Human- versus lightning-ignited large fires are distinctly different in the western US. Human-ignited fires tend to occur in both dry and wet conditions across all seasons, while lightning-ignited fires are more concentrated in dry summers. Both types of large fires are highly probable when dry conditions surpass critical thresholds of vapor pressure deficit (VPDt), but the VPDt thresholds likely differ between them. The distinct flammability thresholds (VPDt), and the associated fire weather risk driven by external natural and anthropogenic forcings is unclear. We quantified their respective VPDt and associated fire weather risk and its changes by leveraging machine learning and Earth system models and agency reported fire records. We found that human-ignited large fires had consistently lower VPDt and greater annual number of flammable days in all western US ecoregions. Flammable days for human-caused large fires increased by 21%, more rapidly than those caused by lightning during 1979-2020. Compared to natural variability and other anthropogenic forcings, greenhouse gas (GHG) emissions had a dominant impact of 81% on increasing human-related flammable days. Our results show clearly that fire weather risk is jointly affected by flammability thresholds and external forcings, indicating likely more flammable conditions for human-ignited large fires in western US.