Underground nuclear explosions (UNEs) produce radionuclide gases that may seep to the surface over weeks to months. The objective of this research was to quantify the impact of uncertainties in hydrologic parameters (fracture aperture, matrix permeability, porosity, and saturation) and season of detonation on the timing of gas breakthrough. Numerical sensitivity analyses were performed, with barometric pumping providing the primary driving force for gas migration, for the case of a 1 kt UNE at 400-m depth of burial. Gas arrival time was most affected by matrix permeability and fracture aperture. Gases having higher diffusivity were more sensitive to uncertainty in the rock properties. The effect of seasonality in the barometric pressure forcing was found to be important, with detonations in March the least likely to be detectable based on barometric data for Rainier Mesa, Nevada. Monte Carlo realizations were performed with all four parameters varying simultaneously to determine their interrelated effects. The Monte Carlo method was also used to predict the window of opportunity for 133Xe detection from a 1 kt UNE at Rainier Mesa, with and without matching the model to SF6 and 3He data from the 1993 Non-Proliferation Experiment. Results from the data-blind Monte Carlo simulations were similar but were biased toward earlier arrival time and less likely to show detectable 133Xe. The estimated timing of gas arrival may be used to deploy personnel and equipment to the site of a suspected UNE, if allowed under the terms of the Comprehensive Nuclear Test-Ban Treaty.