Long-range weather prediction: limits of predictability and beyond

The details of the weather are not predictable beyond 1-2 weeks. At longer time ranges, averages of the weather over space and time can be usefully predicted only to the extent that the variations of the averages exceed the noise produced by the omnipresent, but unpredictable, transient weather. This margin of potential predictability is not large, but parts of it are being exploited in routinely issued monthly and seasonal forecasts. The format and use of these forecasts, the methods by which they are routinely produced, and prospects for improvements are discussed.