Singularity

We’re at that stage where the modelling as a whole is starting to explore new routes more. Not yet enough to greatly alter the means but it shows that there is some genuine potential afoot. Now we wait for the next few updates to see if support will grow much further beyond the potential late Jan Scandinavian high outcome, which never really took hold.

@Mike Poole cluster 1 is a good signal for a -NAO of the Iceland-focused variety which can actually be on par with a full on Greenland high for cold UK weather, though it tends to be in the drier side. Overall, the signal for the long indicated change is a starting to grow as anticipated. Just needs to sustain for a few days in order for confidence to begin increasing.

I approve of the ECM 12z for showing how a strong high with well above normal 850s across the UK can be part of a hemispheric pattern that increasingly disrupts the tropospheric polar vortex. As of D10 the door is finally starting to open for the high over Europe to shift quite a way northward. Strange that GFS was also doing that sort of thing on-and-off a few days back but has consistently failed to sufficiently, ahem, ‘un-flatten’ the polar jet during the past 8 or so runs. Ah well, at least GEFS are starting to explore the idea more.

The Hadley cell is the driver, not the high over S Europe. An expanding trend to that cell is indeed making such highs stronger & a bit more resistant to change, but it hasn’t given them the power to self-manifest or sustain. The net forcing has to be there in the first place. As per Eagle Eye’s post, if the net forcing (both tropical & extratropical e.g. polar vortex state) is against a Euro high then it will vacate regardless. This would well prove to be the case by about a week into Feb. If that wasn’t true, the Met Office wouldn’t have been mentioning easterlies & cold temperatures so much lately. Importantly, the models typically overdo persistence of near term patterns. We notice it often with HLB because of how anomalous that setup is, but it periodically happens with +AO, +NAO setups too. Hence you don’t tend to see me ‘writing off’ a possibility one way or the other based on what the models, even entire ensemble suites, indicate for more than about a week ahead in time. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5022974

The Hadley cell is the driver, not the high over S Europe. An expanding trend to that cell is indeed making such highs stronger & a bit more resistant to change, but it hasn’t given them the power to self-manifest or sustain. The net forcing has to be there in the first place. As per Eagle Eye’s post, if the net forcing (both tropical & extratropical e.g. polar vortex state) is against a Euro high then it will vacate regardless. This would well prove to be the case by about a week into Feb. If that wasn’t true, the Met Office wouldn’t have been mentioning easterlies & cold temperatures so much lately. Importantly, the models typically overdo persistence of near term patterns. We notice it often with HLB because of how anomalous that setup is, but it periodically happens with +AO, +NAO setups too. Hence you don’t tend to see me ‘writing off’ a possibility one way or the other based on what the models, even entire ensemble suites, indicate for more than about a week ahead in time.

Some here seem to view a strong Iberian high as a driver of weather patterns - but this is not so, it’s a response to drivers. In this case the forcing leads to attempts to establish a high over N Europe which are countered by the AO being too positive (likely related to the unusual stratospheric events - who’d have rationally bet on that one?), meaning that in a sense, amplification becomes trapped over S Europe. Due to the MJO & AAM setup, further attempts to shift the high N are likely in the coming weeks (which can happen abruptly even with a strong Euro high). Signs are the first of those in 6 days time will likely fall short & boost the high over C/S Europe instead. Then, however, we are seeing the models tend to raise polar heights as we head into Feb. That improves the chances of the next attempt being successful, regardless of how current modelling paints the picture (they’re not actually that reliable for week 2+, it’s just easier to predict +NAO because it happens way more often than -NAO in N Atlantic climatology). Emphasis on chance here. As Tamara has said many times over the years, there’s no magic bullet for HLB patterns. As I’ve said before, the most likely outcome isn’t always what happens. With this in mind I’ve found forecast model watching a lot less stressful. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5022887

Some here seem to view a strong Iberian high as a driver of weather patterns - but this is not so, it’s a response to drivers. In this case the forcing leads to attempts to establish a high over N Europe which are countered by the AO being too positive (likely related to the unusual stratospheric events - who’d have rationally bet on that one?), meaning that in a sense, amplification becomes trapped over S Europe. Due to the MJO & AAM setup, further attempts to shift the high N are likely in the coming weeks (which can happen abruptly even with a strong Euro high). Signs are the first of those in 6 days time will likely fall short & boost the high over C/S Europe instead. Then, however, we are seeing the models tend to raise polar heights as we head into Feb. That improves the chances of the next attempt being successful, regardless of how current modelling paints the picture (they’re not actually that reliable for week 2+, it’s just easier to predict +NAO because it happens way more often than -NAO in N Atlantic climatology). Emphasis on chance here. As Tamara has said many times over the years, there’s no magic bullet for HLB patterns. As I’ve said before, the most likely outcome isn’t always what happens. With this in mind I’ve found forecast model watching a lot less stressful.

