The Santa Ana Wildfire Threat Index: Methodology and Operational Implementation

Abstract

Santa Ana winds, common to Southern California from the fall through early spring, are a type of downslope windstorm originating from a direction generally ranging from 3608/08 to 1008 and are 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 upon ignition. 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 have been categorized. The Santa Ana wildfire threat index (SAWTI) is a tool for categorizing Santa Ana wind events with respect to anticipated fire potential. The latest version of 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 the University of California, Los Angeles (UCLA). The SAWTI uses several meteorological and fuel moisture variables at 3-km resolution as input to the Weather Research and Forecasting (WRF) Model to generate the index out to 6 days. In addition to the index, a 30-yr climatology of weather, fuels, and the SAWTI has been developed to help put current and future events into perspective. This paper outlines the methodology for developing the SAWTI, including a discussion on the various datasets employed and its operational implementation.

Winds and Gusts during the Thomas Fire

“…Southern California's Santa Ana wind is an offshore flow commonly occurring between September and May [2,4] when high pressure builds over the Great Basin [1,10]. High wind speeds in favored areas can combine with very low humidity to create a significant wildfire threat, especially when the vegetation is dry [9]. Indeed, many of the region's most significant wildfires have started during offshore wind events [8].…”

“…The "Santa Ana" winds of Southern California are a very dry, sometimes hot, strong offshore flow that is most common during the September-May time frame [1][2][3][4][5][6]. These winds can also be locally fast as well as gusty, which means they can play a crucial role in starting and spreading fires and desiccating vegetation [7], substantially elevating the wildfire threat in the region [8,9]. Santa Ana winds have been involved in some of Southern California's largest and/or most notorious wildfires, including the 1961 Bel Air fire, the Laguna fire of 1993, the 2003 Cedar and Old fires, and the Witch and Canyon fires in late October 2007, among numerous others.…”

Twenty-first century California, USA, wildfires: fuel-dominated vs. wind-dominated fires

“…Extreme wind events (with winds speeds over 70 km h −1 ) move rapidly, and knowing the trajectory of the winds early in the event and related parameters could limit losses of lives and property. Progress is being made in prediction capacity (e.g., Rolinski et al 2016;Cao and Fovell 2018), but for this to save lives, it requires effective communication systems, something that also needs further work.…”

Spatial Distribution of Wildfires Ignited under Katabatic versus Non-Katabatic Winds in Mediterranean Southern California USA

“…Owing to the close proximity of densely populated Santa Barbara and surrounding communities, Sundowner-associated fires have been some of the most destructive wildfires regionally, despite relatively smaller sizes (e.g., the 2009 Jesusita Fire consumed 80 homes despite burning only 3534 ha). These conflagrations have often occurred in spring months, suggesting a third key period of fire danger for regional fire management outside of the summer and autumn peaks [17,18]. However, the proportion of the southwestern California fire history attributable to Sundowner events has not been quantified.…”