A Severe Windstorm in the Lee of the Cascade Mountains of Washington State

Abstract

This paper describes a severe, leeside windstorm that struck parts of western Washington State on 23?25 December 1983. It is shown that strong winds, some exceeding 50 m s?1, were mainly limited to swaths downwind of gaps in the Cascade mountains. An examination of the December 1983 event and previous major windstorms indicates that they are generally associated with strong sea level pressure gradients, a stable layer near crest level east of the Cascades which subsides west of the mountains, and light to moderate easterly flow at crest level. The strongest surface winds during the December 1983 event were associated with the period in which the sea level pressure gradient across the Cascades was increasing rapidly. Furthermore, the variation of the leeside winds appears to be modulated by the flow normal to the mountains. It is shown that the horizontal pressure gradients can explain the strong winds and that lee wave amplification, although possible, is probably not of major importance.