Forecasting in the Tropics is a difficult problem. It necessitates a good meteorological and physics background, vast amounts of climatological knowledge, a keen minds eye that can observe the most minute deviation in a mass of nearly homogeneous data, and last, but not least, diligence and dedication in the approach to the forecast.

The types of forecasts in the Tropics are the same as anywhere, in that you encounter flight, route, terminal, operational, general, fleet, local area, and destructive weather forecasts, as well as pertinent warnings and advisories.

Your concern in this chapter is with preparing local area forecasts, including destructive weather warnings and forecasts, and forecasting the movement of the Inter Tropical Convergence Zone (ITCZ) and tropical waves.

The treatment of destructive weather warnings and forecasts is limited to tropical storms and cyclones since tornadoes are largely non-existent in the Tropics. Note also, that thunderstorms are covered in chapter 5 of this manual.

The Composite Warfare Oceanographic Support Modules (CWOSM), Part 1, TM 0492, contains further reading on severe weather features. Information may also be found in the AG2 TRAMAN, volume 1, NAVEDTRA 10370.

LEARNING OBJECTIVES: Analyze upper air features and refer to local area climatology for preparation of surface analyses and forecasts. The importance of local and general area climatology can have a profound impact on operations in the tropics. It is in the preparation of the local area forecast that this knowledge will be most beneficial. During the analysis of the various charts, most forecasters form a mental image of the forecast charts and develop certain fundamental ideas as to the weather in the area of responsibility (AOR) for the next 24 or 48 hours. Climatology serves as a guide for analysis and forecasting within the AOR. The next step in the procedure is to expand and refine these ideas. The ideal approach to a local area forecast is to prog the upper air features first as it is from the upper air charts that the surface chart is eventually prepared. The prognostic surface chart is then used as a basis for the local area forecast. Of course other data must also be considered in preparing the forecast, such as streamline analysis, weather distribution charts, time sections, and climatology.

TROPICAL CYCLONE FORECASTING

LEARNING OBJECTIVES: Recognize synoptic features conducive to tropical cyclone development. Identify situations affecting movement and intensification of tropical cyclones. Interpret tropical cyclone warnings. 

There is at present no one formal procedure for forecasting the development and movement of tropical cyclones. This can be understood when one considers the enormous complexity of the problem, the sparsity of data in the oceanic tropical regions compared to that available in the highly populated continents, and the lack of ship reports from areas of tropical cyclone activity. There are also regional influences to consider. Avery obvious consequence of regional influences can be demonstrated when you compare the North Atlantic area with the North Pacific area. The North Pacific has almost twice the tropical water area and also better than double the average number of tropical cyclones per year.

Forecasting tropical cyclones evolves into the following problems: formation, detection, location, intensification, movement, recurvature, and decay. The factors that enter into forecast preparation are mainly dynamic (relating to the energy or physical forces in motion), but there are also important thermodynamic influences. For instance, tropical storms will not develop in air that is drier and slightly cooler in the surface layers than the air normally present over the tropical oceans in summer. This also holds true in air with a trade inversion and upper dry layer present.

Surface ship temperature and dew point reports, along with upper air data, are extremely valuable in determining whether the surface layers are truly tropical or whether they contain old polar air not yet completely transformed.

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