Winter snowpack was investigated to determine spatial and temporal climate variability in a five-state region (Colorado, Idaho, Montana, Utah, and Wyoming) in the northern Rocky Mountains, covering the period 1951–85. Annual 1 April snowpack (SN) measurements were selected for analyses.
Three basic and persistent patterns of annual SN values surfaced: years with a consistent anomaly over the entire region (wet or dry); years with a distinct north-to-south gradient; and average years. Nearly 90% of the nonaverage annual SN patterns were explained by the frequency of seven 500-mb winter synoptic patterns.
The wet-north-dry-south gradient SN patterns occurred only before 1974, and the dry-north-wet-south gradient SN patterns did not occur before 1973. The long-term wet and dry periods experienced in the northern and southern areas of the five-state region are due to periods when one of the two north-to-south gradient SN patterns occurred and are explained by the changes in the frequency of synoptic patterns.