Abstract
Seasonal forecasts are most commonly issued as anomalies with respect to some multiyear reference period. However, different seasonal forecasting centers use different reference periods. This paper shows that for near-surface temperature, precipitation, and mean sea level pressure, over most regions of the world there is evidence that these differences between reference periods should not be ignored, especially when forecasters combine outputs from several prediction systems. Three methods are presented by which reference periods could be adjusted, and it is shown that the differences between the proposed methods are smaller than the errors that result from not correcting for different reference periods.
