[sryanbruen]

Yeah, there looks to be some kind of warming at 10hPa to the north of Canada whilst the SPV is displaced somewhat from its normal position to Eurasia. However, the forecast is for a strengthening again of the SPV after this temporary weakening and minor warming that is ongoing right now.

For clarity, proper Canadian Warming events have occurred in Nov 1951, Nov 1952, Dec 1954, Nov 1958, Dec 1958, Dec 1959, Nov 1962, Dec 1965, Dec 1966, Nov 1968, Nov 1972, Nov 1974, Nov 1976, Nov 1977, Dec 1978, Nov 1979, Nov 1980, Dec 1981, Nov 1991, Dec 1993, Nov 1996 and Nov 2000.

[sryanbruen]

Originally Posted by sryanbruen

It’s *quite* unusual that all this blocking is tropospheric driven in the high latitudes rather than being driven by the stratosphere. From research, I can spot only a few years that involved tropospheric driven blocking including 2009/10, 2010 and 1985/86. Various factors could be resulting in this including low solar forcing, North Atlantic SSTs and the descent of the easterly QBO that is ongoing etc.

In the space of just over a month, the outlook could not be any more different to what this post would suggest from mid-November. The deceleration in the zonal mean zonal winds did verify at the beginning of December but we have since seen a ramp up (which the GEFS did not forecast) and currently the zonal mean zonal winds are above average though we are nearing the climatological peak anyway. It should be noted that the GEFS (which are of the old legacy GFS and haven't been upgraded unlike the FV3 but are set to get an upgrade in 2020) frequently diverged with the GFS operational run. They frequently suggested a weak to very weak stratospheric polar vortex for mid-December whilst the operational was generally on the other side but even it did not foresee the ramp up that has occurred. Simon's tweet below illustrates this perfectly. Shows how even the stratosphere forecasts have to be taken with a large pinch of salt much like our own forecasts.

https://twitter.com/SimonLeeWx/statu...88324247511040



The Arctic Oscillation for the foreseeable future is set to be positive so a strong polar vortex is to be penciled in over the Arctic. Literally no sign of it being disturbed. Tropospheric patterns look unfavourable too with a flat westerly flow across the Northern Hemisphere.

[BLIZZARD7]

Some signs of a SSW appearing now in the models - Winter to appear late February? Mid February possible with a QTR but Late Feb/Early march if it's the normal ~ 3 week lag impact.

Assuming we even get there...

A few GEFS members showing a reversal -

[sryanbruen]

More GFS members including the operational have gone towards a technical SSW this morning and some even show evidence of a little PV split right at the very end of their runs. Ultimate Fantasy Island and as we’ve seen this winter, stratospheric forecasting can be as unreliable as our own 7-10 day timeframe forecasts but we’ll keep watching.

I think ECM was showing a wave-1 displacement yesterday too with the PV displaced to Eurasia. Let’s see what it shows later today.

[Billcarson]

If we do get a ssw,would I be right in saying 2 things in our favour this yr compared to last yrs ssw is that the qbo is now in its easterly phase and also the fact that the ssw would be happening later then last yr . In Feb the pv should be weaking naturally a bit anyway.

[Mortelaro]

I'll just stick this here will I

https://twitter.com/Petagna/status/1...309427712?s=19

[sryanbruen]

Thanks BB. Marco also posted the ECM 0z output for the strat (which hasn't updated on the Berlin website where I usually get my ECM charts from) and it does show almost a SPV split at the very end of its run with two areas of warmer than average temperatures (North Pacific and northern Europe) attempting to split the vortex into two or more vortices.

Meanwhile, GFS 12z backed away from the split to the very end of its run too at day 16 rather than days 13-14 which it showed on previous runs but the displacement is still at large shown and getting closer any day now. As ever... more runs needed and days to progress.

https://twitter.com/Petagna/status/1221098767438446593

Originally Posted by Billcarson

If we do get a ssw,would I be right in saying 2 things in our favour this yr compared to last yrs ssw is that the qbo is now in its easterly phase and also the fact that the ssw would be happening later then last yr . In Feb the pv should be weaking naturally a bit anyway.

I don't rate the QBO too highly so I wouldn't know much about that but I think it would be fair to say that the later occurrence of this SSW (if it were to verify) would have more of an impact compared to last year's event. Last year's event was also succeeded by a big swave of westerlies in the stratosphere which resulted in record-breaking daily zonal mean zonal winds and it coincided with the westerly QBO growing in amplitude. It's the exact opposite this year... this expected stratospheric warming event follows on from record breaking SPV conditions and a weakening westerly QBO as the easterly QBO descends from the top of the atmosphere.

Remember every SSW event is unique.

[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.

[sryanbruen]

More steps back this morning from the GFS with a fair weakening of the SPV forecast but no major SSW bar a few ensemble members that are clear outliers. Displacement still in the forecast but does not cause any reversal although remember it doesn’t have to be a reversal for impacts to be felt on the troposphere. Splits are better for quick tropospheric responses as well as for more prolonged cold patterns.

These are the games you have to play with Fantasy Island. As for my own opinion, I do not see a major SSW this winter. I have no reason to think there will be. There is a lack of global patterns that would support a major disruption to the SPV.

[sryanbruen]

10 hPa stratospheric temperatures have returned to average or slightly above average for the first time this winter since the early days of December.



An uptick at 30 hPa too but yet to return to average.



https://ds.data.jma.go.jp/tcc/tcc/pr...RAT/index.html

[nacho libre]

Originally Posted by sryanbruen

10 hPa stratospheric temperatures have returned to average or slightly above average for the first time this winter since the early days of December.



An uptick at 30 hPa too but yet to return to average.



https://ds.data.jma.go.jp/tcc/tcc/pr...RAT/index.html

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?

[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.

[sryanbruen]

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.

https://rmets.onlinelibrary.wiley.co...0.1002/qj.3017

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.

[sryanbruen]

The minor warming only got the zonal mean zonal winds back to or slightly below the climatological average and now models forecast a big strengthening again with more record strong date records possibly. This is at a time when they should be weakening naturally too. Similar timing to last year's strong Polar Vortex/cold strat event in Feb/Mar but this looks stronger than that right now.

[nacho libre]

Originally Posted by sryanbruen

The minor warming only got the zonal mean zonal winds back to or slightly below the climatological average and now models forecast a big strengthening again with more record strong date records possibly. This is at a time when they should be weakening naturally too. Similar timing to last year's strong Polar Vortex/cold strat event in Feb/Mar but this looks stronger than that right now.

so the recent modelling of a Scandi High in fi on the GFS will be shortlived. With this upcoming cold spell, it shows a strong pv can be to our advantage sometimes/