Mass fire behavior created by extensive tree mortality and high tree density not predicted by operational fire behavior models in the southern Sierra Nevada

•Most western US frequent-fire forests have been changed by attempted fire exclusion.•Drought and bark beetle tree mortality can exacerbate wildfire behavior and effects.•Dead biomass and tree densities the most important variables predicting fire severity.•Mass fire behavior cannot be predicted by operational fire models.•Forest restoration needed at necessary paces and scales to prevent adverse wildfires.