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There is an enhanced probability (approximately 40%) that the number of tropical cyclones in  the eastern North Pacific during the 2008 peak season (June to September) will be in the below normal category, which is defined as 9 or less named tropical cyclones. There is a 35% probability that the number of named tropical cyclones in this period will be in the normal category (10 to 15 named tropical cyclones). These probabilities are greater than the long-term average probability of 33%. The probability that the number of named tropical cyclones in this period will be in the above normal category (16 or more named tropical cyclones) is 25%.

Fig. 1 - Probability Forecasts for Number of Tropical Cyclones

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

Fig. 2 - Probability Forecasts for Accumulated Cyclone Energy

This forecast is based on the IRI sea surface temperature (SST) forecast with continuing slightly below normal SSTs in the central tropical North Pacific, with slightly above SST anomalies in the eastern tropical Pacific.

Background Information

The mean number of observed eastern North Pacific named tropical cyclones (base period 1971 - 2002) in the peak season (June to September) is 13.3 with a standard deviation of 3.9. The near normal category ranges from 10 to 15 named tropical storms in the June-September peak season. The lowest number of tropical cyclones in the peak season during this historical period was 6 and the maximum was 20. If the peak season climatological median ACE in the period 1971-2002 is defined as 100%, the normal range varies between 82% and 113%. The standard deviation of the ACE index is 47%, but in extreme years the index can exceed 230% or be less than 20%. 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 eastern North Pacific tropical cyclone-like systems in one of our operational atmospheric general circulation models (AGCMs), ECHAM4.5, forced with IRI's predicted sea surface temperatures. AGCMs are not adequate for forecasts of individual tropical cyclones. However, they can have significant skill in predicting the amount of tropical cyclone activity over specific basins, as is the case for the ECHAM4.5 over the eastern North Pacific region. Due to the low-resolution (approximately 2.8 degrees longitude and latitude) model tropical cyclones are weaker and larger than observed tropical cyclones, 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 seasonal distribution of these model tropical cyclones in the eastern North Pacific region is similar to that of observed tropical cyclones in the region. The amount of tropical cyclone activity of both model and observations in the eastern North Pacific region is influenced by ENSO.

The IRI June forecasts for the Eastern North Pacific region is consistent with the NOAA 2008 May outlook for the same region, with a 70% probability of a below normal season.

* 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.shtml).