Decreasing frequency of low and moderate fire weather days may be contributing to large wildfire occurrence in the northern Sierra Nevada

Previous analyses identified large-scale climatic patterns contributing to greater fuel aridity as drivers of recent dramatic increases in wildfire activity throughout California. This study revisits an approach to investigate more local fire weather patterns in the northern Sierra Nevada; a region within California that has experienced exceptionally high wildfire activity recently. The annual percentages of fire season days above 90th and 95th percentile Energy Release Component (ERC) values were very low prior to 1994 (Fig. 3). Since 1994, years with noticeable percentages of exceedances (>20 %) occurred more frequently. However, neither the highest nor second highest annual exceedances occurred after 2012. Starting in the late 1990s and early 2000s there was a decreasing trend in number of fire season days below the 80th and 50th percentile ERC thresholds. Linear trend lines indicated strongly negative declines throughout the study period. The most likely impact of these declines is on the effectiveness of fire suppression, which rely on low to moderate fire weather to “gain ground” on containment. That said, even the combination of both fire weather conditions (greater high fire weather days and lesser low fire weather days) provides an imperfect explanation for the recent increases in burned area. Additional influences, such as dense forest structures, greater fuel continuity, and ignitions (both number and timing of) are likely contributing as well.