The prediction method relies on the use of coupled ocean-atmosphere dynamical models. Due to increased complexity, such a model is run at a coarser resolution than weather prediction models and thus is unable to produce tropical storms similar to those observed in the real world.
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 Enlarge Hurricane Frequency Seasonal Prediction 2013 Enlarge ACE Index Seasonal Prediction 2013 Enlarge
However, it is possible to identify features in the model which are indicative of tropical storms and hurricanes; for example, low central pressure and high relative vorticity. These features can be tracked and counted to arrive at a total number of storms and hurricanes for the season.
For each model storm, we also calculate the individual accumulated cyclone energy (ACE) index and total these values over the whole season to arrive at the seasonal ACE index. The ACE index is calculated as the square of the maximum wind speed for each six hourly period of the storms lifetime. The total number of model tropical storms, hurricanes and ACE index may differ from those values seen in the real world. To make these adjustments we use a calibration procedure based on a fixed historical period