The NWP models are able to provide the solution for seven atmospheric parameters, by solving the momentum, mass and energy conservation equations related to the motion of air and water vapor in the atmosphere. These models are also able to estimate the cloud cover, and incoming solar radiation [11]. By the model GFS (Global Forecasting System) [12], numerical modeling are performed in a (0.5 x 0.5)° earth surface grid with sampling interval of 3h. In order to improve the performance of local forecasts, the data of the global model are assimilated by regional NWP models. By using the hydrostatic model ETA with grid resolution of (0.4 x 0.4)°, Guarnieri [13] obtained a RMSE of 43.9 % and 43.6 % for the daily total of incoming solar radiation, for two different sites in Brazil.