2019 will start off with the #SSW becoming "major" on New Year's Day (this is defined as a reversal of the zonal-mean zonal wind at 10 hPa and 60N). This is now likely to be a vortex split event, rather than the displacement which was originally forecast. pic.twitter.com/95qaGfzVbd

— Dr Simon Lee (@SimonLeeWx) December 30, 2018