Secular Changes of Precipitation in the Rocky Mountain States

Abstract

Long-term precipitation records from Montana, Idaho, Wyoming, Utah and western Colorado were obtained and carefully screened for missing data and station relocations. Each record was divided into five seasonal series and tested for homogeneity. Most of the unhomogeneous records were for the winter season, whereas there were fewest for summer and late summer seasons. Those seasonal records considered to be homogeneous were then used to examine the spatial variation of precipitation anomalies decade by decade (1891?1900, 1901?1910, etc.) from 1941?70 averages. Areas of statistically significant anomalies were identified. Generally, precipitation was high in the 1890's, 1910's and/or 1920's, 1940's and 1960's. Other decades were relatively dry, particularly the 1930's. Springs 1941?70 were wetter than the previous 30 years, but the period 1881?1910 was wetter than the 1941?70 averages. Considering summer precipitation, 1941?70 was probably the most anomalously wet period for at least 110 years and perhaps much longer. Over much of the Rockies the 1960's were extraordinarily wet. Fall precipitation was exceptionally low in the 1950's, but the 1941?60 average was still relatively high, though generally drier than the 1890's, 1910's and 1920's. Winter precipitation in recent decades has been considerably less than was characteristic of the late 19th and early 20th centuries, except in Idaho. Recurrent anomaly patterns, centered over southern Idaho, are characteristic of the winter record. When the entire secular period is considered the recent ?normal? is anomalous; in particular, summer precipitation has been unusually high and winter precipitation relatively low. One hundred years ago the climate of the Rockies was characterized by wetter winters and springs but much drier summers (and in sonic areas) drier falls compared to the post-1940 climate of the region.