North Atlantic tropical storm seasonal forecast 2013

Forecast for June to November 2013

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.

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). This year, 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 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 for further details.

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

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 15 May 2013, predicted the most likely number of tropical storms to occur during the June to November period would be 14, with a 70% chance that the number would 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 was predicted to be 9, with a 70% chance that the number would 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 was predicted as the most likely value, with a 70% chance that the index would be in the range 76 to 184 - which was slightly above normal relative to the 1980-2010 average of 104.

In the event the observed number of storms in the June to November period (13) was within the predicted range, but the observed number of hurricanes (2) and ACE index (31) were outside the predicted range.

Download a report on the forecast verification and analysis of the 2013 season for further details. A public forecast for the June to November 2014 period will be issued in May 2014.

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