Material on Model Output Statistics Background Material on MOS The standard four-panel progs are generated using data from the GFS and NAM dynamic models. By "dynamic", we mean that these models solve mathematical equations in order to predict how the atmosphere will evolve with time (these equations include variables such as temperature, moisture and wind). On average, the dynamic models don't predict surface temperatures very accurately (we can say the same thing about surface dew-point temperatures and surface winds). To give you a sense for why the NAM and GFS dynamic models don't predict surface temperatures very well, we point out that physical processes near the earth's surface (such as the cooling and heating of the ground by radiation gains and losses) are just too complicated to mathematically model very accurately. Instead, meteorologists use oversimplified mathematical parameters and schemes to model complicated physical and radiative processes near the ground. Not surprisingly, these "oversimplifications" lead to forecasting errors. Why, then, are forecasts for daytime high and low temperatures generally pretty accurate? Good