The power system in the Western US has, in recent years, been resistant to drought and heat wave stresses. Meanwhile, renewable and variable generation in the West continues to increase, changing regional power system dynamics. This study combines models from across sectors to evaluate the operation of the Western US power system at high resolution under the combined stresses of extreme heat waves, significant drought, and increasing renewable generation. Rather than project a single year or climate scenario, this work considers a hypothetical power system having 45% variable renewable electricity and operating under a range of extreme heat waves and droughts. The high renewable system is compared to a system representing an approximate 2010 system (about 10% renewable). The various stressors (drought, heat, and variable generation) are studied in isolation and in combination, providing insights on combined effects that enhance or mute individual stressors in water and energy systems. The operation of the system is modeled with an hourly electricity production cost model that simulates the least cost dispatch of generators given individual generator limitations and system reliability requirements. Extreme heat waves are modeled using a multivariate stochastic simulation model applied on 407 locations in the western United States to generate alternative weather years that are consistent with the spatial and temporal trends in the historical period. Exogenous models capture drought impacts on hydroelectric availability and thermal generation and heat impacts on load and thermal generation, which are integrated into the production cost model. The results show that the Western power system remains reliable despite these combined stressors, but that markers of stress such as unmet reserves and regional flow patterns are different with the 45% renewable system compared to the 10% renewable system. This work is an important step in understanding the relative importance of each of the stressors on water and energy systems and how grid vulnerabilities will change under higher penetrations of renewables.