The precipitation estimates provided herein are derived from the NWS WSR-88D Doppler Radar. Radar precipitation estimates can be grossly inaccurate, so radar-based precipitation values are calibrated with the routinely available hourly surface gages. The combined product provides the spatial resolution of radar with the increased accuracy of surface gage networks. These gage-calibrated radar estimates are known as Multi-sensor Precipitation Estimates, or MPE.

There are still errors in MPE. A study by the State Climate Office of North Carolina suggests that MPE compares well with an independent daily precipitation gage network over the Carolinas. Details of this study are available online. The MPE grids used in this tool are routinely produced by the National Weather Service and National Centers for Environmental Prediction.