We propose a method to assess the impact on power system dynamic performance of operational uncertainty caused by variability in the system supply side. Operational uncertainty is not new to power systems, e.g., demand variability. However, with the increased penetration of renewable-based generation, operational uncertainty will extend to a significant portion of the supply side, which may have an impact on system dynamic performance, e.g., frequency or voltage deviations beyond prescribed operational requirements. To address the problem, we propose the use of reachability analysis techniques, which will provide bounds on worst-case deviations of system variables that must remain within certain bounds. The method is illustrated with several examples.