Winter 2023/24 - A Post Mortem

[danm]

 damianslaw I think in terms of meteorological Winter, we are done. Nothing substantial is going to develop over the next two weeks, or unlikely to do so. 

A SSW ofcourse is the wild card that could provide something into March, but really then we're into Spring, and whilst March can sometimes pack a punch with cold weather, many are looking for warmth by then and also due to the strengthening sun, we increasingly need something exceptional to deliver at that time of year. 

[damianslaw]

 danm I guess it depends on what people's expectations are when it comes to cold wintry weather. I agree if after ice days and days of snow cover with mean temps 0 degrees or below, by March such weather is certainly statistically not likely more so further south, unless an exceptional synoptic i.e. 2018. However a few days of maxima in the 0-5 degree mark, run of sub freezing nights and a sprinkle or two of snow is certainly possible first half of March granted more so further north you go. Yes a low bar in context of what many may class as cold and wintry, but many would take it after a snowless winter further south.

Last March gave us our most snow of the season, yes the sun went to work on it by lunchtime but we had a few very snowy days. 

Edited February 12 by damianslaw

[RJBingham]

 raz.org.rain I remember with fondness the 2019 SSW, that brought us the warmest February day on record. 

[danm]

 damianslaw agree, but I think many are looking for a proper cold spell with snow that sticks for at least a couple of days. That is going to be hard to come by now, barring a BFTE type big freeze. 

[damianslaw]

 danm Yes I would agree for majority of population south of a line Preston to the Humber, but in the north unless the sun is abundant and only a few cms, then a good cover can stick easily for a couple of days.. indeed late March 13 proved this. Its important to draw the difference between north and south half of UK especially in March when there are often marked temp differentials. My location I do have a north half UK perspective in the minority I guess..

 

[raz.org.rain]

 RJBingham I find it strange that such a possibility isn't being considered. If anything, we've got a pretty strong case for it occurring again.

[trickydicky]

 damianslaw perhaps its the lack of easterlies in recent years, but it feels to me like the difference in temperatures between north and south has grown. Maybe its just recently bias because all through this autumn and winter it has often been stark. 

[damianslaw]

Perhaps this winter will do a summer 2023 on us, bookended by cold instead of warmth i.e. the core summer period late June to late Aug was predominantly cool and unsettled and un summer like though temps were average following the heat at the start of June and back with vengence by September. Can't really compare the cold in late Nov/ very start Dec on same degree of divergence from norm compared to heat of June 2023.. but some synergies but only if first half of March is cold.

[Metwatch]

At the moment I can't see a February 2019 repeat happening this month, well very unlikely. We would need a more amplfiied jet and heights setting up directly to our east as below.

More info on it if anyone's interested: https://simonleewx.com/2019/02/28/a-winter-heatwave-in-a-warming-world/

 

Current output is as flat as a pancake for the next 10-14 days, mild to average and often wet:

Modelling may change of course, but not today. Anyone's guess what March will bring.

[sundog]

Feb  can be a very depressing time of Yr when the preceeding months have been as poor as they have been for coldies and little on offer for the rest of the month . Another winter down the drain.  Its like being stuck in limbo now and waiting for spring to arrive. 

[Chesil View]

 RJBingham

But not before it had brought me this.

[RJBingham]

 Chesil View What's that white stuff?

[Chesil View]

 RJBingham  

[Weather-history]

 RJBingham

......or a brought a colder second half to January 2019? Often forgotten about this.

6 days of lying snow here during that period for my location.

17th Jan-3rd Feb 2019 CET: 2.2°C

 

 

 

[rwtwm]

 damianslaw I know what you're getting at, much March 2013 probably isn't the best illustration. We had snow cover for about a week during that spell, even down here.

[carinthian]

 sundog Its the same over most Continental Europe as well. A mild fest in the extreme this Feb so far in Austria. Possibly heading for the warmest Feb ever. Gut wrenching charts continue for lovers of cold winter weather. So sad. Seems the longer term drivers have failed to deliver ( so far ) but hey who wants cold in March and April . I bet your bottom dollar it will snow on April Fools Day ! Think I need a pint and put my slush boots on . Drip .. Drip..

