Stratospheric variability is an important potential source of predictive skill of surface weather on sub-seasonal to seasonal (S2S) timescales. In particular, variations of the stratospheric polar vortex can perturb the large-scale extratropical circulation for weeks to months. In this seminar I will show the influence of initializing forecasts during weak and strong Northern Hemisphere polar vortex events, and during the springtime breakdown of the vortex (the so-called "final warming"), on weeks 3-4 predictive skill of near-surface temperature. In general, skill increases and root mean square error decreases across the Northern Hemisphere for forecasts initialized during weak and strong vortex events and early final warming events relative to control forecasts; but in some regions, skill decreases. I will also briefly discuss the most recent disruption of the Southern Hemisphere polar vortex and how it influenced the forecast for the anomalously hot and dry austral spring in 2019. These results come in part from international community efforts within WCRP/SPARC to analyze the S2S project database of historical forecasts in order to better understand how stratospheric information contributes to surface predictive skill.