The role of the stratospheric state in regulating planetary wave activity during sudden stratospheric warming (SSW) events and subsequent surface impacts is still not well understood. Using multiple subseasonal-to-seasonal (S2S) forecast models from the Stratospheric Nudging and Predictable Surface Impacts (SNAPSI) framework, we investigate the 2019 Southern Hemisphere SSW through both free forecasts and stratospheric sensitivity experiments. In the control and nudged experiments, the stratospheric state is constrained toward the zonally symmetric circulation climatology and the observed real-time evolution, respectively, allowing the stratospheric contribution to wave evolution and surface responses to be isolated. Forecasts show that biases in planetary wave activity arise from different processes during different stages of the event. In the early stage, underestimated tropospheric forcing dominates the weak upward wave propagation. However, during the central stage, upper-stratospheric wave activity biases are further regulated by the stratospheric state through changes in the critical line. Moreover, the nudged stratosphere induced a weak but detectable enhanced tropospheric favorable pattern. These results highlight that the stratosphere is not only a passive responder to tropospheric wave forcing but also actively regulates planetary wave evolution. Diagnosis of the three-dimensional wave activity flux (WAF) further reveals a zonally asymmetric downward stratospheric signal in early October, which is forecast in the nudged minus control experiments but with a farther westward shift than observed. Models tend to forecast an earlier than observed negative Southern Annular Mode (SAM)-like pattern in the troposphere and stronger regional precipitation contrasts in the nudged minus control experiments. Several models simulate significantly drier responses in eastern Australia following the downward-propagating SAM in the nudged experiments, while the remaining models show a more dominant role for the lower troposphere. In contrast, the near-surface impacts over South America show large inter-model spread in the nudged experiments, possibly due to the westward-biased forecasts of downward WAF in models.