C

 

[Eagle Eye]

Well this Winter certainly happened, if you can call it that.

[Penguin16]

The thing is even if you want your snow fix now there’s no point booking a ski holiday in advance as you never not what rotten pattern you’ll be stuck in. For my snow fix I’d like it to be actually snowing and not a mild slush fest underfoot. Best to just book it the week it looks good but not many have that luxury including myself. 

[Paul]

I've split out the September stuff from this thread into its own one, for those who really want to discuss it. So this one doesn't get over-run with that. 

https://community.netweather.tv/topic/99881-mild-september-a-mild-winter/?do=getNewComment

 

[Met4Cast]

Lets break this winter down into something a little more digestible in terms of the GSDM/teleconnections and some of the predictions that have been made based on them. I'll be mainly quoting my own posts through the winter but will add in some of Tamaras too if/where appropriate, compare that to the observed state of the AAM/MJO and the corresponding impacts on the weather patterns. I'll discuss the December & in particular the January cold spells too and try and diagnose where things went "off the rails" vs the expectations, particularly with the January cold spell. 

Any bold text within the quoted sections is something I've added to explain certain variables/add context to certain things that otherwise is not explained within the quoted text itself.

Hopefully this post will provide some context on what was expected & why vs the outcomes. Perhaps those that claim it's a load of "rubbish" will gain some much needed context.

Early December:

The opening few days were cold across the UK with blocking extending up through the N Atlantic and into Greenland.

Blocking broke down rather quickly. When did forecasts first start to pick out this colder spell of weather? (I wont be focussing on NWP modelling, mostly teleconnective) here's one of my posts from November 22nd: 

Quote

 

What has happened up until now? A strong westerly wind burst event occurred in the Pacific just west of the dateline, this injected westerly momentum into the overall wind flow budget which sent AAM tendency surging upwards, the prolonged nature of the WWB event and transition east helped to keep AAM tendency positive for a while, this triggered a rise in both Frictional Torque (+FT) and Mountain Torque (+MT) pushing global atmospheric angular momentum (AAM) into positive territory, for a time we had a ocean/atmospheric coupling with the ENSO base state (El Nino).

AAM charts:

If you look at the right AAM chart above, we can see that some of these positive AAM anomalies did transfer northwards towards the pole, albeit not with any particular 'oomph' or real amplification (Atlantic ridge signal). Meanwhile, the MJO was slowly transitioning through a very low amplitude phase 8 which correlates broadly to a, you guessed it, mid Atlantic ridge. Note how broadly similar this MJO phase 8 composite for November is to this weekends pattern;

That is the signal for this weekends Atlantic ridge which models initially struggled with, the tropics & subtropics working in tandem. GFS initially being too amplified and ECM being too flat just to keep it broad. Going into this weekend we will see HP amplify and colder air will sink southwards across the UK. This is now well signposted within broader model outputs and accepted as the evolution going forward. 

The polar jet hitting a roadblock (HP) did amplify the high into Greenland in the end, something that I hadn't expected, this was my own failing of understanding however.

That post from November 22nd went onto discuss what comes after the surge in momentum and the MJO progression continued to cycle on;

Quote

Now.. we're seeing a reversal. Referencing the AAM and Hovmöller charts above, the WWB event has waned and the atmosphere in an attempt to rebalance momentum reverses to easterlies, this tanks AAM tendency and we're beginning to see a fall in overall GLAAM as a result. Meanwhile, the MJO will be transitioning through a relatively high (compared to recent) amplitude phase 1-2 which correlates to HP moving eastwards towards Russia and low pressure from the Atlantic taking over, broadly a typical "zonal" signal, i.e high pressure to the south, low pressure to the north, less south shifted than we've seen recently. 

