Stratosphere watch 2022-23

sryanbruen

This thread is quite late on arrival this year as I have not been bothered to make a new one but I think now is as good a time as any to do so to avoid flooding the FI thread with stratospheric charts and there is some ongoing interest with regards to what is likely in the stratosphere later in January.

First, I won't be doing my in-depth long explanation as I have been doing for the past few years. If you want to learn about how the stratosphere works and why it's of importance to our weather, please see last year's thread here.

Second before I get into the current outlook, I need to correct myself on a recent post in the FI thread with displacements vs splits. Recent analysis was done by James Peacock at Metswift (known as Singularity on Netweather). Initial results of 2m temperature during impact period following a PV displacement or split event showed results you would not expect which I outlined in the FI thread. I somehow missed that the impact period was recalculated subsequently and results were much more in line with what you expect, especially over North America. Both displacement and split events do create blocking in the Atlantic sector but displacements have a tendency more for this blocking to be over towards Canada (west based -NAO) with the storm track further northward into Europe whilst splits have a more well rounded blocking feature over Greenland that is conducive to widespread below average temperatures over Europe. As you'd expect there is a strong signal for colder than average temperatures in the eastern seaboard which could also blow up cyclogenesis and cause that storm track to be further northwards across Europe. Note how strong the cold signal is for Scandinavia following a displacement event though and northern UK is also favoured to be cold.

Meanwhile for the past week, the GFS has been hinting at a significant deceleration in zonal mean zonal winds at 60N 10hPa (which shall be referred to as U-60N from this point in the thread) in the stratosphere in the final week of January. The model has been consistent in showing this and the ensembles are starting to pick up on this somewhat too. Any sign of a major SSW (reversal of the U-60N) is way out into fantasy land towards the final few days of January or even the first days of February. At the moment this all looks largely wave-1 driven and a displacement which as shown above is unreliable for this part of the world if it's severe cold conditions you seek (provided the downwelling occurs in the first place).

The latest ECM ens from Monday also show a significant deceleration but no major SSW for the majority of ensembles. The ECM ens have been catching up with the GFS in this regard rather than the other way round, there was much more stronger than norm U-60N on previous EC46 runs. Still no apparent signal of a major SSW from Glosea but can only go by the Met Office longer range outlook on that as Glosea data is not in the public domain.

Now does the fact the U-60N achieve an arbitrary number of going easterly make a difference in the grand scheme of things? No, not to a great extent anyway. Tropospheric blocking positioning is key as always from other drivers such as the MJO regardless of the stratosphere. However, the more significant the U-60N reversal is, the higher likelihood it is to be of a wave-2 PV split event which as outlined holds the greatest potential for cold, blocked patterns in most of Europe.

Whatever the end result, it's always fascinating to watch the dynamics of the stratosphere unfold. Will it be a major SSW? Will the GFS' call finally be on the ball after numerous failed attempts this winter? Will it be a wave-2 split? Or will the GFS do a 180? Feels like I always end these posts with questions, questions, questions but we're always left second guessing with weather..

One to keep an eye on.

nacho libre

Well on the latest GFS it looks like the Zonal Winds won't reverse at the all important 10 hpa level, so no technical ssw. Whether or not we still get any sort of cold spell cannot be ruled out entirely - there was one in February 1991, which I mentioned before, that had some sort of warming in January, but it fell short of being a ssw. You have to think though it's unlikely we will get one.

In any event, The Mjo in its next cycle could lead to further opportunities for blocking to the North West by Mid February onwards.

nacho libre

According to a poster on Netweather, who has researched 12 Sudden Stratospheric Warmings events in February since 1958, 8 of them failed to deliver a meaningful cold spell to the UK.

sryanbruen

I saw that post. I wonder did he look at the AO because that is the main way of classifying downward propagating (dSSW) and non downward propagation (nSSW) SSW events and if they were dSSW, we would have likely just found ourselves on the wrong side of the blocking as is typically the case. Judging by the fact he said there's been 12 February major SSW events since 1958, he seems to have used the NCEP dataset - various datasets have different major events due to differing reanalysed data on borderline events.

