One fundamental problem we have with weather forecasts is that our input data for the starting point is fairly sparse. GFS calculates the forecast on a grid wit

One fundamental problem we have with weather forecasts is that our input data for the starting point is fairly sparse. GFS calculates the forecast on a grid with 13 km horizontal resolution and 64 vertical layers. We don't have accurate weather information at that resolution from all over the globe, so the starting point for the forecasts is a combination of previous simulations and interpolated observational data. So even if we had a forecast engine that would perfectly simulate everything given some start state, we wouldn't have enough input data to have an accurate start state.