North Atlantic tropical storm seasonal forecast 2021

This forecast was issued on 02 August 2021 and supersedes the pre-season forecast issued in May 2021. Please see the link below for the pre-season forecast.

• Seasonal North Atlantic tropical storm forecast issued in May 2021

Forecast for the whole 2021 season

Issued 02 August 2021 

All values given below represent observed activity up to the end of July 2021 plus the 6-month GloSea6 forecast activity from August 2021 to January 2022. Note, although the forecast includes the month of January, historically very little activity occurs during this month. In the last 30 years there has only been one January storm (Hurricane Alex in 2016).

The most likely number of named tropical storms (winds of at least 39 mph) predicted to occur in the North Atlantic is 15, with a 70% chance that the number will be in the range 12 to 18. The 1991-2020 long-term average is 14. 

The most likely number of hurricanes (winds of at least 74 mph) predicted to occur in the North Atlantic is 6, with a 70% chance that the number will be in the range 4 to 8. The 1991-2020 long-term average is 7. 

The most likely number of major hurricanes (winds of at least 111 mph) predicted to occur in the North Atlantic is 3, with a 70% chance that the number will be in the range 1 to 5. The 1991-2020 long-term average is 3.  

An ACE index of 116 is predicted as the most likely value, with a 70% chance that the index will be in the range 64 to 168. The 1991-2020 average is 123. 

Background

'Tropical cyclone' is the generic term for a low-pressure system over tropical or subtropical waters, with intense convective activity (e.g. thunderstorms) and winds circulating in an anticlockwise direction in the northern hemisphere (clockwise in the southern hemisphere). A tropical storm is a tropical cyclone with mean wind speeds of at least 39 mph. The terms hurricane and typhoon are region-specific names for strong tropical cyclones with wind speeds of at least 74 mph. For further details about tropical cyclones see Met Office tropical cyclone facts. 

The North Atlantic tropical storm season usually runs from June to November, although occasional storms develop before and after these months. The degree of activity over the whole season varies from year to year and is measured in several ways. 

• Total number of tropical storms. The number of named tropical cyclones with winds of at least 39 mph observed over the season. This is the best known measure of the level of storm activity. In this forecast the number of tropical storms also includes those that reach hurricane and major hurricane strength (see below). 

• Total number of hurricanes. The number of tropical cyclones that reach sustained winds of at least 74 mph. 

• Total number of major hurricanes. The number of tropical cyclones that reach sustained winds of at least 111 mph.  

• The Accumulated Cyclone Energy (ACE) index. A measure of the collective intensity and duration of all named tropical storms, hurricanes and major hurricanes during the season. 

The table below shows the number of tropical storms, hurricanes, major hurricanes and ACE index observed in recent years. 

Forecast

At the start of each North Atlantic season the Met Office forecasts the number of tropical storms, hurricanes, major hurricanes and ACE index.  Seasonal forecasts of tropical storm activity have been issued annually since 2007. See our previous seasonal tropical storm forecasts issued in either May or June for 2007, 2008, 2009, 2010 , 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 and 2021.

Method

The forecast is made using information from the Met Office seasonal prediction system, GloSea6, which is an ensemble prediction system based on a fully coupled ocean-atmosphere general circulation model (GCM). This system simulates the ocean-atmosphere processes and interactions that determine tropical storm development. Multiple forecasts are made (using ensemble forecasting methods) to allow estimation of the range of likely outcomes.  The ensemble is then used to produce a best estimate (or most likely) predicted value (the ensemble mean) and forecast range (using the spread in the ensemble outcomes). Further details on the forecast method can be found in Camp et al. (2015). 

Skill

Recent studies have shown that dynamical models have considerable skill predicting the number of tropical storms - for example successfully predicting the change from the exceptionally active season of 2005 to the below-normal activity of the 2006 season. In 2017, the Met Office seasonal forecast system successfully predicted the above-average activity that was observed in the North Atlantic, including the high frequency of tropical cyclone tracks around the northeast Caribbean (Camp et al., 2018). Full details of the skill to predict tropical storm activity around the world has previously been documented by Camp et al. (2015) and for real-time forecasts in Klotzbach et al. (2019).  

Verification

The table below shows the seasonal forecast values issued on 02 August 2021 for the whole of the  2021 season together with the end of season observed values. This season the observed number of tropical storms was outside the 70% forecast range. The number of hurricanes, major hurricanes and ACE Index were towards the upper end of the 70% forecast range.

A public forecast for the June to November 2022 period in the Atlantic will be issued in May 2022.

References