Topographic effects on climate, and particularly on precipitation, are well known in the literature. Nevertheless their separation on the mesoscale from possible anthropogenic effects has proved difficult. A method based on patterns of correlation between local climate elements and indices of the general circulation, which was developed in a study of Australian rainfall, has been applied to precipitation in the State of Washington and surrounding areas where relief is much greater. Patterns are found which account for the major part of some climatic anomalies discussed in the literature and which have previously been ascribed to anthropogenic effects. The wider implications for the study of urban and other anthropogenic effects is discussed with reference to the La Porte anomaly and METROMEX.