Explosive wildfire growth and the west coast thermal trough

The West Coast thermal trough plays a crucial role in Pacific Northwest warm season weather and climate. When a ridge aloft moves over the Pacific Northwest and low-level high pressure associated with it builds inland, east of the Cascade Mountains, offshore flow develops. This flow descends the western slopes of the Cascades, adiabatically warming and drying along the way. This results in a hydrostatic reduction in pressure, thus forming the West Coast thermal trough. Sensible heating at the surface may also play a role in thermal trough development. When thermal trough conditions occur, western Oregon and Washington experience above-average temperatures and low relative humidity. As the thermal trough moves onshore, it often brings an abrupt wind shift, a drop in temperature, and sometimes the onset of cloud cover. As these so-called marine pushes can be quite strong and can occur abruptly, the general public, particularly boaters and the aviation community, must be prepared for potentially hazardous conditions. Thermal trough movement can also have a large impact on wind energy generation, and is associated with the important ramp-up, ramp-down problem.

Weather conditions associated with the position and movement of the West Coast thermal trough are known to drastically increase the potential for the spread of wildfires and therefore put the lives of firefighters at risk. When the thermal trough is west of the Cascades, hot dry winds spread westward out of the Cascades, leading to the fanning and growth of existing wildfires. When thermal troughs move over the mountains, explosive fire growth can occur. In this presentation, several case studies will be analyzed in which a thermal trough passed over an existing wildfire and rapid growth of that wildfire ensued. Output from the Weather Research and Forecasting (WRF) model with 36-, 12-, and 4-km grid spacing will be used to analyze these cases. It is shown that strong vertical motion associated with thermal trough passage can greatly enhance wildfire strength. Important findings regarding the relationship between explosive fire growth and thermal troughs will be discussed, as well as the implications for future fire weather prediction in the Pacific Northwest.