More than 70% of La nina years is associated with Major Sudden Stratospherical Warming (Major SSW), what is introducing phase for strong AO- and NAO- (negative Arctic Oscillation and North Atlantic Oscillation phases).
In late December 2020, Major SSW of the last winter began, what had severe impact to evolution of circulation patterns and temperatures over the world in January and February 2021.
Unusually long AO- / NAO- phase in Winter 2020/2021 was almost 2 months long, with result of extreme coldwaves, in Europe (and Asia and Northern Africa) in January and February both, in the USA mainly in February 2021.
The result of extreme strong AO phase in February are the coldest February 2021 in part of European Russia and Siberia for decades and the coldest February 2021 in the USA since 1989 - for 32 years!
Severe coldwaves are still reported from Northern Hemisphere in the March 2021 /https://mkweather.com/central-siberia-530c-the-coldest-march-temperatures-in-history-la-nina-brings-colder-spring-but-hot-summer-2021/; https://mkweather.com/southern-finland-389c-the-coldest-national-minimum-temperature-in-march-since-1998/; https://mkweather.com/finland-30c-czechia-22c-belgium-10c-siberian-air-is-back// and the next coldwaves were announced /https://mkweather.com/snow-and-frosts-soon-back-europe-on-16-22-march-with-the-next-strong-coldwave/; https://mkweather.com/big-15-day-forecast-for-the-usa-hot-and-sunny-east-and-cold-and-rainy-snowy-west//.
Extreme coldwaves in February 2021 in the USA hit mainly regions of Northern Plains, Midwest and Texas. Extremely cold were too central and western parts of Canada and Alaska.