Issued 17 June 2010
The most likely number of tropical storms predicted to occur in the North Atlantic during the July to November period is 20, with a 70% chance that the number will be in the range 13 to 27. This represents above-normal activity relative to the 1990-2005 long-term average of 12.4.
An ACE index of 204 is predicted as the most likely value, with a 70% chance that the index will be in the range 90 to 319 - which is above normal relative to the 1990-2005 average of 131.
Note: The forecast is for the five full months remaining in the June-November Atlantic tropical storm season.
Our Climate Services for Reinsurance provide expert advice on tropical storms. A detailed report with information on probabilities and the strength and credibility of signals within the forecast for the next six months is produced each month from March to September.
'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 more than 73 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 and ACE index observed in recent years.
|Number of tropical storms||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 for the period July-November. The forecast has been produced following research collaborations with the European Centre for Medium-Range Weather Forecasts (ECMWF).
The forecast is made using information from two dynamical global seasonal prediction systems; the Met Office GloSea4 system and ECMWF system 3. 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. preseason 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 six tropical storms and an ACE index of 60 with a 70% probability range of three to nine storms and an ACE index of 40-80, respectively; in the event, the number of storms was nine and the ACE index was 53.
With increased spatial resolution and improvements to the representation of physical processes in the forecast model - planned for the near future - the prediction skill for tropical storm activity is expected to increase.
The Met Office seasonal tropical storm forecast for the North Atlantic, issued on 17 June 2010, predicted above-normal activity for the July to November 2010 period, with a best estimate of 20 tropical storms (with a range 13-27) and an ACE index of 204 (range 90-319). The best estimate of storm numbers therefore predicted the second most active July to November period on record (behind 2005).
In the event the observed number of storms (19) and the observed ACE index (170) were both within the predicted ranges, and well above the 1990-2005 July to November averages of 12.4 and 131 respectively. In terms of tropical storm numbers only 2005 has recorded more tropical storms in the July to November period.
Download a report on the forecast verification and analysis of the 2010 season (PDF, 1 MB) . The Met Office will issue a public forecast for the June-November 2011 period in May 2011.