Several studies have demonstrated the ability of Numerical Weather Prediction (NWP) models to provide reliable estimates of the global horizontal irradiance (GHI) at the earth surface. However, unlike GHI, the direct normal irradiance (DNI) is not usually a product of these models. Nevertheless, NWP provide a comprehensive description of atmospheric state that can be used to derive DNI in a post-processing step. We present here a methodology to derive the DNI based on the Weather and Research Forecasting (WRF) NWP model and satellite retrievals. Based on this methodology, we evaluate the reliability of three-days-ahead GHI and DNI forecasts in Andalusia (southern Spain). Particularly, hourly time resolution and 3 km spatial resolution estimates were tested against ground measurements collected at four radiometric stations along the years 2007 and 2008. Results showed that the WRF model presents considerable skill in forecasting both GHI and DNI, overall, better than a trivial persistence model. Nevertheless, both MBE and RMSE values presented a marked dependence on the sky conditions and season of the year.