Verification of Subseasonal-to-Seasonal Forecasts for Major Stratospheric Sudden Warmings in Northern Winter from 1998/99 to 2012/13

This study reports verification results of hindcast data of four systems in the subseasonal-to-seasonal (S2S) prediction project for major stratospheric sudden warmings (MSSWs) in northern winter from 1998/99 to 2012/13. This report deals with average features across all MSSWs, and possible differences between two MSSW types (vortex displacement and split types). Results for the average features show that stratospheric forecast verifications, when further averaged among the four systems, are judged to be successful for lead times around 10 d or shorter. All systems are skillful for lead times around 5 d, whereas the results vary among the systems for longer lead times. A comparison between the MSSW types overall suggests larger forecast errors or lower skill for MSSWs of the vortex split type, although the differences do not have strong statistical significance for almost all cases. This limitation is likely to at least partly reflect the small sample size of the MSSWs available.