1. A computer-implemented method of computing a forecast F(t+x) for a second time t+x from at least an observation y(t) at a first time t earlier in time than the second time t+x, where x is a forecast time horizon, the method comprising: obtaining from a computer memory a first plurality of seasonality factors from prior seasons for the first time t; determining a first seasonal adjustment factor I d (t) from the first plurality of seasonality factors; computing a level component L(t) using at least the first seasonal adjustment factor I d (t) and the observation y(t); obtaining a second plurality of seasonality factors from prior seasons for the second time t+x; determining a second seasonal adjustment factor I d (t+x) from the second plurality of seasonality factors; and computing, by execution of software by one or more processors, at least a first forecast F 1 (t+x) at least partially based on the level component L(t) and the second seasonal adjustment factor I d (t+x), wherein the first forecast F 1 (t+x) is considered in determining the computed forecast F(t+x).