A Case Study of a Bora-Like Windstorm in Western Washington

Abstract

This paper examines the meteorological conditions and physical processes associated with the development of strong downslope winds that caused extensive property damage in two areas of western Washington on 28 November 1979. These areas wore located downwind of the two largest and lowest passageways through the Cascade Range: the Columbia River Gorge and the Stampede Pass region. Findings are as follows: 1) The destructive winds, marked by gusts of 25?30 m s?1, appeared in conjunction with the formation of a deep cyclone offshore and the simultaneous development of an unusually powerful anticyclone inland. 2) The pressure gradient was greatly enhanced in the vicinity of the mountain range attaining values as large as 12 mb (100 km)?1. 3) Hydrostatically, the large pressure differences can be attributed to the effect of the barrier in separating cold air on the cast side from warmer air on the West. 4) Trajectory tracing revealed that the temperature difference formed rapidly as a result of the presence of strong subsidence on the Ice side and the absence of low-level subsidence in the confined, inland basin on the windward side. 5) The undisturbed flow normal to the barrier ranged from light easterly at lower levels (5?10 m s?1 at most), to zero in the layer between 600 and 700 mb, to light westerly above. Calculations are carried out to demonstrate that the wind speeds were consistent with the observed pressure differences. The large-scale pressure gradient was well predicted 36 h in advance by the limited-area fine-mesh model (LFM) of the National Meteorological Center.