This paper proposes a methodology to model the external characteristics of a wind farm in service or in planning stage. For system operators, the interested external characteristics of a wind farm are mainly the aggregated available output that the wind farm can feed into power system. In order to accurately and efficiently model the wind production, the wind speed spatiotemporal distribution is characterized. The proposed model takes wake effect and time delay into account, which can improve the spatiotemporal resolutions of the input data provided by numerical weather prediction (NWP). In addition, the layouts and the mechanical characteristics of wind turbine generators (WTGs) are considered in the wind farm output model. How the operational statuses of WTGs influence the wind farm output is discussed. The proposed methodology is validated using the measurements and NWP data of an actual wind farm in China. The results demonstrate the effectiveness of the proposed method for modeling the external characteristics of a wind farm. The wind farm outputs provided by the proposed model can be used as the references for the generation scheduling or for planning of new wind farms