The North Atlantic hurricane season officially runs from 1 June to 30 November. The vast majority of tropical storm and hurricane activity typically occurs during the August-October (ASO) peak of the hurricane season.
Hurricane season statistics for the period 1981-2010 are shown in the table below. During the period 1981-2010, the Atlantic hurricane seasonal averages were 12.1 named storms (maximum 1-minute surface winds between 39-73 mph), with 6.4 of those becoming hurricanes (maximum 1-minute surface winds of at least 74 mph) and 2.7 becoming major hurricanes (maximum 1-minute surface winds exceeding 111 mph, categories 3-5 on the Saffir-Simpson hurricane scale).
Previously, the NOAA seasonal outlooks stated averages for the years 1950-2000 (51 seasons). The seasonal averages from this period were 10.1 named storms, 6.0 hurricanes, and 2.5 major hurricanes. The main difference between the old and new period is seen in the average numbers of named storms, which has increased from 10.1 to 12.2. This increase is thought to largely represent our ability to better identify relatively weak, short-lived systems that simply went unnoticed earlier in the record.
Measuring total seasonal activity: The Accumulated Cyclone Energy (ACE) index
The phrase "total seasonal activity" refers to the collective intensity and duration of Atlantic named storms and hurricanes occurring during a given season. The measure of total seasonal activity used by NOAA is called the Accumulated Cyclone Energy (ACE) index. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named storms while they are at least tropical storm strength.
NOAA uses the ACE index, combined with the seasonal total number of named storms, hurricanes, and major hurricanes, to categorize North Atlantic hurricane seasons as being above normal, near normal, or below normal.
NOAA’s Atlantic hurricane season classifications
Above-normal season: An ACE index well above 103 x 104 kt2 (corresponding to 111% of the 1981-2010 median), or an ACE index slightly above 103 x 104 kt2 combined with at least two of the following three conditions being met: 13 or more named storms, 7 or more hurricanes, and 3 or more major hurricanes.
Near-normal season: An ACE index in the range 66-103 x 104 kt2 (corresponding to 71.4%-111% of the 1981-2010 median), or an ACE index slightly above 103 x 104 kt2 but with no more than one of the following three conditions being met: 13 or more named storms, 7 or more hurricanes, and 3 or more major hurricanes.
Below-normal season: An ACE index below 66 x 104 kt2, corresponding to less than 71.4% of the 1981-2010 median.
The following table shows the 1981-2010 seasonal means and ranges for named storms, hurricanes and major hurricanes during above normal, near normal, below normal, and all Atlantic hurricane seasons. This table highlights the marked differences in activity between the three season types.
||Mean # of
||Mean # of
||Mean # of Major
||Range of Major
||11 to 28
||5 to 15
||2 to 7
||7 to 14
||4 to 8
||1 to 3
||4 to 9
||2 to 4
||0 to 2
||4 to 28
||2 to 15
||0 to 7