The precipitation forecasts are given for six months lead time. Seasonal averages are given as three month rolling means which correspond to the dynamical anomalies given elsewhere. The scale on each figure is percentage of normal, blue indicating above normal and red below normal.

Both the mean (on the left) and trimean (on the right) of the forecasts are displayed. These are both the same forecast. A forecast consists of ten separate runs with slightly different starting conditions; each of these runs is termed an ensemble member thus each forecast is run with ten ensemble members. The mean is the average of all ensemble members of the forecast. The trimean is the average ignoring the extreme ensemble members (both very wet or very dry ensemble members). The trimean therefore expresses the central tendency of the ensemble and may give a clearer indication of the forecast in regions and at times when the normal rainfall is low e.g. during winter it may be that only 1 (out of 10) ensemble members gives an abnormally high value which will skew the mean towards above normal whereas the trimean will ignore it. It is therefore advisable that both the mean and trimean are compared to see if they agree on the forecast.

On the mean forecast map (maps on the left hand side), shading has been introduced to differentiate two levels of confidence. The clear area (ie. where there is no shading) shows where the mean of the ensemble forecasts differs from the model climatology (average climate) at the 90% significance level (according to a students T-test). It indicates where the model is more 'confident' that the forecast is different to climatology. Without any guarantees that a significant deviation will occur, a simple filter showed that the model is more likely to get the sign of the anomaly right than wrong over regions when high T-test scores are found.

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Seasonal forecast

Monthly forecast