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 classifications for overall activity are based on data from the 30-year period 1991-2020. This base period is updated from the prior period, 1981-2010, as is customary with the calculation of normals for many variables. The update to normals is routinely accomplished, every 10 yeras.

During 1991-2020, Atlantic hurricane seasons averaged 14.4 named storms (NS, maximum 1-minute sustained 10 m wind speeds between 39-73 mph), with 7.2 of those becoming hurricanes (H, maximum 1-minute sustained 10 m wind speeds of at least 74 mph) and 3.2 becoming major hurricanes (MH, maximum 1-minute sustained 10 m wind speeds 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. 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 intensity. The 1991-2020 mean value of the ACE index is 122.1 x 10⁴ kt², and the median value is 125.5 x 10⁴ kt². With most of 1991-2020 being in an active era, using ACE values from 1951-2020 is more representative of the possible ranges of activity that can result in similar ACE values. The 1951-2020 ACE median is 96.7 x 10⁴ kt²

The following classifications are based on an approximate tercile partitioning of seasons based on the ACE index, combined with the seasonal number of tropical storms, hurricanes and major hurricanes.

• Extremely active season: An ACE index above 159.6 x 10⁴ kt² (corresponding to 165% of the 1951-2020 median).


• Above-normal season: An ACE index above 126.1 x 10⁴ kt² (corresponding to the 67th percentile of the ACE values from 1951-2020).


• Below-normal season:An ACE index below 73.0 x 10⁴ kt² (corresponding to less than 33rd percentile of the values from 1951-2020 median).


• Near-normal season: Neither the above- nor below-normal season criteria are met.

A near-normal season will typically have an ACE range of 73-126 x 10⁴ kt² (corresponding to approximately 75%-130% of the 1951-2020 median).