A WINDSTORM IN THE LEE OF A GAP IN A COASTAL MOUNTAIN BARRIER

This paper describes a localized windstorm that struck some areas of n orthwest Washington State on 28 December 1990 with winds exceeding 45 m s(-1), resulting in extensive property damage, treefalls, and power outages. Arctic air, originating within the interior of British Columb ia, descended into a mesoscale gap in the Coast/Cascade Mountains and then accelerated ageostrophically to the west. This gap acceleration i s explained quantitatively by a three-way balance among the pressure g radient force, friction, and inertia. The flow maintained its integrit y as a narrow current of high-speed air as it exited the gap and subse quently accelerated over water. Troughing in the lee of the Cascade Mo untains enhanced the horizontal pressure gradient over northwest Washi ngton; this pressure gradient approximately balanced frictional drag r esulting in only minimal acceleration. Farther south the flow decelera ted as the current spread out horizontally.