Specification of Monthly Precipitation in the Western United States from Monthly Mean Circulation

The ability to specify monthly precipitation at 36 stations in Washington, Oregon and California by various indices of monthly mean circulation has been investigated. Most of the circulation indices were derived from eigenvector analyses of 700 mb geopotential height or relative vorticity for 1951–76 or 300 mb relative vorticity for 1963–76. The results indicate that an average up to 60% of the rainfall variance may be specified by same-month average circulation parameters. The 700 mb parameters analyzed within a limited domain in the vicinity of the rainfall network result in the best specification ability. Indices derived from larger domains or from the 300 mb vorticities result in generally poorer specifications. Considerable spatial variation in specification ability is apparent with greatest skills evident in the coastal region of northern California and Oregon and lowest skills in the eastern regions of all three states.