1. A computer-implemented method for forecasting characteristics of a target item comprising: generating, by one or more processors, a forecast function that represents forecasted characteristics of a target item and includes one or more continuous functions and one or more attenuating functions, wherein each of the attenuating functions: attenuates forecast values generated by the continuous functions at periodic time intervals, and is represented as G k sgn(sin(ψ k (t k +γ k ))+L k ), where a function sgn(f(x)) is equal to 0 when f(x)≦0 and is equal to 1 when f(x)>0; determining, by the one or more processors, data values for variables G k , ψ k , t k , γ k , and L k for each of the attenuating functions; and forecasting, by the one or more processors, the characteristics of the target item using the forecast function.