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HOME > Expert Assessments > Atlantic Hurricane Outlook > Background Information

The North Atlantic hurricane season officially runs from 1 June to 30 November. During this period the average number of systems reaching tropical storm status (maximum sustained winds between 39-73 mph), hurricane status (maximum sustained winds of at least 74 mph) and major (or intense) hurricane status (maximum sustained winds exceeding 110 mph, categories 3-5 on the Saffir-Simpson scale) are ten, six, and two, respectively.   However, the vast majority of tropical storm and hurricane activity typically occurs during the August-October period, which is considered the peak of the hurricane season.

Measuring overall activity

The term "overall activity" reflects a combination of intensity and duration of Atlantic tropical storms and hurricanes during a given season. One such measure of seasonal activity is the Accumulated Cyclone Energy (ACE) index, which is calculated as the sum of the squares of the estimated 6-hourly maximum sustained wind speed (in units of knots) for all systems while they are at least tropical storm strength. This index is a robust measure of overall activity and does not suffer the shortcomings inherent in attempting to determine the relative activity of a particular season from the number of tropical storms or hurricanes alone. This index is a slight modification of the Hurricane Destruction Potential (HDP) index used by Dr. William Gray of the Colorado State University, in that the ACE index also includes systems while at tropical storm strength. This index is also highly correlated (0.94) with the Net Tropical Cyclone (NTC) activity index used by Dr. Gray.

The ACE index, combined with the seasonal total number of named storms, hurricanes, and major hurricanes, allows on to idenfity North Atlantic hurricane seasons as being above normal, normal, or below normal.

Above-normal, normal, and below-normal Atlantic hurricane seasons are defined as follows:

The 1950-2000 mean value of the ACE index is 93.7 and the median value is 86.0

Above-normal (very active) season: An ACE index value exceeding 103, which is 120% of the median and 110% of the mean, combined with above-average values of at least two of the following three parameters: seasonal number of tropical storms, hurricanes, and major hurricanes.

Near-normal season: An ACE index value of 65-103, which is 76% - 120% of the median and 70% - 110% of the mean, or an ACE value slightly exceeding 120% of the median but with less than two of the above three parameters being above the long-term average.

Below-normal (relatively quiet) season: An ACE index value below 65, which is 76% of the median and 70% of the mean.

The 1950-2000 seasonal mean and range of tropical storms, hurricanes and major hurricanes during above normal, near normal, below normal, and all Atlantic hurricane seasons is summarized in the following table.

Season  Type Mean # of Tropical Storms Range of Tropical Storms Mean # of Hurricanes Range of Hurricanes Mean # of Major Hurricanes Range of Major Hurricanes
Above-normal 12.1 10 to 19 8.1 6 to 12 3.5 2 to 7
Normal 9.5 6 to 14 5.4 4 to 8 1.9 1 to 3
Below-Normal 6.9 4 to 9 3.6 2 to 5 0.9 0 to 2
All Seasons 9.7 4 to 19 5.9 2 to 12 2.2 0 to 7

The 1950-2000 seasonal mean and range of landfalling hurricanes in the continental United States and Caribbean Islands (including the Caribbean Sea and surrounding land areas) is summarized in the following table.

Season Type Continental United States Caribbean Islands
  Mean        Range Mean       Range
Above-Normal 2.3             0-4 2.3           0-6
Normal 1.8             0-7 0.9           0-3
Below-Normal 0.9             0-2 0.3            0-1

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Page last modified: August 8, 2002
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