The West Coast Thermal Trough: Climatology and Synoptic Evolution

Abstract

Abstract Although the West Coast thermal trough (WCTT) is the most important mesoscale feature over the U.S. west coast during the warm season, its initiation, evolution, and structure are not well understood. Originating in the southwest United States, this inverted trough can extend northward into Oregon, Washington, and British Columbia, with large impacts on temperature, wind, humidity, and air quality. Using NCEP’s North American Regional Reanalysis (NARR), annual and diurnal climatologies of WCTTs reaching the northwest United States were constructed. For the entire year, WCTTs are most frequent along the coast near the California–Oregon border, with weaker maxima west of the Cascade and coastal mountains. Over the coastal region, they occur most often during autumn, while east of the Cascade Mountains, the highest frequency is during summer. There is strong diurnal variability in WCTT frequency during the summer, with little diurnal variation in winter. Though compositing revealed important seasonal differences in WCTT evolution, some common features emerged. An upper-level ridge moves over the northwest United States and associated high pressure builds in the lower troposphere over southwest Canada, resulting in the development of near-surface easterlies and downslope flow over the western slopes of major terrain barriers of the region. Simultaneously, the WCTT extends northward from California into the Pacific Northwest. As the synoptic configuration changes, the WCTT either moves eastward and merges with the larger thermal low over the Great Basin region, which is most common in summer, or it recedes back into California and dissipates, as often happens in winter.