Using reanalysis, observations, and subseasonal to seasonal (S2S) forecasts, the most recent sudden stratospheric warming (SSW) in January 2021 and its predictability are explored. Previous work has shown that SSWs can be forecasted at relatively long lead time if favorable conditions are present. However, favorable conditions were not present for this most recent SSW, which occurred under the tropical westerly quasi-biennial oscillation (QBO) and weak convection over tropical Pacific. In mid-December 2020 and early January 2021, the Ural ridge and East Asian trough were anomalously strong, corresponding to enhanced climatological wavenumber 2. In late December 2020 and mid-January 2021, negative (positive) height anomalies over the North Pacific (Atlantic) enhanced the climatological wavenumber 1. Alternate wave pulses by the wavenumber 1 and 2 finally led to the SSW onset and the long lifetime of the January 2021 SSW. As the composite for QBO westerlies displays a strengthened stratospheric polar vortex, the weak vortex in January 2020 was not attributed to this tropical forcing. Therefore, the predictability for the occurrence of the January 2021 SSW is not beyond two weeks with the required hit ratio >50%. Splitting of the vortex adds further difficulty to the prediction of this event. The cold anomalies over North Eurasia initiated one to two weeks before the SSW onset were likely due to the SSW precursors, which then persisted until mid-January 2021 as the SSW signal began to propagate downward. However, the persistent cold anomalies can only be forecasted in SSW-hit members and SSW-hit S2S models. The observed Arctic sea ice loss is unlikely to extend the predictability of this event in S2S models.
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