At long lead-times, weather models drift towards their own climatology and exhibit bias. You may be tempted by CFSv2 forecasts suggesting a weak stratospheric #PolarVortex this winter. But use with caution: the CFSv2 has an extremely strong early-season weak-vortex bias! pic.twitter.com/DlNpy1jnaW

— Dr Simon Lee (@SimonLeeWx) September 10, 2019