National Weather Service Forecast Offices (WFOs) are responsible for producing Quantitative Precipitation Forecasts (QPF), typically in three to six hour increments, for locations in their County Warning Area. Recently a new methodology, known as QPFHelper, has been developed for generating QPF grids at WFOs. The new methodology is based on an older technique, called Mountain Mapper, developed by NWS River Forecast Centers (RFCs) in the 1990s, but offers some significant improvements.

Mountain Mapper uses a precipitation climatology (PRISM) developed at Oregon State University. Explicit QPF at a select set of points are made by the forecaster. The QPF is compared to the climatological value at that point to create a ratio. The rest of the grid is filled in using an analysis technique that calculates a ratio at each grid point with weights determined by distance between the particular gridpoint and the various explicitly forecast points.

QPF Helper is a software application within the broader capability for generation of gridded forecasts in WFOs known as the Graphical Forecast Editor (GFE). QPF Helper uses the same basic approach as Mountain Mapper but has a number of advantages. (1) Forecasters can select from numerous sources of guidance to initialize the QPF at the explicit points, (2) the influence of the PRISM climatology can be modulated based on the synoptic situation (e.g. less influence in situations with weak orographic forcing), (3) the forecaster can choose among three different analysis schemes to fill in the grid instead of just distance weighting, (4) the forecaster can modify the calculated grid using generic GFE capabilities, and (5) mesoscale detail can be incorporated into the QPF grid because the GFE allows the forecaster to select subset areas of the grid and add additional explicit forecast points on the fly.