Conclusions Severe clear-air turbulence, including serious damage to a DC-8 cargo jet, was caused by strong mountain wave activity. The mountain wave and its temporal evolution during rapidly changing large-scale conditions were uniquely observed with ground-based lidar, wind profilers, and RASS. These observations revealed a localized flow reversal within an upper-level jet that represents a region of mountain wave breaking. A downslope windstorm occurred during the time when the vertical shear of the cross-mountain wind was weakest (i.e., before a strong upper-level jet moved overhead), the mountain top winds were >20 m/s, and a strong stable layer was near mountain top. Significant changes in the wave's vertical structure were observed over less than 3h. Comparisons of the wave observations with numerical simulations suggests that the strong turbulence could be diagnosed from an operational model, and that a high-resolution numerical model could reproduce key wave features even within rapidly changing larger-scale conditions by using time-dependent lateral boundary conditions from a data assimilation system.