In 28 years of seasonal ECMWF guidance issued in September, there was a signal (≥1σ) for an SSW in 4 Decembers before this year.
— Ben Noll (@BenNollWeather) September 18, 2021
An SSW happened in 3 out of 4 of those winters, but there were misses in the early 2000s.
The signal in 2021-22 is the 2nd strongest of all cases 📶 pic.twitter.com/w1QWChSou5