Met4Cast

Bit of a chicken and egg situation really. Trop led patterns triggered the strat warming but then the warming weakened the SPV allowing trop led patterns to amplify a high into Greenland, the weak lower strat perhaps aiding in this. In essence - Without a strong SPV overriding tropospheric drivers, those drivers were able to do their thing to produce the blocking, despite trop led drivers being the reason for the weak vortex in the first place.. Wibbly wobbly..

The trop and the strat have had a wee fender bender but despite a little shock there’s no lasting damage

I’m not sure anyone said the minor warming had no impact? Quite clearly it helped support the Greenland high. Likewise, it’s also now pulling the Canadian vortex east, hence the mild & stormy weather next week. You can see this on the strat plots.

This isn’t your typical SSW bringing down a strong vortex with big surface impacts. We already have a weak vortex & have done for most of winter, this SSW as a result is a bit like adding a glass of water to an ocean and expecting a big change. Despite the warming on the 16th being technically a major SSW, we’ve already seen a typical SSW response (current cold) from the previous minor event. In fact, there have now been 3 warming events this season! Strong vortex’s fall hard.. a weak vortex being punched doesn’t. A quick return to average strength in the next couple of weeks is now very likely, we’ve lost the tropospheric “punching”.

Yes.. Although many in this thread are chasing the cold & snow patterns rather than the “typical winter” patterns. What you’ve said fits well with broader climatic trends of course, but that’s not really where this thread tends to focus on! I don’t think anybody is expecting historic cold spells, just a bit of snow would be nice

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.

Comments like this confuse me and I think it's down to a lack of understanding of "background signals" that trigger it. The teleconnections are what drive our weather and create the weather patterns that we see, NWP modelling is based entirely upon these signals. Understanding teleconnections allows you to view NWP modelling in a much broader context. "Too much emphasis" absolutely cannot be placed on the things that literally drive our weather. If anything, not enough emphasis is placed upon them. I think where people get caught up is that they expected certain drivers to produce specific weather conditions outside their front door & unfortunately, that's not how it works. You can use these drivers to get a flavour of the broader, global scale weather patterns but the micro scale, i.e outside your front door is so local & isolated compared to the northern hemisphere that x + y does not necessarily = a snow day. This current cold period & blocking spell we're in the middle of was well advertised weeks in advance. The MJO cycling through phase 1/2 and slowly through 3 helped to generate a Rossby wave packet within the Pacific which led to wavebreaking in the Atlantic & the Greenland high that has brought about the current cold weather. This, in conjunction with a high total AAM state (increased westerly momentum within the atmosphere) led to a perturbed and amplified jet stream capable of generating the HLB we've seen. A strong +EAMT (East Asian Mountain Torque) event created the Pacific jet extension which developed a downstream trough, causing a jet streak which then created a downstream trough, follow this all the way into the Atlantic with each "wave" amplifying the next. We've got a Greenland high, we've got cold weather across the UK, broadscale pattern? Check. The next step is the micro, i.e small-scale weather systems that may or may not bring about snowfall in specific areas, that's something background/teleconnective signals can never help you predict. Ironically, the "downfall" to this cold pattern & the reason for next weeks milder & stormy weather isn't because of a sudden change in background tropospheric forcing but down to the recent SSW. The SSW has helped to displace the Canadian vortex eastwards towards Siberia which has flattened the pattern. A case of bad timing, again, something teleconnections can never help you predict. Original post: https://community.netweather.tv/topic/99793-model-output-discussion-cold-spell-ending-what-next/?do=findComment&comment=5017705

The state of the stratosphere needs to be taken into account. An unstable profile (such as we have now) often favours higher latitude blocking. If the strat had been in a strong position we'd likely have seen the current blocking fail. SSW's cannot be predicted with any real accuracy at longer ranges so it's always a wildcard. Unfortunately on this occasion instead of amplifying the blocking signal, it's flattened it with a displacement event. You win some, you lose some..

Other modelling appears to be suggesting a similar thing in phase 6 too. Theres certainly consistency between the modelling there! Fortunately, phase 6 in the context of El Nino does still broadly favour Scandi heights. Throw in the tendency for a south shifted jet stream and we could see some decent undercutting, but details details! Not *too* concerned about the MJO forecasts at that range.