Doing a similar analysis to that of Simon Lee using D3-D30 MSLP anomalies following the central date of those 12 February major SSW events, I get the following results. NAO data is via NOAA/NCEP and PV classifications are via Cohen et al. The NOAA NAO data is based more on 500mb heights rather than MSLP so expect some divergence between the reanalysis and the NAO values. I used NAO here as it is more relevant as a means of signifying impact here than the AO. This dataset also tends to underestimate the strong nature of the NAO periods compared to other datasets which are primarily focused on MSLP.

Using these results, I get 5 of the 12 February major SSWE that were succeeded by a -NAO on the NOAA index. These years being 1980, 1984, 2001, 2010 and 2018. 2001, 2010 and 2018 were PV splits whilst 1980 and 1984 were PV displacements. If I'm able to guess, the 4 years that the NW user speaks of are 2018 (obviously), 2010, 2001 and maybe 1979? The first three I am certain on anyway. The NOAA index indicates a +NAO for 1979 but mid-March 1979 featured a severe cold easterly that coincided with St. Patrick's Day. Another year I would add is 1973 which featured an unusually cold and snowy northwest to northerly regime around Valentine's Day which I mentioned in the Irish Weather Statistics thread. March 1966 featured various cold snaps but nothing too remarkable. Mid-March 1980 had a very cold easterly with widespread snow showers between the 17th and 23rd so I would definitely include that one as another. March 1984 was just a cold, very cloudy month whilst 1989 was a nSSW following on from an exceptionally mild winter but April 1989 would later turn out to be notably cold. Early March 1999 featured northerly and easterly winds but it was not severely cold and the month later turned very mild to warm. Late February/early March 2001 brought one of the most wintry spells of the 2000s with a snow event for the east of Ireland and severe frost. 2007 and 2008 were nSSW - in fact, the February 2008 is officially the last nSSW that has occurred. Mid-February to mid-March 2010 was often cold, very frosty and light snowfalls at times during February whilst early March was dry and sunny. 2018 goes without saying so won't go there.

So from that I get 1979, 1980, 2001, 2010 and 2018 as having a notable easterly spell following a February major SSWE. 1973 is a strange and unique case. That's just under half the February events in the database so not the best odds indeed but who's to say this isn't just coincidence and our luck rather than anything deep? Interesting how, according to Cohen, most of them are splits and only 4 of them were displacements.

nacho libre

Well I am even less confident of a cold spell now. You have some memory!

Was there any kind of strat warming at all before the December 2000 cold spell? Matt H seems confident enough that contrary to the seasonal models winter will end cold now. He is a pro, but he likes the cold too, so maybe he is not completely objective. I am just getting the nagging feel this is not our Winter to get deep snowy cold. I know we had the great December cold spell and we had some snow this week, but I mean very cold upppers where we don't have to concern ourselves with things going the wrong side of marginal like what happened last night. Then again it's not all about very low uppers as 2010 showed. The problem last night maybe because we did not have a true Arctic Northerly to begin with.

nacho libre

https://youtu.be/Eix2YFb4_Eg

A bit about the Stratosphere towards the end of the video.

sryanbruen

https://www.boards.ie/discussion/comment/120125250#Comment_120125250

There was a very delayed development of the stratospheric polar vortex in Nov/Dec 2000, see the U-60N below for 2000-01 indicated by the red line. There was no official major SSW at 10hPa as no reversal but it was a significant SSW nonetheless. Pretty much the entirety of December 2000 had slower than average U-60N and even at its strongest in meteorological winter through January 2001, it was just barely around average. This likely had a big hand in 2000-01 being a colder winter than most of the 1990s/2000s up to 2008.

This cross-section shows the NAM/AO from 1-1000hPa during the 2000-01 season, you can see the significant minor SSW that occurred in December 2000 around 10-14 days before the Christmas to New Year cold spell. There was clear downwelling from both this and the major SSW split in the following February which would result in that cold end to February/start of March.

Another one of those minor SSW events that had significant impacts for Europe much like the January 1991 event despite no reversal.

LaoisWeather

Are SSW events more likely under La Nina or El Nino? Or does the ENSO state influence it?

