Winter 2023/24 - A Post Mortem

[Mike Poole]

This thread is very necessary!  And, quite fitting, in a weird way, to have the post-mortem while this excuse for a winter is still alive!

Before giving my thoughts on winter, I just wanted to make a comment on the GSDM - I don’t usually, because I don’t want to get caught in the crossfire between those who are passionate believers and very defensive when challenged, and those who use it as an easy target for criticism when it doesn’t snow.  I don’t have access to the GSDM charts myself, but I do factor into my thoughts the excellent posts on here from those that do.  I have read some papers on it, and @Tamara kindly provided me some useful help on aspects of it a few years ago - it seems sound and very useful, and given it is founded on the principle of conservation of angular momentum, it makes sense to me as a physicist.  I don’t think it is infallible though, forecasts seem to be predicated on the assumption that AAM will rise/fall in a predictable way, and I also have a question mark (in winter) about how intuitions derived from the GSDM fit with things occurring in the vertical dimension i.e. the stratosphere, which I think has been relevant this winter.  It also doesn’t inform on local UK-scale issues which can be so important.

The backdrop to this winter was world weather patterns out of past precedence with extraordinary sea surface temperatures.  My view in the run up to winter, was that any attempt to make predictions based on analogues was therefore pointless, and I put more weight in the seasonal models - because they simply simulate the laws of physics, without knowledge of past patterns, I figured they would be the best guide.  They weren’t!  At best, they could be said to have predicted a weak vortex, which there definitely was, but on pressure patterns relevant to the UK, they have not performed well. 

We have had a very weak strat vortex ever since the Canadian warming (first one for 30 odd years).  The trop has been leading the dance most of winter, and the strat vortex has been behaving really quite oddly.  I thought that surely with that in our favour we must be in with a shout of a decent winter.  Wrong!  But why?  It seems to me that there are two primary possible reasons:

1. Climate change has done something to global weather patterns that have made it next to impossible to get the kind of synoptic patterns which in the past would have delivered cold to the UK - the dreaded ‘Iberian high’ being a symptom (but not cause) of this.
2. We were just very unlucky this year.  Even unfettered by the strat vortex, the trop vortex just never got itself into a favourable position.  I still think with a different roll of the dice we could have ended up with something memorable.

One of the reasons I don’t (yet) believe reason 1, that decent UK winter patterns have become next to impossible, is because they have showed up in the models this year, not just the odd run, but substantial clusters of ensemble runs - we have just not managed to count them down into the reliable.  They would not show up if they were no longer physically possible, for whatever reason, due to climate change.  So, I put most of it down to bad luck.

Which brings me to my final learning point which is disregarding the model output.  It is OK to bin an individual run if the output gets so contorted you know (from looking at charts that have actually verified) that a run is garbage.  But, I think we have been a little quick to dismiss larger swathes of model output on the assumption that ‘the models are misreading the progression of the MJO’, ‘the models always underestimate Iberian heights’, ‘the ECM is prone to over-amplification’, or ‘the GFS is off on one and will come into line’ etc.  I hold my hand up - I’ve done this - and found later that the disgraced model was on to something after all.

All in all, a very disappointing winter, but while we have had no meaningful snow (locally, again, 4 years on the trot now), it has been a (morbidly) fascinating learning experience.  

Edited February 11 by Mike Poole

[BruenSryan]

 Derecho Yes, there was a paper out there that showed central Pacific based La Ninas tend to be particularly front-loaded towards November and December in some cases but not often whilst east Pacific ones have a stronger blocking signature for later in the winter. 

Reanalysis of analogs confirm this. Note though that while CP Ninas tend to be biased to November in terms of -NAO, there's a significant positive z500 anomaly for February extending back to Scandinavia along with a Genoa trough which may or may not result in a continental feed. 

I must mention these analogs were based on ONI rather than MEI. 

[Met4Cast]

 Derecho

On 10/02/2024 at 10:57, Derecho said:

It's an interesting response but the winter months of 2010 saw the complete opposite. Warm air often moving into western Africa and southern Spain with anomalous warmth in the SE North Atlantic. So what made 2023/24 so different?

