The predictability of Northern Hemisphere sudden stratospheric warming (SSW) events is considered in 10 subseasonal to seasonal (S2S) forecast models for 16 major SSWs that have occurred since 1998, a larger sample size than has been considered by previous works. The four factors that most succinctly distinguish those SSWs with above average predictability are a preconditioned vortex prior to the SSW, an active Madden‐Julian Oscillation with enhanced convection in the West Pacific, the Quasi‐Biennial Oscillation phase with easterlies in the lower stratosphere, and the vortex morphology (displacement more predictable). Two of these factors appear to not have been considered in previous works focusing on a large sample of events. Most of these effects are not statistically significant at the 95% level due to the still relatively small sample size, though all would exceed a 90% criteria at least marginally. Combined, however, they account for 40% of the inter‐event spread in SSW predictability, thus indicating that SSWs with favorable precursors are significantly more predictable.

The predictability of stratospheric sudden warming (SSW) events are considered in 10 subseasonal to seasonal (S2S) forecast models for 10 SSWs over the period 1999-2009. The 10 SSWs are divided into those with above-average predictability (in one case exceeding 20 days), below-average predictability, and average predictability. The four factors that most succinctly distinguish the composite 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 SSW, and a strong pulse of wave activity in the week before the event. Other factors, such as El Nino, stratospheric preconditioning, and the morphology (split vs. displacement) are comparatively less important.

<div>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. <br><div></div></div>