These data are estimates of average annual precipitation, in inches, for the climatological period 1971-2000. Parameter-elevation Regressions on Independent Slopes Model (PRISM) derived raster data is the underlying data set from which the data were created. PRISM is an analytical model that uses point data and a digital elevation model (DEM) to generate gridded estimates of annual, monthly and event-based climatic parameters. For further information, the online PRISM homepage can be found at http://www.ocs.orst.edu/prism/.
The data is available as an ESRI GridASCII export file, in latitude/longitude coordinates with a horizontal resolution of 0.00833 degrees, for the entire United States from the Oregon Climate Service web site. The data for Montana is available in Montana State Plane Coordinates, resampled to a resolution of 600 meters, in either GridASCII export format or as a shapefile with polygons representing 27 precipitation ranges.