The west coast thermal trough: structure, evolution and prediction

The West Coast thermal trough plays a crucial role in Pacific Northwest warm season weather and climate. However, its initiation, 3-D structure, and evolution are still poorly understood despite the fact that it is the most important mesoscale warm season feature over the U.S. West Coast. During the warm season, an inverted thermal trough is usually present over the Central Valley of California as a result of strong diabatic heating coupled with subsidence and downslope flow. This thermal trough often extends northward into the Pacific Northwest region, and in this scenario, western Oregon and Washington receive their warmest weather of the year. As the thermal trough moves onshore, it often produces an abrupt wind shift to cooler marine air and the onset of low-cloud cover. The movement of the thermal trough inland also has a major impact on fire weather given the sudden wind shift and steeper lapse rates. These dangerous conditions are known to drastically increase the potential for the initiation and spread of wildfires. The position and movement of the thermal trough also has a large impact on wind energy, particularly over eastern Oregon and Washington where a number of wind farms are located.

In this presentation, a robust definition of the West Coast thermal trough is discussed. This definition is used to generate a climatology of this feature, both interannual and seasonal, using data from NCEP's North American Regional Reanalysis (NARR). Specific dates of thermal trough events pulled from the climatology are used for synoptic compositing to determine the large-scale evolution associated with thermal trough initiation and evolution. Also, evolutions of temperature, wind, relative humidity, and pressure at point locations are analyzed in order to further understand the evolution and passage of the thermal trough. By using model output from the Weather Research and Forecasting (WRF) model with 12- and 4-km grid spacing, as well as NARR data, the 3-D structure of the thermal trough is described.