For broad patterns they have value if not taken too literally, more as a suggestion which way the weather patterns will attempt to go. After all, the MJO is one of many forcing factors. That’s why we don’t see forecast models just following MJO composites even at 7+ days lead time when there here & now becomes increasingly less relevant I.e. persistence of the near term pattern tendency becomes increasingly inaccurate. Long story short, while I can see plenty of scope for a high to set up over Scandinavia by early Feb, I think it’s a lot more up in the air whether it’s positioned & orientated in a way that’s notably cold for the UK (let alone snowy). The HLB dominated seasonal modelling for Feb is very intriguing in this regard - it seems it must be tied to subsequent developments… but can we trust that those will come through? If only it was that easy!

These composites are for all MJO amplitudes however. The ones I’ve referred to are for at least 0.5, maybe even 1.0 amplitude & were produced more recently making them a bit more relevant to the current climate. Not that MJO composites tell all. The AAM cycle adds its own twist, as does the stratospheric polar vortex state if it’s usually strong or weak. I continue to view the week 2 modelling with only slight interest for the time being. The first major opportunity to disrupt the Atlantic trough enough to start migrating high pressure appreciably toward Scandinavia is next Fri so I’m hoping for a clearer picture by Mon on whichever way things will go. These composites are for all MJO amplitudes however. The ones I’ve referred to are for at least 0.5, maybe even 1.0 amplitude & were produced more recently making them a bit more relevant to the current climate. Not that MJO composites tell all. The AAM cycle adds its own twist, as does the stratospheric polar vortex state if it’s usually strong or weak. I continue to view the week 2 modelling with only slight interest for the time being. The first major opportunity to disrupt the Atlantic trough enough to start migrating high pressure appreciably toward Scandinavia is next Fri so I’m hoping for a clearer picture by Mon on whichever way things will go.

I find it helps to set skepticism of runs about as high as it can go when looking at a period of potential Scandinavian high development 7+ days away. Even these days, it remains a weak point for the NWP models; something they really struggle to resolve much in advance. Mainly because of the difficulties resolving the relatively small scale irregularities involved in trough disruption, which is the primary means to get low heights in place over S Europe. This view can even be applied to entire ensemble sets, owing to their forced perturbation only being at the very start; it does little to help resolve the possible development of new small scale features a week or more away. Original post: https://community.netweather.tv/topic/99793-model-output-discussion-cold-spell-ending-what-next/?do=findComment&comment=5019068

I find it helps to set skepticism of runs about as high as it can go when looking at a period of potential Scandinavian high development 7+ days away. Even these days, it remains a weak point for the NWP models; something they really struggle to resolve much in advance. Mainly because of the difficulties resolving the relatively small scale irregularities involved in trough disruption, which is the primary means to get low heights in place over S Europe. This view can even be applied to entire ensemble sets, owing to their forced perturbation only being at the very start; it does little to help resolve the possible development of new small scale features a week or more away.

Ah but here’s the thing - once you’re into Feb, with an El Niño-like (high AAM) background such as is expected next month, the Scandi high signal emerges with MJO phase 6, with 7 becoming the transitional one that’s part Scandi high, part -NAO. So if the models lock onto predicting it to be slow moving in phase 6, I won’t be concerned about all this talk of highs to our N/NE being for nowt. Original post: https://community.netweather.tv/topic/99793-model-output-discussion-cold-spell-ending-what-next/?do=findComment&comment=5018546

Ah but here’s the thing - once you’re into Feb, with an El Niño-like (high AAM) background such as is expected next month, the Scandi high signal emerges with MJO phase 6, with 7 becoming the transitional one that’s part Scandi high, part -NAO. So if the models lock onto predicting it to be slow moving in phase 6, I won’t be concerned about all this talk of highs to our N/NE being for nowt.

A key timeframe is around Fri 26th so 9-11 days from now. This is where model bias tends to show its hand most strongly as it’s far enough out for corrections not to have progressed very far but not so far out that uncertainty largely smothers it. Will be interesting to see how things go the next couple of days. Not saying we’ll definitely see a convergence on something along the lines of the GFS 12z but IMO it was a decent effort given the AAM & MJO situation. Just got a bit overly dramatic late on perhaps as it happened to steer a pocket of particularly cold air across to S UK.

Should trends continue, this will be one of the clearest demonstrations I can recall of how the AAM/GWO diagnostic tool in conjunction with MJO forecasts plus experience of model behaviour can be used to anticipate model bias of the sort that sometimes points entire ensemble suites in the wrong direction. Could it possibly go that smoothly with a high established over Scandinavia by the end of the month… plausibly, yes. Not that I’d go as far as betting on it just yet - the road has a habit of being rocky UK & Ireland-wise, narrow lands as we are! Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5015661

Should trends continue, this will be one of the clearest demonstrations I can recall of how the AAM/GWO diagnostic tool in conjunction with MJO forecasts plus experience of model behaviour can be used to anticipate model bias of the sort that sometimes points entire ensemble suites in the wrong direction. Could it possibly go that smoothly with a high established over Scandinavia by the end of the month… plausibly, yes. Not that I’d go as far as betting on it just yet - the road has a habit of being rocky UK & Ireland-wise, narrow lands as we are!

