Examining Trends in Fire-Weather Conditions across California

Abstract:

Wildfire seasons in recent years have incurred substantial environmental and socio–economic impacts across California. The combined effects from the 2017 and 2018 fire seasons included 130 fatalities, more than 20 billion dollars in costs, and over 3 million acres of burned land. In fact, seasons like 2017 and 2018 may even be becoming more common, with a general perception among firefighting agencies that wildfires are becoming larger, more intense, and more difficult to fight. The question thus arises as to how much extreme fire-weather conditions have changed across the state in recent years. This talk will highlight results from a statistical analysis of the basic synoptic conditions that underpin extreme fire–weather conditions: high surface temperatures, high surface winds, and low relative humidity. These conditions are examined in daily ERA5 reanalyses for each season over a 40-year period between 1979–2019. Over this period, we find statistically significant upwards trends in 2-meter temperature and downward trends in near-surface relative humidity across fire-weather hotspots associated with Santa Ana winds in southern California, Sundowners in Santa Barbara, and Diablo winds in the San Francisco Bay Area. Additionally, surface wind speeds exhibit upward trends over the Sierra Nevada mountains, while decreasing near coastal regions. Furthermore, this talk will also highlight trends in the Canadian Forest Fire Weather Index (FWI), and via multivariate regression, evaluate the relative contribution of temperature, humidity, and surface winds to changes in this index. This talk will thus highlight not only where and to what degree extreme fire-weather conditions have changed across California, but also the factors driving this change.