Interestingly I've read that since 2000, La Nina months out number El Nino months by 116 to 36, whereas from 1979 to 1999 there were 86 El Nino months to 58 La Nina months.

Food for thought.

nacho libre

It seems the mjo in phase 3 can lead to blocking, i thought it would promote a positive NAO. One poster over on Netweather seems fairly confident that we will see height rises to the North West towards Mid Februrary arising out of this. Also that the weakened strat will aid in all of this.

sryanbruen

May as well bank this 18z before it's gone, a wacky major SSW. Can't say I've seen anything like it.

sryanbruen

Still looking like a minor wave-1 displacement is what is expected. No definitive sign of major (reversal) or a split.

The green lines below are the GEFS showing forecast U-60N on the 06z run this morning. Some go for a reversal, most do not but there is a significant group that do in the early days of February.

Gonzo

Just after looking at all the GFS ensembles and if anything today is a downgrade for the SSW. Only 3 or 4 members do a proper split while the others go for a displacement and the PV regains it's strength very quickly rather than a proper weakening.

sryanbruen

There has been no split at all in the outlook as far as what I've seen. If you're hoping for or expecting one, be prepared to be disappointed.

The temperature charts, which I also used to look at myself all the time, are not a good way of seeing if the PV is displaced or being split. They can be good to identify a significant warming occurring. But in terms of identifying the state of the PV, 10mb wind or height charts are far superior. WXCharts has the easiest one to read under Polar - Northern Hemisphere - 10mb Wind but Weatheriscool charts are also good. I am not aware of others at this time. There's numerous sites to view U-60N which are used to identify whether an event is major or minor.

I was also meant to say the latest EC46 on Thursday seemed to show less perturbations going for a major event with a minor displacement still signalled.

If it's blocked, cold setups you are hoping for from stratospheric effects, well then you'd hope the GFS 12z is on steroids for early February as there sure is no quick way out of what it shows with the North Atlantic jet stream raging across to Ireland with masses of explosive cyclogenesis.

nacho libre

I don't think we will see a split without wave 2 type warming. that is why the Vortex keeps rebounding after each wave 1 warming. We just have to hope a displacement and the weakening of the zonal winds t may go in our favour this time. I would not bet the house on it though, especially if the GFS op is correct.

sryanbruen

https://www.boards.ie/discussion/comment/120139461#Comment_120139461

Indeed.

Although perhaps the GFS OP is not as bad as I initially thought as NE Europe still has a lot of high pressure which would in theory send further planetary waves to give more disruption and an increased risk of wave-2 type warming. However, we'd be fast running out of time as spring would be upon us for earliest likelihood of tropospheric response unless a January 2009 scenario were to happen which is another extreme altogether. Gotta admit it gives me January 2014 vibes that 12z with the NE Europe ridging and active Atlantic.

As for the MJO phase 3 being blocked, composite for such shows strong signal for high pressure around Iceland/Greenland with a deep trough in the Atlantic. Looks cold but wouldn't be textbook snowy I'd have thought due to the alignment of the troughs, we would want that trough centred over the Low Countries. Then taking into account the displacement of the PV, would this blocking probably be further south?

nacho libre

Could well be. It would just be our luck that the the mjo produced a block in the right area only for the displacement of the strat to scupper it. I see the US is going to go very cold. Another potential spanner in the works, there always is!

sryanbruen

Any small chance of a major SSW (reversal and displacement) have vanished on the GEFS now. Not a single perturbation showing a reversal from what I’ve seen on the latest run.

