Long-term power system planning is increasingly affected by uncertainty in electricity demand growth, technology costs, transmission expansion, and policy implementation. This paper presents a Morris-based global sensitivity analysis framework integrated with the GenX capacity expansion model for the New York 2040 planning system. The framework evaluates uncertainty across policy, technology, transmission, and demand assumptions while preserving zonal structure within a multi-region planning environment. Multiple policy configurations were evaluated under the Current Policy 2040 scenario using grouped zonal perturbations and repeated GenX optimization runs. Morris elementary effects were computed for both system-wide and zonally resolved outputs, including system cost and zonal cost metrics.

Results show that electricity demand growth remains the dominant driver of statewide planning costs across evaluated policy configurations. Policy activation altered spatial sensitivity behavior, particularly in downstate regions where zonal costs were strongly influenced by natural gas retirement assumptions. The study demonstrates the value of zonally resolved global sensitivity analysis for identifying regional planning risks and uncertainty interactions within large-scale capacity expansion planning models. The proposed framework provides a scalable approach for evaluating uncertainty propagation across policy-constrained multi-zone power system planning environments.