A number of factors degrade the accuracy of quantitative precipitation estimates (QPE) derived from weather radar data. A collaborative effort within the Meteorological Service of Canada (MSC) has examined these factors in detail for a number of events with significant rainfall totals in two areas of Canada. The first area was in southwestern British Columbia with coastal as well as orographic influences while the other was a more continental and flat region in southern Ontario. Doppler radar-based QPE were assessed with observations provided by disdrometers, rain gauges, and hydrological runoff data for a suitably local watershed at each location. The synoptic and mesoscale atmospheric patterns were also considered. Among the effects examined were C-band radar calibration, variations in raindrop size distribution, reduction by clutter filters, time/spatial sampling, precipitation attenuation, and radome wetting. Preliminary results illustrate important differences between the two localities in the relative significance of the effects, including a greater frequency of attenuation and radome wetting at the continental location, and more reductions due to the clutter filters for the coastal location. These are related to the relative occurrence of stratiform and convective storms and their resultant influence on drop size distributions and local wind patterns. The results will guide the development of correction algorithms within the MSC’s operational radar data processing system.