Still looking like this displacement will help produce an intensified vortex over Greenland which will increase the likelihood of stormy episodes.

nacho libre

https://www.boards.ie/discussion/comment/120141322#Comment_120141322

Yes, indeed it's likely Glosea foresaw a ssw not taking place , which is why the UKMO long range was confident about a return to unsettled conditions with no mention of blocking in any previous updates over the last couple of weeks . It's the double whammy of a weak strat enforcing westerlies on an already mobile trop pattern. It is hard to see how the Mjo in phase 6 or 7 can override this to deliver deep cold. Maybe if we are lucky we could get conditions like last week at some stage. It could be though that the seasonal model end up being right about February. It's a grim outlook, unless some wave 2 warming shows up in the future, but then we will looking to March

sryanbruen

https://www.boards.ie/discussion/comment/120141778#Comment_120141778

Fair play to Glosea is all I can say.

sryanbruen

There's the confirmation from Butler that any small chance of a major SSW has diminished.

https://twitter.com/DrAHButler/status/1617606274778267648?s=20&t=6d2rlUZcDeVwH0nFP8Xxrg

sryanbruen

Some stunning nacreous or polar stratospheric clouds showing in Scotland where the stratosphere temperature is sub -78C which is required for visibility of these clouds. Ireland last seen them in 2016 to my knowledge with some great displays in early February 2016, could be sighting of some in the north today/tomorrow as the core of the coldest stratosphere temps gets displaced towards our shores. Shame there looks to be too much cloud cover with rain expected to approach later today, typical ain't it.

https://twitter.com/highlandweather/status/1619745530632105985?s=20&t=VUrlu-a-SalKP7XhIPoWhA

https://twitter.com/The_Cullenloon/status/1619747956000960518?s=20&t=VUrlu-a-SalKP7XhIPoWhA

sryanbruen

Some slight signs perhaps of a renewed warming during second half of February from the EC and GFS as presumably wave activity occurs from Ural blocking being forced by the MJO. Another case of watching to see what happens but it's a long way away and hypothetically if a major SSW event did occur that time of year, effects wouldn't likely be felt until the start of spring has passed.

As expected, too cloudy to see any possible nacreous clouds this evening over Ireland.

nacho libre

Yes, if we do get an early final warming it could end up being a very cold March. The CFS in recent runs has been showing just that scenario.

pauldry

Saw a bit of sky yesterday evening in Sligo and it was quite green like the one above. Any chance tonight as there are somewhat clearer sky's about away from NW

sryanbruen

https://www.boards.ie/discussion/comment/120181954#Comment_120181954

There's a possibility this evening of a small last chance of seeing them in the north of the country but those cold temperatures are quickly dissipating and going above -78C at 10hPa. If it was green you did likely see nacreous, just very faint type-1 structure.

nacho libre

An early final warming?

sryanbruen

https://www.boards.ie/discussion/comment/120188442#Comment_120188442

Doubt it. A February final warming has never happened before. The record earliest final warming is March 2016 when the U-60N reversed on 5th March. It resembled a mid-winter major warming by how extreme it was but the U-60N never recovered to westerly so it's regarded as the final warming of the 2015-16 season. This is a month and 10 days early. The average date of the final warming is April 15th.

sryanbruen

GFS yet again up to its old tricks right at the end of its run. Posting just for archival sakes. It shows a major displacement. We well know how the GFS goes by now with its new version since early December. Saying that, knowing our luck one will happen and we get a very blocked March. One hasn't happened since 2018..

nacho libre

Yes it would be having chased deep cold all winter, watch it arrive right on cue for spring. The CFS was showing a lot of blocked runs throughout March recently.

Gonzo

Just took a look at the latest CFS extended and if it verifies we are in for a cold Spring with plenty of cold spells and these carry on into the start of June.

sryanbruen

Another det run showing a major displacement and it looks prolonged too rather than a short affair.

The reversal starts on the 15th and ends the run with -19.9 m/s on the 18th.

Just realised the GEFS 06z had a significant cluster reversing too but the spread is large, as you'd expect at such a timeframe. The GEFS have been performing better than the OP for the stratosphere again this winter as you'd expect - though not necessarily so for our weather!

EC46 update to come this evening... Monday's update as a reminder was supportive of a significant weakening again in the second half of the month but not quite going to reverse like the GEFS above.

This possibility has more of a base to it than the previous two with more support but does that mean it's any more likely to occur? No but I'm probably going to be like a broken record saying this for numerous posts, it'll be typical for one to occur and result in a blocked March or mid-spring and sometimes the effects from a major event can continue into the final month too as U-60N take a deep dive through the spring season naturally and meridional patterns are much more likely.