Issued 15 May 2013
The most likely number of tropical storms predicted to occur in the North Atlantic during the June to November period is 14, with a 70% chance that the number will be in the range 10 to 18. This represents slightly above 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 9, with a 70% chance that the number will be in the range 4 to 14. This represents above normal activity relative to the 1980-2010 long-term average of 6.
An ACE index of 130 is predicted as the most likely value, with a 70% chance that the index will be in the range 76 to 184 - which is slightly above normal relative to the 1980-2010 average of 104.Tropical Storm Frequency Seasonal Prediction 2013 Hurricane Frequency Seasonal Prediction 2013 ACE Index Seasonal Prediction 2013
'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). This year, 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 10 tropical storms and an ACE index of 90 with a 70% probability range of 7 to 13 storms and an ACE index of 28-152, respectively. In the event, the number of storms was 17 and the ACE index was 123. Download a report on the forecast verification and analysis of the 2012 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: 15 May 2013