LOCAL NEWS
What Does This Year's Hurricane Season
Have In Store For The Caribbean?
by David Jones of the Caribbean Weather Center
Professor Gray and his colleagues at Colorado State University recently issued their latest forecast of this year's hurricane activity in the Atlantic Basin, which includes the Caribbean. In summary, it is thought that, although the 2000 Hurricane season is likely to be less active than those in recent years, it is still expected to be about 25% more active than the long-term average. The likelihood that a major or intense hurricane will make landfall in the Caribbean this year is about 10% above the long term average. Looking further ahead, Gray says that the current indicators suggest that next year's (2001) hurricane season may be fairly quiet.
Gray now uses both a statistical model as well as an "analogue years" approach to develop the forecast. The forecast is therefore a compromise but provided better results when this method was used for the first time last year. Gray admits, however, that he feels less certain about this year's forecast than he did for 1999. Of the 13 statistical predictors used in developing this year's forecast, 8 indicate a risk of heightened activity and 5 suggest a lower level of activity.
1. Rainfall in the Sahel Region of West Africa. This is a crucial factor because it determines the amount of moisture in tropical waves and therefore the likely level of convection activity. The greater the amount of convection activity in tropical waves, the greater the chance of tropical cyclone formation. In the Sahel Region, January-March is the drought period. The rainy season begins in Spring with the heaviest rainfall often experienced from July to September. Gray is predicting that the rainfall in the crucial months of August and September will be marginally above average leading to increased activity. This is at odds with the International Research Institute's (IRI) forecast that there is a 75% chance that Sahel rainfall will be at or below average during those months.
2. La Ni–a/El Ni–o. In 1999, there was an unusually strong La Ni–a, the cool phase of the La Ni–a/El Ni–o phenomenon during which there is often enhanced tropical cyclone activity in the Atlantic Basin. On the other hand, during El Ni–o years, activity is reduced because a persistent westerly upper level jet stream helps to inhibit convection activity by shearing off the tops of convection clouds. Gray predicts that there will be no return to El Ni–o during 2000 which would have led to a reduction in forecasted activity. This contrasts with the latest and predicted sea surface temperatures off the Pacific coast of South America in the forecasting model used by the Caribbean Weather Center which shows that a warming trend is expected to continue over the next few months. There is also a strong, westerly upper level jet stream over the Caribbean and the tropical Atlantic at the present time. Both of these factors are reminiscent of a typical El Ni–o situation. 3. Sea Surface Temperatures (SST). Warm (at least 79¡F), moist sea water in the tropical Atlantic is an important factor in tropical cyclone formation. Although SST's in both the tropical Atlantic and the NE Caribbean, currently 78¡F, are 1-2¡F below normal at the present time, Gray believes these will be above normal during the main part of the hurricane season thanks to warm sea water returning from the North Atlantic during the summer months. This is supported by IRI which forecasts a rapid warming in tropical Atlantic SST's in August and September.
4. Sea level atmospheric pressure (SLAP). Lower than normal SLAP in the tropical Atlantic can enhance hurricane activity and based on past patterns, Gray believes these will be slightly below normal during the hurricane season which is an enhancing factor. It is interesting to note, however, that the E Caribbean experienced unusually high atmospheric pressure at times during the past winter months, particularly in March, which would be an inhibiting factor. However, the downside is that when this situation occurs, there is an increased risk of more intense systems forming at lower latitudes in the tropical Atlantic and thus pose a greater potential threat to the Caribbean. A study by the Caribbean Weather Center in 1998 showed that one-third of intense hurricanes forming in the far eastern tropical Atlantic moved through the Caribbean. The Caribbean Weather Center will carefully monitor these and other climate variables over the next few months. Like Professor Gray, I feel that this yearâs hurricane season is very difficult to predict, particularly in view of the unusual weather patterns we have been experiencing in the Caribbean over the last six months or so including Hurricane Lenny. It may well be that we could be lulled into a false sense of security by perhaps a quiet early start to the season only to find ourselves facing a flare up of activity in August and September. Remember, it only takes one!
David Jones is ZBVI's Weatherman and founder of the Caribbean
Weather Center