 53%, or 47%. The probability of having a decade pass without a major hurricane landfall during an AMO warm phase is thus remarkably low; 0.05%. 

While the current stretch of no landfalls might seem beneficial, the result of a decade of dormancy can lead to the incorrect perception that the risk of a US major hurricane landfall has decreased or dissipated. This assumption couldn't be more incorrect; numerous academic studies have showed that major hurricanes are forming, they've simply just not hit the US, and have impacted Mexico and the Caribbean instead. There's no apparent reason for the lack of US landfalls, other than fortuitous steering currents and serendipity. 

Eventually, our luck will run out, and the US will experience a major hurricane landfall once again. The year could be 2016; with this year's El Niño expected to run its course by spring of next year, Atlantic hurricane activity could return to near normal or above normal levels. The recent past should not be used as a metric of future US hurricane risk.

Category: Climate/natural disasters: Disaster risk, Floods/storms, Resilience

Location: United States

1 Comment

Monica Ningen - 2 Dec 2015, 1:12 p.m.!

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