The Influence of Terrain-Induced Circulations on Wintertime Temperature and Snow Level in the Washington Cascades

Abstract

This paper describes the mesoscale distribution of temperature and snow level in the Washington Cascades. During the winter months, when the Cascades separate relatively cold continental air to the east from warmer marine air to the west, easterly gap flows draw cold air into the major mountain passes, producing snow levels that are several hundred meters lower than over nongap locations west of the Cascade crest. Rapid warming episodes occur in these passes when winds shift from easterly to westerly, resulting in dangerous avalanche conditions. Synoptic composites indicate that such warming episodes are usually associated with the passage of a frontal system and associated pressure trough. An observational and numerical modeling case study details the mesoscale aspects of one such rapid warming event that occurred during the landfall of the January 1993 Inauguration Day cyclone. Ahead of the landfalling cyclone, synoptic-scale warm advection increased temperatures west of the Cascade crest while cold air damming held temperatures nearly steady to the east. During this period, the synoptic-scale cross-barrier pressure gradient drew cold air from eastern Washington into the mountain passes, locally lowering temperatures and snow levels. Adiabatic cooling further reduced the temperature of this cold easterly flow as it ascended into the passes. Following passage of the cyclone and the attendant occluded front, a shift in pass-level winds from easterly to westerly produced a 5°C temperature rise in Snoqualmie Pass in less than 1 h. The ability of the nonhydrostatic Pennsylvania State University/NCAR mesoscale model to simulate these local effects is discussed. Although the model produced a physically realistic simulation of the mesoscale distribution of wind and temperature during this event, the simulated cold dome east of the Cascades modified and eroded too quickly. Improvements in the initialization of the cold dome and representation of boundary layer and radiative processes may be needed to improve forecast skill.