With all of this in mind, the GFS doesn't seem particularly plausible (GFS at the time was suggesting prolonged blocking), but the ECM solution does. (ECM at this time showed a collapse in blocking/return to Atlantic weather) I doubt the ECM has it exactly right re: details but the signal from the EPS is very much in line with everything I've discussed here. Cold next week, yes. Wintry? Perhaps in places, but a prolonged spell of high latitude blocking looks very unlikely and I suspect the GFS will end up with some egg on it's face. It's not too dissimilar to the ECM it's just probably over-doing the amplification. 

(Blocking as expected, was not a prolonged event and weakened quite rapidly into early December as above re-analysis charts show)

The signal going forward through early December at the moment is overwhelmingly to one of Atlantic driven weather with the worst of the unsettled weather in the north. I suspect we will eventually see NWP outputs beginning to reflect this once the reversal to easterlies in the Pacific starts to feed back into NWP initialisation data in real time.

So.. the signal on November 22nd was for colder weather across the UK late November/early December but teleconnections, in this case the MJO & AAM were towards a return of the Atlantic. By the end of week 1 of December blocking had gone and Atlantic weather systems were across the UK. A prediction that proved right 2 weeks in advance using the teleconnections. I'd class this as a win. 

In that same post from November 22nd I went onto discuss the expected progression through December in terms of the background state flagging late December/early January as a potential time period where blocking/cold weather could manifest, an attempted prediction more than a month ahead.

Quote

Beyond that? Well, now it starts to get exciting. On the Hovmöller above we can see the next MJO wave expected for December beginning to appear in the Indian ocean. This could be more amplified, trigger another more substantial rise in AAM and cycle round into phases which support proper blocking in a hopefully a more amplified state. Late December/early Jan holds a lot more potential.

Did this prediction hold true? In terms of the AAM, yes. Through December we did indeed see a substantial rise in total AAM thanks to the MJO progressing through the Indian ocean and into the Maritimes. Unfortunately, as we know the MJO began to collapse as it passed through the Maritimes, this is something that happens fairly frequently during El Nino winters and particularly in the context of a strongly +ve IOD which, despite weakening, was in place during December. 

FIRST FAIL:

The expectation of a colder late December was a fail, the MJO cycle did not progress as anticipated and due to collapsing this left little "oomph" to generate the blocking that was anticipated/mused upon. I wouldn't say this was a fail in teleconnections but a failure in forecast modelling, indeed models had been overly enthusiastic about a more amplified cycle (albeit GFS/ECM were less enthusiastic than the ridiculous output from the BOM model at the time)

Here is the complete MJO cycle, the blue line represents Decembers progression & then subsequent collapse.

This meant, late December was mild with little in the way of blocking, despite the AAM/GWO responding through December as expected thanks to the very same MJO progression. +Frictional Torque led to +Mountain torque adding westerly momentum into the atmosphere and driving total AAM upwards, as depicted here;

MJO moves eastwards creating +FT, this, via rossby waves transfers northwards increasing +MT and we see the substantial total AAM rise following these two events. The collapse in MJO however led to this late December pattern;

Interestingly, the +EAMT during December helped to trigger the minor SSW in late December. +EAMT increases heatflux into the stratosphere. So, prediction of the AAM proved correct but collapsing MJO failed in the late December expectation & indeed we missed the boat for early January, JUST! 

Indeed, contrary to the belief in this thread that "People who use the GSDM never explain when it fails", I did quite an in-depth post discussing why anticipated late December blocking had perhaps failed and indeed discussed my own personal failings in understanding, you can read that here;

Early - Mid January

Through early to-mid January we saw what was technically a 2 week cold spell, the first week was largely dominated by high pressure (with a brief less cold spell in the middle) with the 2nd week of the cold spell being much colder. Here are a few re-analysis charts to show what happened;

This cold spell was the subject of much debate on teleconnections at the time which had seemingly favoured a continuation of cold beyond the 20th (when the breakdown ultimately came) with expectations very much being that wedges of high pressure could continue to deflect the jet stream southwards & keep the cold in place, despite the waning of high latitude blocking, naturally, this didn't come to pass and instead we saw a quick return to mild & indeed stormy weather with 2/3 (I forgot) named storm systems. 

