North Atlantic tropical storm seasonal forecast 2014

Forecast for June to November 2014

Issued 16 May 2014

The most likely number of tropical storms predicted to occur in the North Atlantic during the June to November period is 10, with a 70% chance that the number will be in the range 7 to 13. This represents slightly below normal activity relative to the 1980-2010 long-term average of 12.

The most likely number of hurricanes predicted to occur in the North Atlantic during the June to November period is 6, with a 70% chance that the number will be in the range 3 to 9. This represents near normal activity relative to the 1980-2010 long-term average of 6.

An ACE index of 84 is predicted as the most likely value, with a 70% chance that the index will be in the range 47 to 121 - which is slightly below normal relative to the 1980-2010 average of 104.

Our Risk modelling collaborations provide expert advice on seasonal prediction of tropical storms. Please email [email protected] or contact our Contact us for assistance.

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.

The North Atlantic tropical storm season usually runs from June to November. 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 tropical storms observed over the season is the best known measure of the level of storm activity. However, the total number of storms tells us little about variations in the intensity and lifetime of storms from one season to the next.
• Total number of hurricanes. A certain proportion of tropical storms will attain hurricane strength each season. These are storms with sustained winds of at least 74 mph.
• The Accumulated Cyclone Energy (ACE) index. This is a measure of the collective intensity and duration of all tropical storms over the season and thus includes storm lifetimes and intensities as well as total numbers over the season.

The table below shows the number of tropical storms, 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 and the value of the ACE index. Previously, this was for the period July-November. However, since 2011 the forecast has been issued for the full season (June-November). The forecast has been produced following research collaborations with the European Centre for Medium-Range Weather Forecasts (ECMWF). Since 2013 a forecast of the number of tropical storms reaching hurricane strength (winds at least 74 mph) has also been issued.

Method

The forecast is made using information from two dynamical global seasonal prediction systems; the Met Office GloSea5 system and ECMWF system 4. Both systems simulate 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. In contrast to the dynamical methods used in this forecast, statistical prediction methods, which have traditionally formed the basis of most published predictions, do not model atmospheric processes. They rely on past relationships between storm numbers and preceding observed conditions (e.g. pre-season SST patterns).

Seasonal forecasting of tropical storms

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. Last year the Met Office forecast was for 14 tropical storms, 9 hurricanes and an ACE index of 130 with a 70% probability range of 10 to 18 storms, 4 to 14 hurricanes and an ACE index of 76-184, respectively. In the event, the number of storms was 13, the number of hurricanes was 2 and the ACE index was 31. Download a report on the forecast verification and analysis of the 2013 season for further details.

See our seasonal tropical storm forecasts for  2007, 2008, 2009, 2010 , 2011, 2012 and 2013.

With increased spatial resolution and improvements to the representation of physical processes in the forecast model the prediction skill for tropical storm activity is expected to increase.

Verification

The Met Office seasonal tropical storm forecast for the North Atlantic, issued on 16 May 2014, predicted the most likely number of tropical storms to occur during the June to November period would be 10, with a 70% chance that the number would be in the range 7 to 13. This represents slightly below normal activity relative to the 1980-2010 long-term average of 12. The most likely number of hurricanes was predicted to be 6, with a 70% chance that the number would be in the range 3 to 9. This represents near normal activity relative to the 1980-2010 long-term average of 6.  An ACE index of 84 was predicted as the most likely value, with a 70% chance that the index would be in the range 47 to 121 - which was slightly below normal relative to the 1980-2010 average of 104.

In the event the observed number of storms (8), hurricanes (6) and ACE index (65) in the June to November period were all within the predicted ranges.

Download a report on the forecast verification and analysis of the 2014 season for further details. A public forecast for the June to November 2015 period will be issued in May 2015.

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