We started with predicted 500-mb maps that came from computer models. The computer models use surface and upper-air observations to characterize current conditions, then solve equations to simulate how the atmosphere evolves. Forecasters use some model output directly, and supplement it with Model Output Statistics (MOS) that link model solutions to historically observed surface weather variables that people care most about, such as temperature, dew point, wind speed and direction, cloud cover, and probability of precipitation.