Topographically Forced Convergence in Western Washington State

Several times a year when the low-level winds from off the Pacific Ocean are within a narrow range of speed and direction, air passes both north and south of the Olympic Mountains of Washington State and is forced to converge in Puget Sound by the north-south oriented Cascade Range. This phenomenon, termed the Puget Sound convergence zone, often results in a band of cloudiness and precipitation in northern and central Puget Sound with clear, subsiding air to the north and south. This paper presents the results of a series of case studies in which the structure of the zone, its meteorological manifestations, and the environmental conditions necessary for its formation are explored. It is shown that the convergence zone is skillfully forecast by surface coastal winds and undergoes a strong annual and diurnal cycle, being most frequent during the late spring and early summer months and during the afternoon and early evening. It is also found that the zone is structurally similar to a shallow cold front and has a significant influence on the precipitation climatology of the region.