Wildfire Ignition-Day Vapor Pressure Deficit Trend and Its Weakening Atmospheric Circulation Control Over the Western United States

Vapor pressure deficit (VPD) is a key fire weather indicator linked to increased burned areas in western US. Despite a strong increase in regional VPD due to climate change, we find no significant trend in VPD on fire ignition days (VPD_F). This discrepancy is due to a decreasing climatological mean (VPD_F-m), driven by the expansion of fires into higher-latitude and higher-altitude regions with climatologically lower VPD, and a relatively stable anomaly (VPD_F-a). The weak trend in this VPD_F-a is, in turn, a result of two opposing trends: a significant increase in the thermodynamic contribution from background warming, offset by a weakening of the dynamic contribution from circulation patterns. This lowering of the circulation “bar” for ignition, where less extreme weather is now sufficient to start large fires, explains the observed spatial expansion of fire risk, meaning regions historically less prone to fires now face heightened risk.