Effects of Nocturnal Atmospheric Drying Events on Fire Weather Conditions in Complex Terrain

The San Francisco Bay Area is known for its complex terrain and the corresponding impact it has on local weather. One such geographical feature is the Diablo Range which extends south along the East Bay to heights above 1,000 m. This isolated region has its own unique weather phenomenon that significantly enhances wildfire development. During typical nocturnal hours, boundary layer temperatures decrease, relative humidity values increase, and fire behavior is limited. However, in the Diablo Range there have been several cases where, in the middle of the night, humidity levels suddenly decrease and exacerbate fire weather conditions. One such drying event resulted in one of the largest fire shelter deployments in U.S. history. To provide the firefighting community with knowledge of this hazardous phenomenon, the synoptic and mesoscale features that influence the occurrence and intensity of such events are investigated.

The two interacting forces that determine humidity fluctuations are strong subsidence aloft and a stable marine layer. The subsidence responsible for these drying events is associated with negative vorticity and upper level convergence, most often the product of an upper level shortwave trough. While subsidence aloft can persist on the scale of days, the nocturnal nature and sudden fluctuations of drying events depends on the depth of the marine layer and diurnal flow of the land-sea circulation. An 11-year analysis found an average frequency of 9.1 cases per fire season with occurrences five times more likely in early morning hours than afternoon hours. Nocturnal atmospheric drying events have also been observed as far south as the San Gabriel Mountains and as far north as the Klamath Mountains.