My post on Christmas Eve below, highlighting some excitement re: potential for cold patterns through January. 

Quote

 

MERRY CHRISTMAS!!

AAM tendency has gone through the roof, we're seeing a massive uptick in +ve MT, notably (and finally) a large +EAMT event which can be a strong precursor/trigger to SSW. This was all largely expected but has been delayed somewhat due to the weakening of the MJO cycle so we've missed the boat on Christmas but things are now unfolding very nicely for the new year.

AAM tendency spiking with +ve momentum now shifting polewards, total AAM is now at the highest level since July/August and will likely continue to rise on the back of the current very strong MT event. 

With the MJO moving through phase 1 seemingly rather slowly, all this feeds back into an increasing likelihood of high latitude blocking developing early-mid January, taking a lag of 14-20 days into account. On top of this we have a potential SSW unfolding which could have further implications later into January depending on downwelling etc. 

All in all.. despite the "delay", things for this winter are shaping up very, very nicely. 

 

Now, some IMPORTANT CONTEXT: This was around the time a SSW was unfolding, originally modelling had predicted it to be a major SSW (technical reversal) but modelling backed away from this idea. My post on the 27th December highlighted this;

Quote

 

The GFS has been gradually backing away from a SSW in recent runs, particularly last nights 00z with a fairy substantial shift away, following yesterdays 12z EPS suite.

Tonights EC46 will certainly make for interesting viewing, I'd not be surprised to see this trend away from the idea too given more recent modelling.

 

Ironically on the same day (27th December) I had urged caution re: a SSW disrupting what at the time teleconnectively was a favourable tropospheric pattern for cold weather patterns.

Quote

 

Personally I'd prefer a SSW doesn't happen (though I would 100% take a very, very weak SPV)

We've got some very, very favourable trop led drivers coming into play during January without the help of a SSW, whilst a SSW could potentially amplify the cold signal/prolong any cold/blocking we end up getting, it could also "reshuffle" things into something unfavourable for us. 

SSW's are always risky.. exciting when nothing else is going our way but always a concern in situations like the one we're looking at going into January. An extraordinary amount of luck and January could be very, very special.. 

 

So.. these two posts 11 days before the first day of the January cold spell (beginning on the 8th January). 

Tropospheric & teleconnective drivers pointed towards blocking but I was cautious re: the SSW potentially making things unfavourable instead. So, yet another example of teleconnections leading the way in terms of broadscale predictions. In actual fact, during early January the mod thread was awash with comments about sinking highs and the likelihood of cold collapsing, those of us subscribing to and applying the GSDM knew this wasn't going to happen, here are a couple of posts from the 3rd January where I tried to reassure that this wasn't what was expected.

Quote

 

Back to reality.. 

The GEFS show the broad evolution, much colder next week before slightly less cold air (at least aloft, the surface will remain cold throughout) filters in with the high re/shuffling before a second bite of the cherry later in the run as the high retrogresses. 

Still a potential for wintriness in the E/SE from the initial UK based high, how much amplification still needs ironing out. 

 

And;

Quote

 

It was never “near 0” and the high isn’t “sinking”. A sinking high is a high that goes into Europe and allows the Atlantic in, that’s not what is being modelled here. 

A UK based high has been favourite for days now. What is up for grabs is how much amplification can be squeezed out of it. 

 

Indeed, as the re-analysis charts above show, we did NOT see a sinking high & we saw retrogression week 2 of the cold spell. Another win for teleconnections vs modelling.

Now, lets fast forward a little bit. We know how the cold spell evolved & the thinking that was being put forward at the time. I.E talks of 1979 and 1947, teleconnectively this was being advertised & indeed we saw some absolutely stunning modelling output in the extended at the time,remember that SSW I just talked about? That's about to bring us resoundingly back into reality. The impact of this SSW could not have been forecasted beyond a week & unfortunately, the late December SSW came back to haunt us. 

