A Better Way To Measure Seasonal Hurricane Activity

Most people like to gauge how active a given hurricane season is by the number of named storms, hurricanes and major hurricanes.  Those numbers may be misleading because a system could be designated a tropical storm (or hurricane) for a few hours and still be counted the same as a tropical storm (or hurricane) that lasts for several days.

A much more robust measure of total seasonal activity is something called the Accumulated Cyclone Energy (ACE) Index.  The ACE index is a wind energy index, and is defined by the National Oceanic and Atmospheric Administration (NOAA) as the sum of the squares of the maximum sustained surface wind speed (in knots) measured every six hours for all named systems while they are at least tropical storm strength.  The ACE index accounts for the intensity and duration of storms and hurricanes.

Yesterday’s Atlantic hurricane season outlook from Drs. Klotzbach and Gray called for an ACE of 185 (104 knots2).  This is nearly double the average ACE value from 1950-2000 which is 96 (104 knots2).

Last week’s Atlantic hurricane season outlook from NOAA estimated a 70% chance that the 2010 seasonal ACE range will be 155%-270% of the median.  The median in this case is simply the value of 1950-2000 ACE values below and above which there is an equal number of values.  According to NOAA, an ACE value above 117% of the median reflects an above-normal season.  If that is confusing, just know that NOAA considers an ACE value above 175% of median to reflect an exceptionally active or hyperactive season.

Fact:  The El Nino that, in part, decreased Atlantic hurricane activity in 2009 is transitioning to neutral conditions.  We will likely encounter neutral or La Nina conditions during the 2010 hurricane season.  There are typically more U.S. landfalls in neutral and La Nina years.

It would seem wise to take preparedness messages for this hurricane season seriously.


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