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Deadly late-season Atlantic hurricanes growing more frequent

By: Dr. Jeff Masters, 1:40 PM GMT on November 17, 2010

What may be the final tropical disturbance in the Atlantic (94L) has fizzled, due to dry air and increasing wind shear, and there's an excellent chance that this will be final "Invest" of 2010 in the Caribbean. The 2-week wind shear forecast from the GFS model is showing a dramatic increase in wind shear over the Caribbean in the coming weeks, which should put an end to the Caribbean hurricane season. However, it's been another remarkably active November in the tropics this year. The formation of Hurricane Tomas this month marks the fourth consecutive year in the Atlantic with a hurricane occurring November 1 or later. We had Category 1 Hurricane Noel in 2007, Category 4 Hurricane Paloma in 2008, Category 2 Hurricane Ida in 2009, and now Category 2 Tomas in 2010. This is the first time since beginning of reliable hurricane records in 1851 that there have been four consecutive years with a late-season November or December hurricane in the Atlantic. The previous record was three straight years, set in 1984 - 1986. It used to be that late-season hurricanes were a relative rarity--in the 140-year period from 1851 - 1990, only 30 hurricanes existed in the Atlantic on or after November 1, an average of one late-season hurricane every five years. Only four major Category 3 or stronger late-season hurricanes occurred in those 140 years, and only three Caribbean hurricanes. But in the past twenty years, late-season hurricanes have become 3.5 times more frequent--there have been fifteen late-season hurricanes, and five of those occurred in the Caribbean. Three of these were major hurricanes, and were the three strongest late-season hurricanes on record--Lenny of 1999 (155 mph winds), Paloma of 2008 (145 mph winds), and Michelle of 2001 (140 mph winds). Of course, the number of storms we are talking about is small, and one cannot say anything scientifically significant about late-season Atlantic tropical cyclone numbers, unless we include storms from late October as well. This was done, though, by Dr. Jim Kossin of the University of Wisconsin, who published a 2008 paper in Geophysical Research Letters titled, "Is the North Atlantic hurricane season getting longer?" He concluded that yes, there is an "apparent tendency toward more common early- and late-season storms that correlates with warming Sea Surface Temperature but the uncertainty in these relationships is high". The recent increase in powerful and deadly November hurricanes would seem to support this conclusion.

Figure 1. Damage on St. Lucia from Hurricane Tomas. Image credit: St. Lucia Star.

Deadly late-season Atlantic tropical cyclones are growing more frequent
In the 500+ years people have been encountering hurricanes in the Atlantic and recording these encounters, we are sure of only twenty November and December storms that have caused loss of life. If we restrict our time window to the past 70 years, when we have a fairly reliable data base of hurricane mortality thanks to the yearly storm summaries published in Monthly Weather Review, we find only ten late-season storms that killed people. Of those ten, seven occurred in the past twenty years, including the second deadliest late-season tropical cyclone on record, Hurricane Gordon of 1994, which killed 1145 people on Haiti. Only the Great Cuba Hurricane of 1932, which killed 3107 people in Cuba, the Cayman Islands, and the Bahamas, was a deadlier late-season tropical cyclone. Hurricane Tomas ranks as the 6th deadliest late-season hurricane since 1851 in the Atlantic. The storm left 30 dead or missing on Haiti, at least nine dead on St.Lucia, two on Curacao, and one in the Virgin Islands. The storm has caused hundreds more indirect cholera deaths in Haiti, by spreading contaminated water.

Figure 2. Number of days a named storm existed in the Atlantic during November and December between 1950 and 2010. Years when an El Niño event occurred are not included, in order to make the plot smoother (El Niño events tend to dampen Atlantic hurricane activity during all portions of the season.) There has been a general increase in late-season tropical cyclone activity over the Atlantic in recent decades.

The increase in deadly late-season storms in the past twenty years is primarily a Caribbean phenomena. Only two deadly late season hurricanes have affected the U.S. in the past century: Hurricane Kate of 1985, which killed five in Florida, and the 1925 Florida Hurricane, which hit southwest Florida as a Category 1 hurricane on December 1. This remarkable storm was the latest landfalling hurricane in U.S. history, and killed 60 people.

Kossin, J., 2008, "Is the North Atlantic hurricane season getting longer?", Geophysical Research Letters, Vol. 35, L23705, doi:10.1029/2008GL036012, 2008.

A list of the 20 deadly late-season tropical cyclones: http://www.wunderground.com/hurricane/deadly_nove mber.asp

My blog post, Is the Atlantic hurricane season getting longer?

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Jeff Masters

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