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April 2008 IRI Typhoon Activity Forecast

There is a slightly enhanced probability (approximately 40%) that the number of named tropical cyclones in the western North Pacific during the 2008 peak season (July to October) will be in the above normal category, and a 35% that the number of cyclones will be in the normal category. These probabilities are slightly greater than the long-term average probability of 33%. The normal category is defined as between 17 and 20 named tropical cyclones, the below normal category 16 or less named tropical cyclones and the above normal category 21 or more named tropical cyclones.

Fig. 1 - Probability Forecasts for Number of Tropical Cyclones

The accumulated cyclone energy (ACE*) index during these months has an enhanced (approximately 40%) probability of being in the normal range and 35% of being in the above normal range.

Fig. 2 - Probability Forecasts for Accumulated Cyclone Energy

 Furthermore, a slight northwestward shift in the average longitude and latitude of tropical cyclone tracks is predicted, compared to the climatological mean. This forecast is forced by continuing below normal sea surface temperatures anomalies in the eastern tropical Pacific, as shown in our SST forecast, for the JAS and ASO periods.

Background Information

The mean number of observed western Pacific named tropical cyclones (1971-2002) in the peak season is 18.4 with a standard deviation of 3.4. The lowest number of tropical cyclones in the peak season during this historical period was 13 and the maximum was 28. If the peak season climatological median ACE in the period 1971-2002 is defined as 100%, the normal range varies between 89% and 118%. The standard deviation of the ACE index is 40%, but in extreme years the index can exceed 200% or be less than 50%. The historical variability in the ACE index is proportionately larger than the variability of the number of named tropical cyclones, as it takes into account not only the number of tropical cyclones but also their intensity and duration.

This outlook was produced by tracking western North Pacific typhoon-like systems in one of our operational atmospheric general circulation models (AGCMs), ECHAM4.5, forced with IRI's predicted sea surface temperatures . While low-resolution (approximately 2.8 degrees longitude and latitude) AGCMs are not adequate for forecasts of individual typhoons, they can have significant skill in predicting the amount and location of tropical cyclone activity over specific basins, as is the case for the ECHAM4.5 over the western North Pacific. Model tropical cyclones are weaker and larger than observed, but have an identifiable signature with many observed tropical cyclone characteristics. The model skill is due to the variability of the tropical cyclone activity being mainly determined by large-scale variables that affect that activity, such as sea surface temperatures and vertical wind shear, which can be predicted using AGCMs. The spatial and temporal distributions of these model tropical cyclones in the western North Pacific are similar to those of observed tropical cyclones in the region. The average tracks and genesis locations of both model and observed western North Pacific tropical cyclones are also strongly influenced by ENSO. These locational variables have an important impact on the percentage of tropical cyclones which make landfall. In El Niņo years there usually is an east-southeast shift in the average track and genesis position, while in La Niņa years a west-northwest shift usually occurs.

The IRI April 2008 forecast for number of named tropical cyclones differs from the Tropical Storm Risk March 10th 2008 statistical forecast of western North Pacific tropical cyclones for the calendar year 2008. This forecast called for a below normal number of tropical cyclones and ACE.

* The ACE index is calculated by summing the squares of the estimated 6-hourly maximum sustained wind speed in knots for all periods in which the tropical cyclones are either of named storm or typhoon intensity. Reference: Bell, G.D., and Coauthors, 2000: Bull. Amer. Met. Soc., 81, S1-S50 (see also http://www.cpc.noaa.gov/products/outlooks/background_information.html).