Comments like this confuse me and I think it's down to a lack of understanding of "background signals" that trigger it. The teleconnections are what drive our weather and create the weather patterns that we see, NWP modelling is based entirely upon these signals. Understanding teleconnections allows you to view NWP modelling in a much broader context. "Too much emphasis" absolutely cannot be placed on the things that literally drive our weather. If anything, not enough emphasis is placed upon them. I think where people get caught up is that they expected certain drivers to produce specific weather conditions outside their front door & unfortunately, that's not how it works. You can use these drivers to get a flavour of the broader, global scale weather patterns but the micro scale, i.e outside your front door is so local & isolated compared to the northern hemisphere that x + y does not necessarily = a snow day. This current cold period & blocking spell we're in the middle of was well advertised weeks in advance. The MJO cycling through phase 1/2 and slowly through 3 helped to generate a Rossby wave packet within the Pacific which led to wavebreaking in the Atlantic & the Greenland high that has brought about the current cold weather. This, in conjunction with a high total AAM state (increased westerly momentum within the atmosphere) led to a perturbed and amplified jet stream capable of generating the HLB we've seen. A strong +EAMT (East Asian Mountain Torque) event created the Pacific jet extension which developed a downstream trough, causing a jet streak which then created a downstream trough, follow this all the way into the Atlantic with each "wave" amplifying the next. We've got a Greenland high, we've got cold weather across the UK, broadscale pattern? Check. The next step is the micro, i.e small-scale weather systems that may or may not bring about snowfall in specific areas, that's something background/teleconnective signals can never help you predict. Ironically, the "downfall" to this cold pattern & the reason for next weeks milder & stormy weather isn't because of a sudden change in background tropospheric forcing but down to the recent SSW. The SSW has helped to displace the Canadian vortex eastwards towards Siberia which has flattened the pattern. A case of bad timing, again, something teleconnections can never help you predict.

I know some are downbeat about this current colder period due to the lack of snowfall (in southern areas, anyway) but the teleconnections advertised this period very well. This current cold spell isn't over yet but already it seems a return to blocked/colder conditions is the broad direction of travel into February. The mechanism and drivers for this to occur are already in motion. The MJO looks to progress through the Maritimes into the western Pacific, with this frictional torque has risen with +MT following on behind. MT always follows FT. Westerly momentum is transferred from the earths rotation into the atmosphere and the response is AAM tendency goes up, driving total AAM upwards. Indeed, AAM tendency has recently turned positive once again in response to the MJO progression eastwards. As the MJO continues into phase 6 (and possibly 7, this is more uncertain) we see a configuration favourable for high latitude blocking. It seems probable we'll see a period of mild (perhaps exceptionally so) weather as high pressure builds northwards through the UK but there is an increased likelihood of Scandinavian blocking & thus, colder easterly winds 6th - 15th February as a broad timeframe. The starting pistol, to borrow a phrase from Tamara has been fired. NWP modelling isn't quite in range of this period yet however we're already seeing signs of pressure rising to the NE via a route through the UK, I suspect these runs are a little progressive in terms of timing but do advertise the broader direction of travel. Eyes down.. the next chase is about to begin. Original post: https://community.netweather.tv/topic/99793-model-output-discussion-cold-spell-ending-what-next/?do=findComment&comment=5017688

I know some are downbeat about this current colder period due to the lack of snowfall (in southern areas, anyway) but the teleconnections advertised this period very well. This current cold spell isn't over yet but already it seems a return to blocked/colder conditions is the broad direction of travel into February. The mechanism and drivers for this to occur are already in motion. The MJO looks to progress through the Maritimes into the western Pacific, with this frictional torque has risen with +MT following on behind. MT always follows FT. Westerly momentum is transferred from the earths rotation into the atmosphere and the response is AAM tendency goes up, driving total AAM upwards. Indeed, AAM tendency has recently turned positive once again in response to the MJO progression eastwards. As the MJO continues into phase 6 (and possibly 7, this is more uncertain) we see a configuration favourable for high latitude blocking. It seems probable we'll see a period of mild (perhaps exceptionally so) weather as high pressure builds northwards through the UK but there is an increased likelihood of Scandinavian blocking & thus, colder easterly winds 6th - 15th February as a broad timeframe. The starting pistol, to borrow a phrase from Tamara has been fired. NWP modelling isn't quite in range of this period yet however we're already seeing signs of pressure rising to the NE via a route through the UK, I suspect these runs are a little progressive in terms of timing but do advertise the broader direction of travel. Eyes down.. the next chase is about to begin.

