Downward propagation of easterlies in the stratosphere (blue top right) weakens and stalls in EC as daughter vortex east Canada weakens and fills. GFS not that bothered with pushing anomalies lower either... pic.twitter.com/OglCicZRLy

— Chris Fawkes (@_chrisfawkes) December 28, 2018

It's clear models are struggling with this forecast, however. The GEOS5 is much weaker and shorter in duration of the +NAM at the surface than GEFS. I suspect the CMC and ECMWF look more closer to the GEFS prediction, however. pic.twitter.com/Es6Mj1adR3

— Michael Ventrice (@MJVentrice) December 28, 2018

Still a lot of uncertainty about the exact impact of the ongoing SSW.

The latest ECM data looks much more promising than yesterday's in terms of the impact of the zonal wind reversal. It is being forecast to propagate down into the troposphere and remain fairly strong.

Day 4 http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwfzm_u_f96.gif

Day 10 http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwfzm_u_f240.gif

The forecasts are moving about quite a bit day by day which suggests a low degree of confidence at the moment.

The latest 0z GFS parallel run shows the strat vortex taking a real beating over the next couple of weeks.

The first wave of warming, which is elongating the vortex, causes a small piece to split off by 1 Jan

http://modeles.meteociel.fr/modeles/gfsp/runs/2018122900/gfsnh-10-84.png?0

By T180 the main piece of the vortex has split in two and we now have three remnant pieces 

http://modeles.meteociel.fr/modeles/gfsp/runs/2018122900/gfsnh-10-180.png?0

By 7 Jan we have a secondary warming commencing over Siberia

http://modeles.meteociel.fr/modeles/gfsp/runs/2018122900/gfsnh-10-216.png?0

This forces the European piece of the vortex west and it recombines with the piece that was over the western Atlantic. Also by this stage the secondary warming is becoming quite substantial and pushing into western Russia

http://modeles.meteociel.fr/modeles/gfsp/runs/2018122900/gfsnh-10-300.png?0

By the end of the run the PV has been shunted over to the east coast of the US / Canada

http://modeles.meteociel.fr/modeles/gfsp/runs/2018122900/gfsnh-10-384.png?0

It would be surprising if all this does not have some impact on the troposphere at some point. Exactly when and where is impossible to say but the suggestion from the experts that second half of Jan and Feb could be interesting seems about right to me. Everything has been pushed back a bit compared to the thoughts in mid December. Second half of winter could be very interesting - but no guarantee that any blocking sets up in the right way to impact the UK. Patience is the watchword for now.

Brilliant post GW.  Hope and patience in equal measure. 

The secondary warming event in the stratosphere is due to peak this evening

http://modeles.meteociel.fr/modeles/gfsp/runs/2019010600/gfsnh-10-18.png?0

This one doesn't really push up towards the Pole. Instead it drifts west and forces the main lobe of the PV, currently over the UK / North Sea to drift west and recombine with the piece over the NE of the US / Canada

http://modeles.meteociel.fr/modeles/gfsp/runs/2019010600/gfsnh-10-78.png?0

Then the PV splits again but this time the main lobe drifts west to sit over Canada

http://modeles.meteociel.fr/modeles/gfsp/runs/2019010600/gfsnh-10-186.png?0

So lots still going on in the stratosphere

Zonal winds remain generally negative in the stratosphere between 60N and 90N and between 2hPa and 30hPa

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwfzm_u_a12.gif

The secondary warming causes the reversal to strengthen again in the coming days. Here is the chart for 10 Jan

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwfzm_u_f120.gif

By day 10 the reversal has weakened to a nearly neutral state

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwfzm_u_f240.gif

Here are the 10hPa charts for today, 10 Jan and 15 Jan

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwf10f24.gif

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwf10f120.gif

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/figs/ecmwf1/ecmwf10f240.gif

The impact of all this, if any, on the troposphere will therefore be ongoing for some time. Hence it is hardly surprising that the model ensembles post 15 Jan are currently all over the place, albeit there is a definite cooling trend. We might expect to see that cooling trend in the second half of the month firming up over the next few days, if blocking sets up in the right place. Patience is still required. Unfortunately these days most people seem to expect instant reactions to everything. All the building blocks are in place for a potential spell of extended severe cold weather somewhere in Europe.

Another great post GW thanks.

I am interested in the extent to which the SSW event of late December has contributed to the forecast cold weather ahead.

Is the famed two-week window for tropospheric/ surface responses that well -defined and proven? 

If it is, is there an indefinite timescale for sustenance of cold conditions? Can they last a month after the event? Two months?

Could the cold spell forecast be part of normal variability, enhanced by the SSW somehow?

I have no issue with the uncertainty relating to the correlation of an SSW event and the surface conditions in any particular longitude- I have posted earlier the caveat about assigning any really cold spell in a SSW winter to the SSW.

Interest still out there?

Seems the Beeb answered my question-how kind!

https://www.bbc.co.uk/weather/features/47006603

SSW effects somewhat nullified for us due to swings in NAO between +ve and -ve phases.

Finishes with a teaser for 'later in the season'.

Nice clear explanation for the non pro.

They don't really explain anything, though - just saying that the Azores High hasn't buggered off this year without explaining *why*.

I'd have been far more impressed if, for example, they mentioned that last year saw three reversals in quick succession, followed by a rapid recovery (which helped "flush down" the easterlies). This time it's been a far slower recovery, with two reversals - the latter of which was at a bare minimum level, rather than a relatively strong one as last time.

They could have mentioned as well that last time we saw a Canadian warming, which effectively zapped the vortex from above and meant it couldn't take up its main residence where it usually does - whereas this time any warming which made it round to Canada was much weaker, with less of an effect on the Canadian part of the vortex.

Instead they just waffled on about how each SSW is different and that the Azores High is still there (and the chart showing a negative NAO only illustrated a weak NAO - i.e. weaker Azores High/Icelandic low, rather than a strong negative NAO, i.e. an actual low near the Azores and a ridge or high over Iceland.)

With the polar night Jet still holding its 'limp peanut' shape I think we only really have the 'Final warming' , and any anoms it might drive, to come?

Otherwise whatever you have thought the 'reset' pattern to be this winter will just plod on!

This is a neat animation of the vortex over the past few weeks.

There and back again: a PV view of the breakdown and recovery of the Stratospheric Polar Vortex from 23 Dec 2018 to 21 Feb 2019. pic.twitter.com/6pW78eRq9V

— Mathew Barlow (@MathewABarlow) 25 February 2019