BACKGROUND INFORMATION: NORTH ATLANTIC HURRICANE SEASON

The North Atlantic hurricane season officially runs from 1 June to 30 November. The vast majority of tropical storms and hurricanes are typically seen during August-October (ASO), the peak months of the hurricane season.

Hurricane season averages and strength classifications are based on data from the 30-year period 1981-2010. This shorter base period is used (rather than the longer 1950-2010 period) because it is more accurate and better represents our ability to identify relatively weak, short-lived systems that went unnoticed earlier in the record (Landsea et al. 2010).

During 1981-2010, Atlantic hurricane seasons averaged 12.1 named storms (NS, maximum 1-minute surface winds between 39-73 mph), with 6.4 of those becoming hurricanes (H, maximum 1-minute surface winds of at least 74 mph) and 2.7 becoming major hurricanes (MH, maximum 1-minute surface winds exceeding 111 mph, categories 3-5 on the Saffir-Simpson hurricane scale).

Measuring total overall seasonal activity: The Accumulated Cyclone Energy (ACE) index

The phrase "total overall seasonal activity" refers to the combined intensity and duration of Atlantic named storms and hurricanes occurring during the season. The measure of total seasonal activity used by NOAA is 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

Extremely active season: An ACE index above 152.5 x 10⁴ kt² (corresponding to 165% of the 1981-2010 median), with at least two of the following three conditions: 13 or more named storms, 7 or more hurricanes, and 3 or more major hurricanes.

Above-normal season: An ACE index above 111 x 10⁴ kt² (corresponding to 120% of the 1981-2010 median), with at least two of the following three conditions: 13 or more named storms, 7 or more hurricanes, and 3 or more major hurricanes.

Below-normal season:

• a) An ACE index below 66 x 10⁴ kt² (corresponding to less than 71.4% of the 1981-2010 median), or
• b) An ACE index above 71.4% of the median with the following three conditions: 9 or fewer named storms, 4 or fewer hurricanes, 1 or fewer major hurricanes.

Near-normal season: Neither the above- nor below-normal season criterion are met. A near-normal season will typically have an ACE range of 66-111 x 10⁴ kt² (corresponding to 71.4%-120% 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.