The Cascade Mountain range, located about 250 km inland from the Pacific Ocean, extends from north to south through the entirety of Washington and Oregon. This barrier is continuous except for a handful of gaps, of which only the Columbia Gorge provides a near sea-level conduit. Because of this, gap flow is common in the Columbia Gorge and plays a profound role in defining the climate of the surrounding region, which includes the city of Portland. Examples of phenomena associated with Columbia Gorge gap flow are damaging winds, frigid cold, snowfall and freezing rain.

A detailed study has been undertaken to improve existing knowledge of the impact of the Gorge and the dynamical processes occurring within it. This was achieved through three separate sub-projects. A thorough climatological study assessed quantitatively the influence of gap flow through the Gorge on weather phenomena in the study area. Synoptic composites were produced for different Gorge influenced weather events to identify and analyze associated synoptic patterns. Finally, a detailed case study of a Gorge gap flow event, including both observational analysis and high-resolution modeling, was performed. The case study analysis is the primary focus of this poster presentation.

Between December 11 and December 15, 2000 an easterly gap flow event in the Columbia Gorge brought a sustained period of moderate winds and freezing temperatures to the western exit of the Columbia Gorge. Overrunning precipitation falling into the gap outflow through the latter part of the event resulted in a mix of frozen precipitation in the Portland metropolitan area. An observational analysis of the case was undertaken using traditional surface-based data assets, as well as more recently developed observing platforms. In particular, extensive use was made of ACARS data from commercial aircraft arriving and departing Portland International. Radial velocity data from the NWS WSR-88D radar was also utilized. In addition, the case was modeled at high resolution (grid spacing down to 444 meters was used) using the Penn State MM5 mesoscale model. The purpose of the modeling part of the study was twofold. The first objective was to verify that the Columbia Gorge region can be modeled using a high-resolution mesoscale model and determine what vertical and horizontal resolution is required to accurately represent the airflow through it. The second was to use model output with the observational analysis to define the structural characteristics of the flow within the Gorge and in the exit region and gain a better obtain insight into the dynamical processes governing the flow.