Starting in 1992, National Weather Service (NWS) River Forecast Centers (RFCs) have been using a program referred to as Stage III which creates mosaics of hourly radar derived precipitation. In areas where two or more radars overlap, Stage III has optionally used the mean or maximum value when creating the RFC-wide precipitation mosaic. Up to this point, no information about whether a particular area within the normal range of a radar is beam-blocked by intervening terrain has been used in deciding what value to assign to a given grid point in the mosaic. This has created a significant error in the Stage III estimates in mountainous areas. A new mosaicking algorithm has been developed which will select the radar with the lowest unobstructed coverage in the overlapping area.
To determine areas which are climatologically beam blocked, an interactive technique has been developed which analyzes the mean value and frequency of WSR-88D derived precipitation at each grid point over a several year period. Areas that are beam-blocked or not well observed due to the beam overshooting the precipitation will have lower radar rainfall frequencies than those areas which are well sampled.
The technique of choosing the lowest unobstructed tilt in the overlapping area still leaves grid points that are not covered by any radar in the rainfall mosaic. An optimal estimation technique will be used to fill in the missing areas by weighting nearby raingage and radar estimates.