Originally Posted by Billcarson
Was the main problem with last yrs ssw that there wasnt enough downwelling? I presume the ssw of Feb 2018 was more textbook even though as you say every ssw is different.
Yes, it failed to downwell so there was a lack of -NAM and in fact, it went very positive in February at the same time that there was a big ramp up in the zonal mean zonal winds.
The February 2018 SSW was abrupt and perfect to developing a Beast from the East type setup as the initial warming occurred over Siberia (this allowed a Siberian High to develop once downwelling was successful) and then we saw a secondary warming around Greenland a few days later which resulted in a split of the SPV. The vortices were placed to the west of America and southeast Europe.
We saw the first signs of downwelling very quickly by the 22nd (10 days after the initial date of the major SSW event) when an anticyclone was evolving to the north and northeast of Scandinavia before becoming an actual Scandinavia High. There was warm air advection in the North Atlantic despite the vortex around Greenland which allowed the high to boost and an exceptionally cold airmass push around the high into Russia and then eventually Europe before reaching us by the 26th and 27th. All the building blocks were in place as they say.
There were some Scandinavian Highs in November and December 2018 that would have likely resulted in the major SSW event in late Dec/early Jan but as mentioned, there was no downwelling and was the first nSSW (non-propagating classified) since February 2008. As some strat experts mentioned, the run of successfully downwelling (also classified as dSSW events), PV split major SSW events of January 2009, February 2010, January 2013 and February 2018 was highly unusual.
I should mention that according to a study by Karpechko et al in early 2017 in the link below, though not publicly accessible, entitled "Predictability of downward propagation of major sudden stratospheric warmings", we could have anticipated the 2018/19 major SSW failing to downwell based on wave activity propagation to the stratosphere during the days immediately following the central or initial date of the dynamic event.
This is a good figure showing past major SSW events classified as dSSW (propagating) and nSSW (non-propagating) from the paper. 2018 and 2019 not here because it was released in 2017 but we know that 2018 was a dSSW and 2019 was a nSSW.
Originally Posted by nacho libre
Is this just part of the natural occurrence at this time of year? Can you recall any notable wintry events in recent times from a vortex displacement?
Displacements are nothing unusual and frequently occur with minor SSW events (which is where the zonal mean zonal winds at 60N 10hPa do not reverse) which happen a few times on average every winter although this winter, this one will be the first since early December or late November as mentioned.
I'd say there have been a fair few but it'd be tedious to go through every example and take a good bit of time since they're not classified. In terms of major SSW events associated with displacements, the only one of note I can see that downwelled was that of early December 1981 which was immediately followed by the coldest December of the 20th century and a severe cold spell in January that everybody knows. The NAM wasn't persistently negative either which I'd expect with a displaced vortex.
Originally Posted by Kermit.de.frog
Given the time it takes for warming at 10hpa to propagate down through the layers of the atmosphere even if an unusually steep rise in temperatures occurred right now you'd be lucky to see the practical effects on surface level pressure patterns by the end of February. The lag is weeks.
Even then it is no guarantee of cold weather in any case.
Missed the boat this year.
All we can hope for is to sneak something out of a slowing of the zonal pattern. More traditional route but not as spectacular.
Depends on the type of event and the state of the atmosphere at the time. Some events can have very quick tropospheric responses like the 2018 one but these tend to be associated with splits rather than displacements. The split signal is almost pretty much vanished now from models with the GFS continuing to backtrack.
The displacement of the stratospheric polar vortex into eastern Europe *could* allow for some height rises close to the northwest if some propagation takes place. It's always "woulds" and "coulds" or "ifs" and "buts" with the weather of course.