Surface Prog Charts are forecasts of surface conditions over the contiguous United States that show pressure systems, fronts and precipitation. Areas of precipitation expected at the valid time of the forecast are shaded in colors according to the type and likelihood of precipitation. These charts provide a "snapshot" of weather elements expected at the specified valid time and can be used to obtain an overview of the progression of surface weather features. The Weather Prediction Center (WPC) updates the 12 and 24 hour forecasts four times a day.