NOAA / National Weather Service / National Centers for Environmental Prediction / Weather Prediction Center (WPC)
Forecasters at the National Weather Service's Weather Prediction Center (WPC) and its predecessor organizations have been making Quantitative Precipitation Forecasts since 1960. Quantitative Precipitation Forecasts, or QPFs, depict the amount of liquid precipitation expected to fall in a defined period of time. In the case of snow or ice, QPF represents the amount of liquid that will be measured when the precipitation is melted. Precipitation amounts can vary significantly over short distances, especially when thunderstorms occur, and for this reason QPFs issued by the WPC are defined as the expected "areal average" (on a 20 x 20 km grid) in inches.