Wildfires have significant hydrological and ecological impacts in the western U.S. Using a decades-long high-resolution regional climate simulation and wildfire observations, we investigate the antecedent hydrometeorological conditions (AHCs) of wildfires in the western U.S. during 1984-2018. During the warm seasons (April to September), the wildfire AHCs feature higher temperatures, drier soil, or stronger shortwave (SW) or longwave (LW) radiation. K-means clustering classified wildfires into four types with distinct AHCs: compound-type with high temperature, high LW and SW, and dry soil, LW-type and SW-type featuring high LW and high SW, respectively, and wet-soil-type, each representing 15%-32% of all the wildfires. The relationship between wildfire and their antecedent meteorological, hydrological, and vegetation conditions is examined based on these four types, revealing unique AHCs on the different fire types. Meanwhile, this classification reveals stronger connections between the seasonal wildfire occurrence and the seasonal AHCs at regional scales, potentially improving wildfire prediction. Increases in wildfire occurrence during 1984-2018 are dominated by increases in the compound- and LW-type, while the wet-soil-type has decreased, consistent with the long-term drying in the western U.S. Our results summarize the variability of wildfires in the western U.S., and the classification and regressions established in this study can be used to better quantify wildfire risks in a changing climate.