Description: March average precipitation calculated from data collected during 1981 - 2010. From PRISM Climate Group: "There are many methods of interpolating precipitation from monitoring stations to grid points. Some provide estimates of acceptable accuracy in flat terrain, but few have been able to adequately explain the extreme, complex variations in precipitation that occur in mountainous regions. Significant progress in this area has been achieved through the development of PRISM (Parameter-elevation Regressions on Independent Slopes Model). PRISM is an analytical model that uses point data and a digital elevation model (DEM) to generate gridded estimates of monthly and annual precipitation (as well as other climatic parameters). PRISM is well suited to regions with mountainous terrain, because it incorporates a conceptual framework that addresses the spatial scale and pattern of orographic precipitation. Precipitation was modeled monthly. For this application, PRISM was parameterized to use existing 1971-2000 mean monthly precipitation grids as the predictor grids in the interpolation. The PRISM weighting functions for distance, "elevation" (now the 1971-2000 climatology), topographic facet, atmospheric layer, orographic effectiveness, and coastal proximity were all retained. This allowed observed deviations from normal to be interpolated with sensitivity to physiographic factors. An annual grid was produced by summing the monthly grids. "