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August 2003 IRI Atlantic Hurricane Activity Forecast

There is an enhanced probability (approximately 40%) that the number of tropical cyclones in the Atlantic during the 2003 peak season (August to October) will be in the below normal category, which is defined as 5 or less named tropical cyclones. There is a 35% probability that the number of named storms in this period will be in the normal category (between 6 and 9 named storms). These probabilities are greater than the long-term average probability of 33%. The accumulated cyclone energy (ACE*) index during these months has an enhanced probability (approximately 45%) of being in the normal range, with a probability of 35% of being in the above normal category. The combination of enhanced probabilities for a below normal number of cyclones and for an above normal ACE may be interpreted as implying that the cyclones that do form will be comprised of proportionally more intense cyclones than normal, or will last for a greater number of days than normal. However, this interpretation should be considered cautiously in view of uncertainties and known imperfections in the models. This forecast is consistent with the near-neutral conditions in the tropical Pacific sea surface temperatures (SSTs) and slightly above normal SSTs in the tropical Atlantic, as shown in our SST forecast.

Background Information

The mean number of observed western Pacific named tropical cyclones (1971-2001) in the peak season is 8.1 with a standard deviation of 3.2. The near normal category is between 6 and 9 named tropical cyclones in the August to October peak season. The lowest number of tropical cyclones in the peak season during this historical period was 3 and the maximum was 15. If the peak season climatological median ACE in the period 1971-2001 is defined as 100%, the normal range varies between 68% and 129%. The standard deviation of the ACE index is 71%, and in extreme years the index can exceed 300% or be less than 20%. The historical variability in the ACE index is proportionally 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 Atlantic hurricane-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 hurricanes, 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 North Atlantic. 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 temporal distribution of these model tropical cyclones in the North Atlantic is similar to that of observed tropical cyclones in the region. The amount tropical cyclone activity of both model and observations in the Atlantic is strongly influenced by ENSO.

The IRI August 2003 hurricane activity forecast is somewhat different from, but not at odds with the NOAA 2003 Atlantic Hurricane Outlook Update , which calls for above-normal season, both in the number of hurricanes and in the ACE index.

* 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).