Here's a post I made on January 9th.

Quote

 

FWITW the 00z ECM is highlighting where I broadly expect things to head as outlined in the quoted post from yesterday.

Cold air across the UK vs incoming disruptive lows = high impact snowfall events. 

I do think the ECM is being a little too progressive with the lows though, I suspect the first couple will "miss" and pass to the south, again still looking towards the 20th - 20th as a timeframe, although a day or two before can't be discounted given current modelling. 

J

 

Just for context, here's what actually happened on the 16th & 17th:

The low missed southern England by just a couple of miles and our friend the southern Euro high was further north than previous modelling had anticipated.  The conversation within outputs between the 10th - 15th was largely focussed around the tracking of that low pressure system. Zooming back out to teleconnections though and the previously mentioned SSW, when precisely did the wheels fall off? Remember, the cold spell came to an abrupt end on the 20th January, on the 9th things were ticking along nicely.

On January 12th, @Tamara posted this;

Quote

 

The suspicion I continue to have is that these poleward eddies that the models propose to increase polar jet momentum, will get sheared off and this whilst this still leaves a flat jet stream coming from upstream, the shearing off process will cut off the supply to the polar jet in mid stream and force it to backtrack into the sub tropical jet. Much the theme of discussion of late. 

That process, leaving some of the residual ridging between Greenland & Iceland intact and/or re-programming re-emergence of wedge type ridging and subsequent disruption of Canadian lobe energy in mid Atlantic. This leaves southerly jet kayaks to resume progress and changes the extended ensemble means that propose a flat westerly pattern across NW Europe & inflating Azores High.

It will be very interesting (as I see it) to watch and monitor if this does actually transpire.

 

And my subsequent response;

Quote

 

Sounds like what you’re expecting is something similar to the 00z GEM, i.e a more active jet into the Atlantic meeting residual wedges of high pressure to the north causing the jet & subsequent low pressure systems to disrupt SEwards. 

What you advertise is appearing within model outputs albeit as a lower probability outcome currently within the modelling. 

I’m not sure models will be handling this particularly well at extended ranges, resolving split flows & disrupting energy is something modelling always struggles to handle so what initially looks like a flat, zonal-esq pattern often corrects as the lead time shortens. 

No doubt that less cold air will push in as blocking wanes, but that doesn’t necessarily mean a mild zonal onslaught is on the way. 

 

Expectation on the 12th January, 8 days before the cold spell ended was for residual heights to remain around Iceland/N of the UK, it was clear the main blocking would weaken and indeed less cold air was being advertised but as Tamara explained, a southerly tracking jet was still a possibility underneath wedges. 

By the 13th, it was largely game over for a continuation of cold, here's what I posted;

Quote

 

I think the hope of wedges remaining to the north with colder air across the UK beyond the 20th can be discounted now, this is a very strong signal from the EPS for a return to milder, wetter weather. 

The Canadian PV lobe being pulled towards Siberia (likely thanks to the recent warming) is the dominating signal and will override any potential blocking. 

Late January into early February however does still hold potential for a return to more blocked conditions with this next momentum surge. 

Before all of that though - A week of cold weather across the whole of the UK with the potential for snowfall just about anywhere. 

 

I theorised at the time that the recent warming in December had perhaps reshuffled things out of favour with blocking & indeed subsequent analysis (especially when factoring in what would be a technical SSW on January 16th) would show this to be the case. The warming shifted the Canadian PV eastwards into the Atlantic sector which caused the entire pattern to flatten and pulled the jet northwards, any hopes of residual highs to the north were gone.

This is NOT something teleconnective forecasting can predict, this is not something that was foreseen at long lead times but was actually a very short term change which changed the entire outlook going forward and we all know what happened beyond the 20th. On the 18th, I posted this.

Quote

 

Nothing is ever definite in weather, the atmosphere is chaotic, complicated and unless you can model every single molecule of air you'll never have 100% accuracy. However; as described above teleconnections have led the way in terms of the broader forecasts but of course that's never 100%. Unexpected things crop up (such as the SSW impacts) that you just can't account for. 

