Synoptic climatology of the Western United States in relation to climatic fluctuations during the twentieth century

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Abstract
An objectively derived catalogue of daily pressure patterns for 1899‐1974 has been prepared for the western United States. The temperature and precipitation characteristics of the major types are described and a more detailed analysis for Colorado in extreme cold and warm months shows that the sign of the anomalies for each type is generally consistent with expectation based on the probable airflow patterns. A regression analysis of type frequencies versus temperature and precipitation anomalies for 1899‐1970 at stations east and west of the Continental Divide in the Rocky Mountain states shows that useful explanation of variance is obtained only for temperatures in the transition seasons and for precipitation west of the Divide in winter and east of the Divide in spring. Within‐type variability of the climatic characteristics is one source of the discrepancy. The results underline the problems encountered in trying to link climatic anomalies with atmospheric circulation characteristics.