According to GEOS & ECMWF analyses, 10 hPa 60°
N zonal-mean zonal winds turned back to westerly on August 31 indicating the start of the 2019-20 stratospheric #PolarVortex season. Assuming it is replicated in ERA reanalysis, this would be the latest on record since 1979 by 1 day. pic.twitter.com/sqKyfYgTQr

— Dr Simon Lee (@SimonLeeWx) September 1, 2019