A Windstorm in the Lee of a Gap in a Coastal Mountain Barrier

This paper describes a localized windstorm that struck some areas of northwest 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 Columbia, descended into a mesoscale gap in the Coast/Cascade Mountains and then accelerated ageostrophically to the west. This gap acceleration is explained quantitatively by a three-way balance among the pressure gradient force, friction, and inertia. The flow maintained its integrity as a narrow current of high-speed air as it exited the gap and subsequently accelerated over water. Troughing in the lee of the Cascade Mountains enhanced the horizontal pressure gradient over northwest Washington; this pressure gradient approximately balanced frictional drag resulting in only minimal acceleration. Farther south the flow decelerated as the current spread out horizontally.