Dynamics of Orographic and Thermal Forcing on the Environment Conducive to the Formation of the Tubbs Fire (2017)

The Weather Research and Forecasting Model (WRF-ARW) was used to simulate the downscale evolving atmospheric dynamical processes conducive to the intensification and propagation of the Tubbs Fire (2017), which impacted Napa and Sonoma Counties, CA. As this wildfire spread quickly and erratically through complex mountainous terrain, due to the Diablo Winds, the wildfire led to multiple fatalities and over 36,000 acres burned. The simulations and supporting observations during the pre-Diablo Wind period indicated a well-defined inverted surface trough in northern CA.’s Central Valley along with a strong amplifying trough in the middle troposphere producing cold frontogenesis over the Sierra Nevada. Jet streak diagnostics, along with simulated and observed soundings from Reno, Nevada, indicated an indirect exit region descending circulation in conjunction with cold air advection caused by the southwestward low-level jet under the jet’s entrance region. As the organizing jet streak departed, a favorable environment for mountain wave amplification became apparent. Leeside sinking motion consistent with wave steepening intensified, resulting in strong winds in northern California, seen in cross-sectional analysis. Sensitivity tests consisted of removing the Sierra Nevada Mountain Range (NoSN), complete removal of terrain east of the coastal mountains (NoMN) and removing the diurnal heating cycle (NoDH). Each test produced unique results which help explain the complex terrain influence on the Diablo Winds, as well as the influence on bore genesis off the coast of CA.