A tool is presented that generates stochastic input parameters consisting of simulated short-term wind power and load forecasts required by a stochastic scheduling model. In combination with additionally generated time-series describing the occurrence of forced outages of conventional power plants, the demand for reserves in dependency of the forecast horizon is determined. The distribution of the stochastic input parameters is represented by the use of scenario trees. Results of an exemplary application are discussed showing the dependency of simulated forecast errors on the assumed forecast error standard deviations as well as the impact on required reserves.