We compare 18 years of hourly mesoscale weather (wind and dryness) observations to daily fire emissions across California’s forest and savanna fuels (~80,000 “days of fire”). We find that severe weather and fire are insufficient but necessary conditions for large, rapidly spreading wildfire. Wind and dryness differentially affect different fuels across differing scales of fire emissions. Overall, the difference between wind and dryness leads to two different regimes in fire emissions: dryness dominates moderate emissions fires and wind dominates already-dry large emissions fires. The wind and dryness experienced during very infrequent, very high emissions fires are not individually as infrequent as the fires themselves. Compared to the background state of the two ecoregions, both regions need similarly anomalous concurrence of wind and dryness to support high emissions fires. However, some relative and absolute differences in wind and dryness experienced across the fuel types and across the lifecycle of the fires exist.
Additionally, we denote thresholds in the weather for both fuel types above which fires may spread rapidly. These thresholds are applied to a 41 year meteorological dataset. The spatiotemporal variation of these thresholds across California is described as are trends in those thresholds. Trends in covariate wind and dryness threshold exceedance suggest that (in the fall) there has been an increase of up to around one week in the time reaching fire-weather thresholds compared to nearly a half-century ago. However, wind and dryness individually display generally inverse trends. The increase in the number of days during which fires can start or become severe may be further enabling severe fire in California.
