Additive Season, No Trend

This Time Series model is partially equivalent to the simple exponential smoothing model; however, in addition, each forecast is "enhanced" by an additive seasonal component that is smoothed independently (see The seasonal smoothing parameter  ). This model would, for example, be adequate when computing forecasts for monthly expected amount of rain. The amount of rain will be stable from year to year, or change only slowly. At the same time, there will be seasonal changes ("rainy seasons"), which again may change slowly from year to year.
To compute the smoothed values for the first season, initial values for the seasonal components are necessary. The initial smoothed value S0 will by default be computed as the mean for all values included in complete seasonal cycles.