Recent Escalation of Winter Fire-Weather Risk in the Southwestern United States: Roles of Anthropogenic Warming and the Negative Pacific Decadal Oscillation.

Winter wildfires in the southwestern United States (SWUS), especially Southern California, are less frequent than summer and autumn fires but can be highly destructive, as demonstrated by the January 2025 events. How winter anthropogenic warming and internal climate variability shape winter fire weather risk in the SWUS remains unclear. Using historical observations, reanalysis, and model simulations, we show that winter fire weather risk in the SWUS has nearly doubled during 1984-2024. An attribution analysis associates approximately 33% of this increase with anthropogenic ocean warming and 53% with internal Pacific Decadal Oscillation (PDO) variability. Winter North Pacific midlatitude warming associated with anthropogenic forcing and the negative PDO strengthens westerly winds and favors an eastward shift and southward extension of winter blocking anticyclones. While warming and prolonged drought from autumn into winter precondition fuels, winter midlatitude North Pacific blocking anticyclones extending into the western United States intensify winter surface air warming and vapor pressure deficit, and reduce precipitation and soil moisture in the SWUS, thus exacerbating winter fire weather risks in the SWUS. CMIP6 projections support the escalation of winter fire weather risks under continued warming.