IMO it’ll be worth keeping an eye out for any adjustments toward a more northward direction of travel for the big surge of very mild air being modelled the week after this coming one. That would serve as a trigger for shifting high pressure fully northward through the UK. I only flag this due to the expected AAM/MJO situation not really being conducive to a massively pumped up high centred south of the UK (such would be fitting if AAM was a lot lower). In other words, it’ll take a lot to force such an outcome rather than open a path for northward migration of the high at some point during that week. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5012261

IMO it’ll be worth keeping an eye out for any adjustments toward a more northward direction of travel for the big surge of very mild air being modelled the week after this coming one. That would serve as a trigger for shifting high pressure fully northward through the UK. I only flag this due to the expected AAM/MJO situation not really being conducive to a massively pumped up high centred south of the UK (such would be fitting if AAM was a lot lower). In other words, it’ll take a lot to force such an outcome rather than open a path for northward migration of the high at some point during that week.

Based on the high AAM state limiting the strength of westerlies, I’d not be surprised to see the shift to a more settled outcome next weekend into the week after be repeated again tomorrow such that UKM & ECM keep wet weather north of most or all of the UK, perhaps even with the south holding onto light winds. Regardless, the high across W or NW Europe should tend to migrate further N or NNE during the final third of Jan as the new high AAM cycle plays out in tandem with an active MJO setting out across the Pacific. In Feb the response to a Pacfic-crossing MJO becomes most strongly in favour of HLB out of the winter months. No surprise that the seasonal models have been so keen on a HLB dominated month. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5010756

Based on the high AAM state limiting the strength of westerlies, I’d not be surprised to see the shift to a more settled outcome next weekend into the week after be repeated again tomorrow such that UKM & ECM keep wet weather north of most or all of the UK, perhaps even with the south holding onto light winds. Regardless, the high across W or NW Europe should tend to migrate further N or NNE during the final third of Jan as the new high AAM cycle plays out in tandem with an active MJO setting out across the Pacific. In Feb the response to a Pacfic-crossing MJO becomes most strongly in favour of HLB out of the winter months. No surprise that the seasonal models have been so keen on a HLB dominated month.

The ultimate level for week 2 forecasting is correctly anticipating the impacts of model bias, which can strongly skew entire ensemble suites. Alas, that’s a lot easier said than done! I sure can see how GFS & ECM might be incorrectly presuming a more traditional route for the westerlies associated with the initial stages of rising AAM (a.k.a. lagged impacts of AAM having recently fallen to near neutral). In short, if enough residual heights hold on at the high latitudes via lingering easterly momentum anomalies, the strengthening polar jet will be redirected into S Europe instead of N. It is inevitably an ‘if’ but the point is, it has more chance of happening than the ensemble products currently suggest. In other words, best keep an open mind as to what may happen even on the broad scale later next week & through the one after. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5008298

The ultimate level for week 2 forecasting is correctly anticipating the impacts of model bias, which can strongly skew entire ensemble suites. Alas, that’s a lot easier said than done! I sure can see how GFS & ECM might be incorrectly presuming a more traditional route for the westerlies associated with the initial stages of rising AAM (a.k.a. lagged impacts of AAM having recently fallen to near neutral). In short, if enough residual heights hold on at the high latitudes via lingering easterly momentum anomalies, the strengthening polar jet will be redirected into S Europe instead of N. It is inevitably an ‘if’ but the point is, it has more chance of happening than the ensemble products currently suggest. In other words, best keep an open mind as to what may happen even on the broad scale later next week & through the one after.

The mid-stratosphere modelling has made for an interesting watch this past week and probably has played a role in the uncertainty regarding how strong the tropospheric polar vortex lobe over N. America will be, hence how strongly the Atlantic lows attempt to push cold weather out of the UK next week. First off there's a week split this Wed-Thu, followed by a partial merger on Fri, followed by a more pronounced split Sat-Tue. It remains to be resolved how strong the vortex above N. America is, hence the uncertainty with what goes on below. Incredibly, there are hints it may attempt to split yet again later next week, though the latest run wasn't as interested as the 00z. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=5001066

The mid-stratosphere modelling has made for an interesting watch this past week and probably has played a role in the uncertainty regarding how strong the tropospheric polar vortex lobe over N. America will be, hence how strongly the Atlantic lows attempt to push cold weather out of the UK next week. First off there's a week split this Wed-Thu, followed by a partial merger on Fri, followed by a more pronounced split Sat-Tue. It remains to be resolved how strong the vortex above N. America is, hence the uncertainty with what goes on below. Incredibly, there are hints it may attempt to split yet again later next week, though the latest run wasn't as interested as the 00z.

The increased interest from models in raising heights to the N of Scandinavia in 8-10 days time has caught my eye. One of the difficulties before any such development is how far NE the main trough sets up over Scandinavia / NW Asia. We’re very peripheral in that sense. A high to its north encouraging that trough to ‘string out’ would really aid driving of a deep cold airmass toward NW Europe during 3rd week Jan. Worth looking out for any increased ensemble support for it in the next few days. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=5000334