How reliable are seasonal forecasts?

What is it about?

How good, or reliable, on a scale of 1 to 5 (5 is very good), are forecasts of winters and summers around the globe? These forecasts are made 1-4 months ahead of time and are expressed as the probability that it will be warmer or cooler and wetter or drier than normal. We find a wide range of goodness rankings, depending on region, variable (temperature or rainfall) and time of year. Forecasts of summer rainfall over Northern Europe are exceptionally poor.

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Why is it important?

Seasonal forecasts are used for a broad range of applications. For example, information about rainfall and temperature for the growing season can influence a farmer's decision about which crops to plant, or a humanitarian organization's strategy for anticipating food shortages in drought-prone regions of the developing world. Information about the reliability of our seasonal forecasts may guide policymakers like the UK Government when they consider options for investment in science.