The Santa Ana Wildfire Threat Index

Santa Ana winds, common to southern California during the fall through early spring, are a type of katabatic wind that originates from a direction generally ranging from 360°/0° to 100° and is usually accompanied by very low humidity. Since fuel conditions tend to be driest from late September through the middle of November, Santa Ana winds occurring during this time have the greatest potential to produce large, devastating fires when an ignition occurs. Such catastrophic fires occurred in 1993, 2003, 2007, and 2008. Because of the destructive nature of such fires, there has been a growing desire to categorize Santa Ana wind events in much the same way that tropical cyclones and tornadoes have been categorized. The Santa Ana Wildfire Threat Index (SAWTI) is an index that categorizes Santa Ana wind events with respect to anticipated fire potential. The index was officially made available to the public in September of 2014, and has since been featured on The Weather Channel and local news media. The SAWTI was designed to help fire managers be better informed when making decisions regarding the mobilization and allocation of fire resources, and it also helps the public to be better prepared for a potential catastrophic fire. The index has been a result of a three and a half year collaboration effort between the USDA Forest Service, the San Diego Gas and Electric Utility (SDG&E), and UCLA. The SAWTI uses several meteorological and fuel moisture variables informed from 3km resolution Weather Research and Forecasting (WRF) model forecasts to generate the index out to 6 days. In addition to the index, a 30-year climatology of weather and fuels data, along with the index itself has been developed to help put current and future events into perspective. The methodology for developing the SAWTI, which includes a discussion on the various datasets employed along with the challenges of incorporating fuel moisture information into the index, will be presented.