Long-range forecasting is essentially a "best guess", but I don't think teleconnections/background signals are too over stated or relied upon, you just can't expect 1+2 to always = 3, sometimes you end up with 2+2 unexpectedly.

 

I stand by that. Hopefully I have shown in this (admittedly very long post) that teleconnective forecasting has done well throughout this winter and has signposted the broader changes within northern hemisphere patterns. What is hasn't done is tell you that it will snow in your location on a specific day, or promised snow to anyone. 

Rubbish the science as you wish, but it has proved a valuable tool throughout this winter, short term changes within the configuration of the polar field cannot be predicted at longer ranges but can have a big impact on the weather/broader patterns regardless of teleconnective forcing, this, as stated, cannot be predicted or incorporated particularly well within the GSDM.

Once February is over I will do another post analysing what happened teleconnectively through this month, expectations for a cold February were high during January, evidently that hasn't come to pass.

Edited February 13 by Met4Cast

[damianslaw]

 Met4Cast A very good post, any further surmisings on the reason for last week's low pressure to behave why it did after many models were expecting it to go east not north west!

[Singularity]

I concur with most of the above, the big exception being the stratosphere's role in what took place for the final third of Jan. The 'major SSW' was not a true event, owing to there actually being substantial reflection of the vertical wave activity flux back down toward the troposphere. A large proportion of it did so, while the rest was absorbed.

When that happens, the energy transfer works in favour of the polar vortex - hence zonal regimes for the North Atlantic - and against high latitude blocking. It's rare that this occurs with the 60N 10 hPa zonal wind reversing but on this occasion that was made possible by the minor SSW that took place in early Jan. It seems just enough absorption occurred to nudge the already displaced polar vortex just far enough off the pole to cause that reversal of the mean, before the effect of the reflection kicked in. 

Since then, the middle third of Feb has been all about troposphere-led forcing and, it seems, the discovery that when you have record warm tropical and eastern subtropical North Atlantic sea temps enhancing the Hadley cell, Atlantic trough behaviour is strongly skewed away from tilting negatively and disrupting - the very process required for sustaining blocking high pressure at high latitudes. Never before have I seen so many opportunities for that fail to come to pass within a single season. Even when it has happened it's been at a higher latitude than usual.

There is a very serious implication to this - more than just the UK experiencing relatively little cold, snowy weather: The pattern of anomalously dry, warm weather across Iberia is able to self-perpetuate to a high degree. It may well be that a strong cooling of the aforementioned areas of the N. Atlantic is required for a sustained escape from this pattern. Trouble is, when exiting an El Niño event as we're expected to this spring, there's usually a warming effect on those areas. This is giving me a horrible feeling that the hot season could be absolutely brutal down there this year.

[raz.org.rain]

7 minutes ago, Singularity said:

 

There is a very serious implication to this - more than just the UK experiencing relatively little cold, snowy weather: The pattern of anomalously dry, warm weather across Iberia is able to self-perpetuate to a high degree. It may well be that a strong cooling of the aforementioned areas of the N. Atlantic is required for a sustained escape from this pattern. Trouble is, when exiting an El Niño event as we're expected to this spring, there's usually a warming effect on those areas. This is giving me a horrible feeling that the hot season could be absolutely brutal down there this yea

 

It could very well impact our own shores too, I really wouldn't be at all surprised if we see atmospheric blocking that's favourable to dragging heat into NW Europe. Potentially a very hot and dry summer ahead.

[mountain shadow]

 raz.org.rain Indeed, and when the Atlantic does try and push in, the potential for extremely severe thunderstorms increases hugely.

[Bristawl Si]

 raz.org.rain it was meant to happen last Summer, remember?

Plume after plume were forecasted; instead post-June we had a July that was dire, an August that was not quite as bad as July, and late August/early Sept the sun and warmth eventually returned for a week or so.

Last Summer, post-the June spell, was a complete bust, as far as expectations and forecasts were concerned.