Recalibrating the Canadian Fire Weather Index for Southern California’s Downslope Wind Driven Fire Regimes

Wildfire danger in Southern California is strongly influenced by episodic offshore wind events and extreme atmospheric dryness, conditions that are not fully captured by the standard Canadian Fire Weather Index (CFWI), which was developed for boreal forest environments using noon-based meteorological inputs and has been widely applied by various climatic and ecological regimes. This study presents a regionally calibrated adaptation of the CFWI to improve representation of wind-driven fire danger in San Diego County’s chaparral-dominated landscapes. Using high-resolution observation-based meteorological data, the CFWI was recalibrated by replacing noon observations with daily maxima of temperature, relative humidity, and wind speed, applying a region-specific day-length factor, and introducing conditional modifiers based on wind direction and humidity thresholds. Evaluation over the 2000–2009 period, including extreme fire years such as 2003 and 2007, shows that the recalibrated CFWI substantially improves the ability to distinguish large wildfire days, particularly during peak Santa Ana wind seasons. A 25% Initial Spread Index adjustment triggered by easterly (negative zonal) winds and low relative humidity provided the greatest improvement, especially in autumn and winter. Results also highlight strong seasonal and spatial variability, supporting a seasonally adaptive calibration strategy. This work demonstrates that physically motivated recalibration is needed for application of the CFWI to wind-dominated fire regimes, providing a more reliable tool for operational fire danger assessment and wildfire preparedness in locations prone to extreme wind driven wildfires.