Authors: A. LeRoy Westerling; Matthew Hurteau; Shuang Liang; Joseph CrockettIn the arid western US, considerable biomass is concentrated in montane forests, where orographic effects and lower temperatures favor winter snow accumulation. Warming and more variable precipitation are fostering more frequent drought, earlier snowmelt, and greater climatic extremes, extending fire seasons and increasing the frequency and size of large forest wildfires. While large forest wildfire frequencies are regionally sensitive to timing of spring snowmelt, western US forests exhibit diverse responses to warming. Forests with historic mean snow-free periods of approximately two to four months and high cumulative spring and summer AET have been most sensitive to changes in spring timing. Mid-elevation forests in the US Rocky Mountains and the Sierra Nevada have had the largest forest areas with the most drying associated with early spring snowmelt timing compared with late spring snowmelt timing since the mid-1980s, and show the greatest increases in large-fire frequency from early to late snowmelt seasons. Wildfire frequency and burned area in Pacific Northwest forests have increased more rapidly, albeit from a low base, in the most recent two decades. Projected changes in forest vegetation and wildfire likewise indicate that continued warming may similarly elicit diverse responses in regional forests.