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.Tropical Storm Frequency Seasonal Prediction 2014 Hurricane Frequency Seasonal Prediction 2014 ACE Index Seasonal Prediction 2014
'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.
The table below shows the number of tropical storms, hurricanes and ACE index observed in recent years.
|Year||Tropical storms||Hurricanes||ACE index|
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.
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).
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 (PDF, 1 MB) for further details.
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.
Last updated: 16 May 2014