Just to go back to this, here are the SST's during early January 2010 compared with the current SST's.

 

both El Nino years and both had anomalous warmth in the SE Atlantic, one major difference however is the strength of the tripole, much colder waters stretched across the central Atlantic in 2010 vs this years, I suspect this perhaps helps to explain why we saw a cold winter in 2010 vs this year. 

Not to say the anomalous warmth is the entire reason for this winters broadly mild weather, but certainly an argument could be made that the warmth helped to inflate the Iberian high this year. 

MJO was also broadly more favourable through February in particular.

Edited February 11 by Met4Cast

[Don]

8 hours ago, North-Easterly Blast said:

I think that even though it is a long way off, the earliest background signals for winter 2024-25 are already bad news from a cold perspective for the UK, with a westerly QBO, and being close to solar maximum, and the likelihood of a La Nina.

Perhaps another 'PV of doom' winter similar to 2019/2020?

Edited February 11 by Don

[North-Easterly Blast]

 Don I hope that next winter we do not see anything as awfully stormy and wet without a sniff of cold like 2019-20 or 2013-14, but even at this early stage, background signals like being close to a solar maximum, a westerly QBO, a possible La Nina do not sound to bode well for cold outbreaks from HLB, developing in the UK during winter 2024-25.

Shortly after the last solar minimum, I always thought that winter 2020-21 was the window of opportunity for a cold winter for the UK, and the pattern did try to set up more favourably for cold for the UK, but the pattern never really came together in its full potential for cold, and it did turn cold at times in Scotland, but for the rest of the UK a cold spell was never sustained for more than a small number of days, and was rarely anything especially potent.  I always now regard 2020-21 as a "failed cold winter" when the UK missed the opportunity to see a truly cold winter.

Another "failed cold winter" further back that I would regard as so further back is 2005-06.  That winter had a fair amount of blocking over Scandinavia and eastern Europe, and those areas saw a cold winter that year, but for us in the UK, the really cold air never really made it and the UK was left with often close to average conditions for most of the time.  2005-06 was another so called failed cold winter for the UK, when the opportunity for a cold winter for the UK was there but the pattern never came together in its full potential to get the UK cold.

[damianslaw]

 Don Big difference to winter 19-20 and this winter, the PV was much stronger then and sent storm after storm our way. This winter has seen shortlived episodic stormy spells thankfully and at least a couple of shots of cold, unlike 19-20 which was relentlessly mild. 

I've just re-read and you are comparing to possible expectations for mext winter. No doubt we'll see a brutal cold winter against expected signals! Wouldn't be surprised given how chaotic things feel..

Edited February 11 by damianslaw

[raz.org.rain]

 North-Easterly Blast quite the concerning thought really. It makes you wonder if we really have just missed the last opportunity for a notably cold winter? By the time the next perfect setup comes around, the effects of climate change would arguably be more severe. I know some of us froth at the mouth at the mere suggestion of climate change as a factor, but the statistics are quite clear regarding the changes. There's a theory that the synoptics of the early 2010s is something that occurs every ~50 years or so. If that's the case, I'd expect the next occurrence would be nothing spectacular with the added warming. Depressing conclusion.

[Dark Horse]

My cousin lives in Shetland and locals say they've had a decent and snowy winter this year. Quite a bit of snow cover right down to sea level. That does seem to be the case north of Inverness it would seem.

Edited February 11 by Dark Horse

[North-Easterly Blast]

 damianslaw 2013-14 was just the same as 2019-20.  So far winter 2023-24 was a short cold interlude at the start of December, then very mild after the first week of December to the opening days of January - then a benign HP spell, a five day cold outbreak from the north, then very mild again from January 21st until now, and likely until at least next weekend.

[Don]

 damianslaw I was taking about next winter 24/25, not this one.  As you say very different altogether!

