Scientists can't forecast exactly what the climate will be like in the future, but they can forecast the likelihood of various future climate states. Although climate forecasts can be for different quantities (e.g., total rainfall, average temperatures), and be made and presented in different ways, they are all, in essence, probabilistic. In other words, a forecast might say that one state is more likely, but there is always a chance that a different state will occur. Although there are many things that you need ask about a forecast, a good start is to understand precisely what is being forecasts—that is, what are the possible climate states and how likely they are.

For example, tercile rainfall forecasts, which are common (e.g., prepared by the IRI), explicitly give the probability of rainfall falling in one of three categories: below normal, near normal, and above normal. They often apply for rainfall over a period of three months, and the meaning of these categories in terms of rainfall amounts depends on you location. A forecast of a 50% chance of below normal rainfall, 30% chance of near normal rainfall, and 20% chance above normal rainfall means that the most likely category is below normal rainfall—but there is still a 50% chance of rainfall being near or above normal.

It is also useful to be aware of what the forecast can and can't tell you. For example, a forecast of a higher probability of above normal rainfall over the next three months doesn't tell you anything about whether the season will start early or finish later, when rainfall will occur during the three months, or whether the rainfall will be a little above normal or very much above normal.

Links:

• IRI Website Tutorial: Seasonal temperature and precipitation forecasts for any location
• El Niño and La Niña: Current forecasts and information