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May-June 2010

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The 2009 Atlantic Hurricane Season: A Slow Summer, Quiet Fall

The 2009 Atlantic hurricane season was marked by below-average tropical cyclone activity with the formation of nine tropical storms and three hurricanes, the lowest numbers since the 1997 Atlantic hurricane season. Two of the hurricanes strengthened into major hurricanes, or Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale. The numbers of tropical storms and hurricanes were below the long-term averages of 11 and six, respectively, although the number of major hurricanes equaled the long-term (1966 to the present) average of two. In terms of the accumulated cyclone energy (ACE) index,1 2009 had 60 percent of the long-term median ACE, also the lowest value since 1997. In addition, Tropical Storm Ana's formation on August 11 marked the latest formation of a tropical storm in a season since 1983, when Hurricane Alicia formed on August 15. The below-normal activity appears to have been the result of strong vertical wind shear and large-scale sinking in the tropical atmosphere, associated with the development of El Niño during the summer months.

ROBBIE BERG are hurricane specialists at the National Hurricane Center (NHC) in Miami, Florida. The cyclone summaries are based on Tropical Cyclone Reports prepared by the authors and other specialists at NHC, including Jack Beven, Eric Blake, Michael Brennan, Daniel Brown, John Cangialosi, Todd Kimberlain, and Richard Pasch. The reports are available online at http://www.nhc.noaa.gov/2009atlan.shtml.

LIXION AVILA are hurricane specialists at the National Hurricane Center (NHC) in Miami, Florida. The cyclone summaries are based on Tropical Cyclone Reports prepared by the authors and other specialists at NHC, including Jack Beven, Eric Blake, Michael Brennan, Daniel Brown, John Cangialosi, Todd Kimberlain, and Richard Pasch. The reports are available online at http://www.nhc.noaa.gov/2009atlan.shtml.

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