The Northern California Fires of October 2017: A Terrain-Induced Downslope Windstorm Well Predicted by Mesoscale Models

During the evening of October 8, 2017, fast-spreading fires developed in the high terrain of Sonoma County and, driven by strong winds, descended into nearby heavily populated regions, including the urban area of Santa Rosa. As a result of these fires, 44 people lost their lives, approximately 9000 structures were destroyed, and total economic loss has been estimated to exceed 15 billion dollars. Winds gusting to 60-90 kts were observed downstream (southwest) of the crests of the regional domain and appear to have both initiated (by stressing the electrical distribution system) and driven the wildfires that evening.

This presentation will describe the meteorology of the Northern California fires, showing that the winds were associated with strong downslope winds. Known locally as the Diablo Winds, this strong flow is connected with high pressure building into the intermountain west, which results in strong offshore pressure gradients, a stable layer near and above crest level, and a critical level above crest level. Both observations and the output of a high-resolution WRF simulation for this event will be described. The realism of current high-resolution modeling will be evaluated for this wind/firestorm, and the potential use of such modeling for reducing the impacts of such events will be examined. Of particular importance is the excellent performance of current operational models, such as HRRR.