Lack of openly available power system data due to market regulations or corporate policies hinders the development of models for power system planning and operation studies. This work presents an open source model of the Nordic synchronous area (the Nordic490 model), which can be used for power flow calculations. To reach an accurate modelling of the Nordic transmission grid, the production and consumption time series are disaggregated based on demographics, and the values of certain network parameters and generation characteristics are investigated, by crosschecking various data sources. The resulting model is validated using open market data.