The GFS 12z deterministics idea of a Scandi high is a huge outlier amongst the ensembles.. but it’s an interesting one. It’d usually be rather easy to ignore given it’s an outlier but what it shows does fit with the background forcing that has been discussed to death in recent weeks. MJO progression east > +FT > +MT > Rising AAM tendency with the MJO eventually pushing into the Pacific, tropics & sub tropics then singing together for high latitude blocking. I wonder if the GFS Det is picking up on this signal but being too progressive with it? Evidently there’s zero support currently but it’s a very curious one. The next few days could be interesting.

Difficult to argue against the signal for milder & unsettled weather looking at the ensembles but the longer term prospects still favour a return to cold/blocked patterns.

Really? This week has been largely below average with blocking. High pressure is now shifting NW towards Greenland ushering in much colder air for this coming week. That’s 2 weeks of below average & blocked conditions which has been well advertised within the GSDM/teleconnections. Is it widespread snow? No, perhaps but, but that wasn’t really advertised or promised. What was advertised is an increased likelihood of blocking & colder weather and in fairness that’s exactly what we’ve seen. Teleconnections can give a broad sense of the larger patterns but will never be able to tell you if X will see snow. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5010899

Really? This week has been largely below average with blocking. High pressure is now shifting NW towards Greenland ushering in much colder air for this coming week. That’s 2 weeks of below average & blocked conditions which has been well advertised within the GSDM/teleconnections. Is it widespread snow? No, perhaps but, but that wasn’t really advertised or promised. What was advertised is an increased likelihood of blocking & colder weather and in fairness that’s exactly what we’ve seen. Teleconnections can give a broad sense of the larger patterns but will never be able to tell you if X will see snow.

I'm talking about the warming in early January, not the Canadian warming in December.

Perhaps this next round of warming will help keep the strat away from Canada & give the next renewed blocking potential more of a chance!

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.

If we look at the GFS strat forecasts I think we can find an answer to where this zonal push within the modelling is coming from. The Canadian lobe of the sPV appears to get sucked towards Siberia by the stronger SIberian lobe, this occurs 17th-20th, around the time we're seeing modelling flatten the Atlantic pattern out and return the tropical jet northwards. This could perhaps be a response to the minor SSW we've recently seen, perhaps a case of the worst timing ever? (I did muse that a SSW minor or otherwise could potentially disrupt any cold spell a while back). Once that transition has occurred & based on the current background forcing I see no reason why we wont see high pressure beginning to re-establish itself in a more favourable place for further cold weather outbreaks, Tamara has outlined this far better than I can in a few of her recent posts on this thread.

Seems unlikely now. The vast majority of the EPS, MOGREPS and GEFS keep this system south and the envelope is shrinking rather quickly. I think for us in the south we need to start looking for smaller disturbances within the flow coming down from the north next week.

Not quite..

Because of a single run? I think we need a little perspective here. We currently have below average/cold conditions across the UK and by the end of next week we would have had (apart from a couple of days) 2 weeks of colder, blocked conditions. Many southern areas saw snow (albeit not a lot) last Monday. We go into next week with much colder air moving south across the UK with increasing snow risks. Background forcing has done it’s job, the broadscale pattern is as advertised & expected, high latitude blocking is becoming established. The UK is a small island though so despite everything being favourable, blocking & cold conditions in place snow can still be elusive, but I don’t think it will be. Next week looks very unstable and open to small-scale features developing and bringing widespread snow risks, these may not be picked up until 12hrs before they arrive. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5009289

Because of a single run? I think we need a little perspective here. We currently have below average/cold conditions across the UK and by the end of next week we would have had (apart from a couple of days) 2 weeks of colder, blocked conditions. Many southern areas saw snow (albeit not a lot) last Monday. We go into next week with much colder air moving south across the UK with increasing snow risks. Background forcing has done it’s job, the broadscale pattern is as advertised & expected, high latitude blocking is becoming established. The UK is a small island though so despite everything being favourable, blocking & cold conditions in place snow can still be elusive, but I don’t think it will be. Next week looks very unstable and open to small-scale features developing and bringing widespread snow risks, these may not be picked up until 12hrs before they arrive.

Although worth noting the increase in ridging on the GFS, it's certainly a step towards the UKMO even it it hasn't quite managed to go the whole 10 yards.