Terrain-driven Winds and Gusts During the Thomas Fire

The Thomas fire began on December 4th, 2017 and, driven by a prolonged Santa Ana wind event, spread rapidly to become the largest wildfire in modern California history (since 1932). It reportedly started near Steckel Park and Thomas Aquinas College, located north of Santa Paula, at 6:32 PM, and was finally declared contained only after 39 days and almost 282,000 acres burned. Apparently, a second fire started independently about 30 min later on Koenigstein Rd, a few miles north of the main ignition, and merged with the Thomas fire later that night. According to Cal Fire, the Thomas fire destroyed more than 1000 structures and damaged almost 300 others. Mudflows caused by heavy rains the following month added an additional 20 fatalities to the two killed directly by the fire.

The fire started in an area lacking many meteorological stations. During the fire onset, some of the nearest stations, in fact, were reporting winds that were suspiciously light and variable. As a consequence, we turn to numerical modeling to estimate winds and gusts occurring before and during the fire. WRF simulations employing domains telescoping to 2/3 km resolution over the fire area were made, using a model configuration verified and calibrated against wind observations obtained from this and other Santa Ana events (cf. Cao and Fovell 2016, 2018; Fovell and Cao 2017). The simulations suggest that strong but locally focused winds occurred at the suspected Thomas and Koenigstein ignition points around the fire onset time, resulting from a downslope windstorm. In contrast, most of the surrounding stations are found to be sited in wind shadows formed by the terrain.