Hurricane Drought Over? Not So Fast

After a relatively quiet start, the 2016 Atlantic hurricane season grabbed the attention of the insurance industry during September and October. On October 6, all eyes fixed on Hurricane Matthew, a Category 4 storm barreling towards Florida and presenting the greatest threat to the U.S. insurance industry since Sandy in 2012. Despite Matthew’s high winds and floods, caused by both storm surge and rainfall, the storm’s offshore track certainly spared Florida and the southeast U.S. from a potential worst-case scenario.

Matthew was preceded by Hurricane Hermine, a Category 1 storm that made landfall along Florida’s Panhandle on September 2. These storms ended a nearly 11-year period where no hurricanes affected the state of Florida.

While the soft insurance market is expected to weather the effects of Hermine and Matthew, modelers at RMS are investigating whether these events provide valuable clues about future, near-term Atlantic hurricane frequency. Did either of these storms finally end the United States’ well-publicized hurricane drought?

It’s The Major Storms That Matter

The term “hurricane drought” first appeared in Geophysical Research Letters in research by Tim Hall and Kelly Hereid, and is defined as a lack of major hurricanes, ranking as Category 3 or greater on the Saffir-Simpson Hurricane Wind Scale, making landfall on the U.S.

Based on this definition, which itself ignited academic debate and drew ire from some meteorologists – the drought continues. Although a Category 4 hurricane on its approach to Florida, Matthew did not officially make landfall in the U.S. until striking South Carolina at Category 1 intensity.

Scientists agree that the Atlantic Basin entered a prolonged period of above-average hurricane frequency in 1995. To insurers, this translates into a period of increased likelihood of elevated damages and loss. As the drought stretched into record territory, scientists and insurers alike wondered whether this era had come to a close. Colorado State University’s Phil Klotzbach points out that it’s the major hurricanes, those driving the drought, that could provide us with the first clues.

Annual frequency of Atlantic Basin and U.S. landfalling major (category 3-5) hurricanes over known periods of high frequency (1926-1969, 1995-2011) and low frequency (1900-1925, 1970-1994).

An average of 2.7 major hurricanes have formed each year in the Atlantic Basin since 1950. With three major hurricanes – Gaston, Matthew, and Nicole – the 2016 season was the first since 2011 to exceed this average. But the four preceding seasons featured below-average major hurricane activity. You have to go back to the last low period of hurricane activity, around a quarter of a century ago, to see such a run of quiet years.

Annual counts of Atlantic Basin major (category 3-5) hurricanes since 1995. Dashed lines represent the 1950-2015 average (black, 2.7 hurricanes), the 1995-2011 average (red, 3.8 hurricanes), and the 2012-2016 average (blue, 1.8 hurricanes). The 2016 season is inclusive of all activity up to the end of October.

The Most Intriguing Statistic

This four year period is important to RMS modelers monitoring the medium-term rate (MTR), our scientific reference view of hurricane landfall frequency, looking ahead five years. It is the product of 13 individual forecast models; the contribution of each of those models is weighted according to its ability to predict the historical fluctuations in activity.

These forecast models include “shift” models that support the theory of cyclical Atlantic hurricane frequency. These shift models identify the seasons since 2011 as statistically distinct from periods observed since 1950 which are acknowledged as more active, based on the lack of recent major hurricanes.

It Ain’t Over ‘Til It’s Over

One month remains in the current season, so it is possible that more hurricanes could inform our view of the bigger picture.

The final month of the hurricane season, November has produced some notable Atlantic hurricanes, proving that the season’s latter stage requires close observation. Our attention to cyclogenesis turns primarily to the Gulf of Mexico and Caribbean, as evidenced by these past events:

  • Hurricane Kate, the latest landfalling U.S. hurricane in modern history, peaked in intensity as a Category 3 storm before weakening ahead of its landfall near Mexico Beach, Florida on November 21, 1985.
  • Hurricane Lenny, a Category 4 hurricane forming in 1999, is best known for its “wrong way” track that crossed the Caribbean Sea from west to east.
  • Hurricane Michelle, the most intense hurricane of the 2001 season, made landfall in western Cuba as a Category 4 storm on November 4, causing over $2 billion in economic damage.
  • Hurricane Paloma, following a track just to the east of Michelle, reached Category 4 strength at the height of its lifecycle, later impacting the Cayman Islands and Cuba in November 2008.

Shift models inform only one driver of activity considered by the MTR methodology. RMS plans to publish the findings of its annual medium-term rate forecast review in early 2017, after considering the most recent activity and other drivers of near-term hurricane behavior. This forecast will contribute to the updated RMS view of hurricane risk, forthcoming in spring 2017 as part of the version 17 software release.

Senior Product Manager, Model Product Management
Tom is a Senior Product Manager in the Model Product Management team, focusing on the North Atlantic Hurricane Model suite of products. He joined RMS in 2009 and spent several years in the Client Support Services organization, primarily providing specialist peril model support. Tom joined RMS upon completion of his B.S. and M.S. degrees in meteorology from The Pennsylvania State University, where he studied the statistical influence of climate state variables on tropical cyclone frequency. He is a member of the American Meteorological Society (AMS).

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