[North-Easterly Blast]

 raz.org.rain No, the synoptics of the late 2000s / early 2010s had not occurred for the first time in 50 years.  There were excellent examples of northern blocking and northerlies / easterlies in 1978-79, and at times in 1981-82, 1985, 1986, 1987 and to some extent more briefly in the mid 1990s.

[damianslaw]

 North-Easterly Blast There are similiarities to last winter, albeit the early cold not as intense as Dec 22 or long lasting, the Jan cold was a bit colder than last Jan cold.. very mild latter Dec into Jan both years and now what appears to be a very mild latter third Jan through Feb as last year..Last March delivered a cold shot first part of month.... 

[Weather-history]

Another solar example:  winter 2000-01 was close to solar maximum (November 2001) but I think of a lot of members would take that winter with both hands over the last 2 or 3 winters. For my location, it was one of best winters between 1997 and 2009. For the Manchester Winter Index, it;s index was higher than any of the winters that occurred around the following solar minimum.

 

 

Edited February 12 by Weather-history

[North-Easterly Blast]

 Weather-history I think that winter 2000-01 is another one of those winters that tries to set up a pattern to deliver a cold winter to the UK, but where the pattern never really comes together in its full potential to deliver the UK a proper cold winter.  I think that in 2000-01, Scotland saw frequent cold spells but much of the time England never really saw significant cold for more than short periods - 2000-01 was in many ways like a similar version of 2020-21.

In fact I would have thought that winter 2000-01 is a similar solar match to 2023-24 (just before solar maximum) but you are correct that it brought much more in the way of cold outbreaks than 2023-24 has done and now looks to do.

Edited February 12 by North-Easterly Blast

[trickydicky]

Just to go back to all these drivers, the AAMs and the MJOs and whatever else. I think the problem, if you want to call it that, in the model thread is that it is kind of split into 2 camps. Or maybe 3. The main camp of contributors are those who look at the models - the GFS, the ECM and UKMO - and from them try and make an educated guess at what the weather will do beyond the current spell that we are in. So in general they are looking for cold when outside it is mild (in winter). The next camp are those who can’t read the models (like me) and just want to know if a cold spell is somewhere on the horizon. We generally just lurk and observe. Then there is a camp who talk about the atmosphere and what it theoretically might do. This camp don’t generally talk about weather in a specific place at a specific time. There is a mutual frustration on both sides. The atmospheric drivers people are annoyed with the people who just keep talking about snow and the snow people are annoyed that the thread gets filled with enormous posts that barely mention the weather. There is some crossover between the camps but there are also fundamentalists on both sides. 
 

Any post mortem of the winter depends massively on your location. It will probably range from pretty decent in Northern Scotland through the decent side of average in the far north of England to absolutely desperate in the south of England. We have pretty much been at the boundary of the cold air all winter, without it ever leaving some and ever reaching others. 

Edited February 12 by trickydicky

[mulzy]

Has anyone got the years of the 10 mildest winters in the CET series?  Are we in danger of 23/24 joining that list?

Edited February 12 by mulzy

[Penrith Snow]

 trickydicky Yes in Northeen Scotland the winter has been ok with frequently northerly outbreaks, in northern England it's been poor but with some snowfall and overall better then 2020, 2019, 2017,2014,2007.

However, Midlands southwards its been up there with the worst, even Birmingham which lies above 450 feet has had literally no snow, that's bad for such a inland relatively high location.

Southern England had no chance.

Awful

Andy

[lawrenk]

 trickydicky That’s a very good summing up in my opinion.

Tolerance is the key. I’m very much in “camp two”, would like to progress to “camp one” but, frankly, it’s a slow process! The third camp is, and always will be, beyond me, as I don’t have the time or inclination to learn to that degree. Truthfully, I skip past the very long and technical posts. That’s not to disrespect those who produce them at all and I admire those who put that amount of effort in.

I do think, though, that it’s perfectly legitimate for posters to challenge each other, where an outcome that the “science” suggested fails to materialise, as long as it’s done in a non confrontational way.

Finally, the Mods do a great job, given the range of contributors you have highlighted.

[danm]

Just spent half an hour reading through this entire thread, it has got a bit feisty in here!

