Inter-annual weather variability can significantly impact load profiles, renewables output, and plant operations and should be considered in planning for decarbonization to ensure system resiliency. Yet capacity expansion models typically consider weather from only a single historical year. This project completes a quantitative exploration of weather variability across historical weather years (1980-2019) and generates synthetic “extreme” weather years that impact hourly load as well as solar and wind output. This analysis then uses EPRI’s U.S. Regional Economy, Greenhouse Gas, and Energy Model (US-REGEN) to conduct scenarios to quantify and compare implications of weather variability for optimal capacity investments and dispatch in the near future (2035). The research highlights the difficulty of defining “extreme” weather years and finds that weather year selection can have material impacts on capacity expansion and system resiliency.