Windstorms along the Western Side of the Washington Cascade Mountains. Part I: A High-Resolution Observational and Modeling Study of the 12 February 1995 Event
This paper describes the leeside wind event of 12 February 1995 in which a swath of strong low-level easterly flow, with gusts to 35–40 m s−1, extended downwind of a major gap in the Cascade Mountains. Over the Puget Sound lowlands, low-level easterly flow approaching the Olympics was decelerated to less than 5 m s−1, resulting in enhanced (over 30 cm) snowfall along the windward side of the barrier. In addition to conventional synoptic observations, high-resolution data sources were utilized: profilers on both sides of the Cascades, a WSR-88D Doppler radar located in northern Puget Sound, and nonhydrostatic model simulations down to 1-km resolution. With the exception of underpredicting heavy snowfall to the east of the Olympics, many of the mesoscale features of this event were realistically simulated, including cold-air damming over eastern Washington, strong winds downwind of the gaps, a mountain wake downstream of Mount Rainier, and flow splitting around the Olympics. A major objective of this study was to determine the role of the three-dimensional topography of the Cascades on the leeside windstorm and the relative importance of gap flow and mountain wave accelerations. Low-level model trajectories showed a tendency for the air to pass westward through the major gaps in the Cascades. However, additional simulations, in which the major Cascade gap was slowly removed, indicated that filling the lower passes of the gap (<1600 m) had only a small effect on the leeside winds due to the considerable depth of the cold air (∼2800 m). When the lowest terrain was increased to 2000 m, the barrier intersected the lower portion of the stable layer aloft and the leeside winds were dramatically reduced. A simulation without the Olympics showed that early in the event, upstream blocking by this barrier decelerated the winds over and to the west of central Puget Sound. As cold air pooled within the Puget Sound basin, a structure similar to a hydraulic jump was enhanced above the Cascade lee slopes, weakening the winds along the lower Cascade foothills substantially.