Overall a very disappointing winter, away from perhaps northern Scotland. It's a shame as this winter held out a lot of promise, but failed to deliver. 

My perspective on the use of medium range forecasting tools - everything that falls into the teleconnections bucket (GSDM forecasting, analysing MJO cycles, global wind flow budgets, AAM etc) - is that they are useful, and we should continue to progress our knowledge of how these factors can influence broad scale synoptic weather patterns, that could then influence our own weather. However, we also shouldn't make firm predictions of UK weather based off of them. They give us a first step to understanding whether the atmosphere is pre-disposed to supporting a blocked, cold, snowy set up or not, but nothing more. It can't be used to say 3/4 weeks out that the UK will or will not experience a particular type of weather, simply that the likelihood of it occurring may or may not increase. Surely though, that is all longer range forecasting is supposed to do? Provide a probabilistic view on what may happen. 

At the end of the day, so much needs to fall into place for our little island on the eastern edge of a large ocean, influenced by warm ocean currents, with a prevailing wind that originates from the sub tropics that there is little point getting emotionally invested in model output beyond about 96 hours or teleconnection data several weeks away. Absolutely view them, understand the drivers, try and make predictions, but there are far too many people here who literally hang off every single suite of models, attempting to count down from day 10 to day 14, hoping nirvana charts will deliver. We all hope they verify, and there is nothing wrong with looking at or discussing them, but I do think a few people get so engrossed in them that when they often fail to verify they literally spiral into a cycle of doom. Why put yourself through it? I have learned over 20 plus years of model watching not to get invested until a cold spell is modelled to within 4 to 5 days at the absolute maximum, and even shorter than that when it comes to snowfall. 

With all of the advances in weather forecasting, the technology we have available, and other tools that are giving us a much greater understanding of atmospheric drivers, the same is still true for our small part of the world - forecasting a particular type of weather is fraught with difficulty beyond 5 days. 

Edited February 12 by danm

[jellybaby1969]

Northern Scotland has had a decent winter..... here in coastal ayrshire we had a 4 day period of lying snow.  Central Scotland south has been a very poor winter apart from the odd localised event ( N Ireland / Cumbria) feeling mainly mild but not extremly mild  tho near 20c was recorded in Northern Scotland. The galling thing is the amount of rainfall that has fallen and will continue to fall as we go thru February. The Atlantic still in charge and no matter the teleconnections ... pv split .... ssw events, we in the UK for the majority just and cant see a more wintery pattern.

[Bristawl Si]

I reck this is the 4th Winter in a row where post-mortem wise on NW, peeps have said that all the background signals were there, right synoptics, etc  BUT

They didnt 'deliver'. Just saying

[Don]

 Weather-history you’re not wrong regarding 2000/01!

[jules216]

If anyone is interested the UK lowest temperature this winter is from start of December here in Foregin - North West of Carrbridge -17.5°C, this is unoficial of course from Scottish Frost hunters. i was Happy to assist in pin point this location in advance of station instalation. 

[raz.org.rain]

I'm not convinced that the forecasted SSW will have any noteworthy impact on our weather, but it seems to be considered the last chance saviour for a pretty dismal winter. If anything, heights over Iberian continue to demonstrate a strong push northwards in the ensemble outputs. I can't imagine that we'd get anything other than a few days worth of average to chilly weather before the southwesterlies kick back in.

[damianslaw]

Sneaky feeling we may be premature with this thread. and winter could bite back hard by months end and into March, certainly not late enough to pack a punch.

I sense much dispondency how it has panned put by and large, largely due to promising signals of colder weather perhaps beating off the atlantic more so second half of winter which since 20 Jan has not been the case.

We have had many instances of cold wintry very tail ends to winter and starts to spring.

Why am I saying all this, namely due to expected SSW affects, and given northerlies and easterlies tend to show there face come late Feb.

This is not a forecast for wintry nirvana by months end, just that I feel we shouldn't close the book on winter weather until 2 weeks time if none is on the horizon. 

Edited February 12 by damianslaw