1. A system for monitoring a monitored computer system, the monitoring system comprising: a computer data repository that stores a first plurality of seasonality factors for a first time t from prior seasons for the monitored computer system and a second plurality of seasonality factors for a second time t+x from prior seasons for the monitored computer system, where x is a forecast time horizon; and a processor programmed, via executable program code, to at least: determine a first seasonal adjustment factor for the first time t based at least in part on the first plurality of seasonality factors; deseasonalize an observation y(t) of a metric of the monitored computer system at the first time t based at least in part on the first seasonal adjustment factor; determine a level component based at least in part on the deseasonalized observation of the monitored computer system; determine a second seasonal adjustment factor for the second time t+x based at least in part on the second plurality of seasonality factors; and compute a first forecast F 1 (t+x) based at least in part on the level component and the second seasonal adjustment factor for the second time t+x, wherein the first forecast F 1 (t+x) is considered in determining a computed forecast F(t+x) for the metric of the monitored computer system.