The predictability of sudden stratospheric warming (SSW) events are considered in 10 subseasonal to seasonal (S2S) forecast models for 16 SSWs over the period 1998 - 2021. The four factors that most succinctly distinguish those SSWs with above average predictability are an active Madden-Julian Oscillation with enhanced convection in the West Pacific, the Quasi-Biennial Oscillation phase with easterlies in the lower stratosphere, a strong pulse of wave activity before the event, and the morphology (displacement more predictable). However none of these effects are statistically significant at the 95\% level using a two-tailed t-test due to the relatively small sample size. Strong events are more predictable if one focuses on the question of whether the models predict a SSW, though not if one focuses on the absolute error of the anomalous stratospheric easterlies. Other factors, such as El Nino and stratospheric preconditioning are comparatively less important.
