Singularity

Perhaps GFS still has some worth after all as the ECM 12z has picked up the slight trough disruption for Friday that GFS started exploring a few runs back. Goes to show what a large scale cold airmass can do when the polar jet isn’t particularly strong, though it remains to be seen whether it actually occurs.

We need to wait and see just how the blocking high over the Urals/Barents-Kara seas behaves next week. I’ve noticed a bit of a trend to keep it more or less in place for longer, due to a weaker polar jet across the Atlantic-Europe sector. With that, the chance of a SSW occurring sometime end of Jan to mid-Feb is increasing IMO. Still far from odds on, mind!

EC42 support for a SSW hasn’t totally gone but has reduced from about 20% of the pack to near 12%. I expect it’s highly sensitive to just how long high pressure remains across Barents/Kara with some coverage of adjacent lands to drive cross-mountain flow hence vertical wave activity flux. The faster it gets pushed east, the sooner that flux subsided and the less likely the warming is to be sufficient for fully displacing the polar vortex and triggering a SSW. Theres also uncertainty regarding the Aleutian low, just how long that persists for. The longer it continues the better the odds of a very large wave-1 (displacement) or even a wave-2 (split type) event. There’s a reason why SSWs aren’t reliably picked up on at over two weeks lead time. The possible one for early Feb seems a coin-toss to me, we can only guess which way it will actually go. Original post: https://community.netweather.tv/topic/98196-model-output-discussion-new-year-and-beyond/?do=findComment&comment=4786067

EC42 support for a SSW hasn’t totally gone but has reduced from about 20% of the pack to near 12%. I expect it’s highly sensitive to just how long high pressure remains across Barents/Kara with some coverage of adjacent lands to drive cross-mountain flow hence vertical wave activity flux. The faster it gets pushed east, the sooner that flux subsided and the less likely the warming is to be sufficient for fully displacing the polar vortex and triggering a SSW. Theres also uncertainty regarding the Aleutian low, just how long that persists for. The longer it continues the better the odds of a very large wave-1 (displacement) or even a wave-2 (split type) event. There’s a reason why SSWs aren’t reliably picked up on at over two weeks lead time. The possible one for early Feb seems a coin-toss to me, we can only guess which way it will actually go.

MJO looks to reach phase 7 by this weekend but with uncertain amplitude, EPS much keener than GEFS in that regard. Seems we will after all see some significant influence on the weather patterns by Thu with a high manifesting over Scandinavia, but the timing relative to the AAM cycle, along with lower stratospheric polar vortex status, is not conducive to a sufficiently weak zonal flow across the Atlantic for cold air to reach the UK directly. We could feasibly see a bit drawn around the N flank of a low breaking away from the Atlantic trough akin to yesterday’s GFS 06z but it’s a long shot. AAM changes will become more conducive to a slower, south-shifted polar jet during week 2, hence the ensemble modelling suggesting such things despite the MJO becoming indistinct.

Essentially we’ve spent the past week or so tracking whether the MJO might be just about strong enough, in the face of climatology and an ongoing La Niña, to force a sufficient northward advance of high pressure across Europe for a Scandinavian resting point. Modelling has gradually backed off that and unusually it was ECM/EPS that started out keenest on it (fast & relatively strong), something which had me seeing more of a chance of sufficient MJO activity than I would if it had been GFS/GEFS (they’re usually the overly keen model runs). The 00z runs look to me to be another step away. In fact, the GFS 00z is more fitting of a return of enhanced convection to the Indian Ocean, something I’ve noticed gaining a little traction in the ensembles lately. It’s not out of the question that 1st half Jan as a whole plays out very differently to what longer term ensemble means have been suggesting, with a mobile pattern and any colder weather coming via rear flanks of lows crossing the UK as high pressure stays suppressed across southern Europe. Remains to be seen how the MJO actually unfolds, mind - there’s huge spread in the ensembles on that matter. Enough runs - possibly including the GFS 06z by the looks of it as of +192 hours - are still finding sufficient phase 6-7 MJO action to keep that outcome on the table… just about. Kind of annoying really as I’d like to be able to settle on a route forward for a while! Original post: https://community.netweather.tv/topic/98132-model-output-discussion-christmas-week-and-beyond/?do=findComment&comment=4778948

Essentially we’ve spent the past week or so tracking whether the MJO might be just about strong enough, in the face of climatology and an ongoing La Niña, to force a sufficient northward advance of high pressure across Europe for a Scandinavian resting point. Modelling has gradually backed off that and unusually it was ECM/EPS that started out keenest on it (fast & relatively strong), something which had me seeing more of a chance of sufficient MJO activity than I would if it had been GFS/GEFS (they’re usually the overly keen model runs). The 00z runs look to me to be another step away. In fact, the GFS 00z is more fitting of a return of enhanced convection to the Indian Ocean, something I’ve noticed gaining a little traction in the ensembles lately. It’s not out of the question that 1st half Jan as a whole plays out very differently to what longer term ensemble means have been suggesting, with a mobile pattern and any colder weather coming via rear flanks of lows crossing the UK as high pressure stays suppressed across southern Europe. Remains to be seen how the MJO actually unfolds, mind - there’s huge spread in the ensembles on that matter. Enough runs - possibly including the GFS 06z by the looks of it as of +192 hours - are still finding sufficient phase 6-7 MJO action to keep that outcome on the table… just about. Kind of annoying really as I’d like to be able to settle on a route forward for a while!

So the very important MJO action has corrected a bit weaker during the weekend, reducing the strength of push toward a Scandinavian high - hence we’re seeing a lot of Euro highs in the modelling as the amplifying ridge becomes trapped over central parts of the continent. Still enough spread across stronger MJO outcomes that we could yet see a Scandi high emerge, but probably of the ‘saggy’ variety that requires a narrow range of orientations in order for cold continental air to reach the UK much - or at all. Failing that we have the SSW possibility but honestly, that has to be mainly wave-2 driven to capture my interest. When wave-1 dominates as modelling currently suggests, the resulting displacement type SSW tends to put notably cold weather across N America while we get at most a 10 day cold spell but are just as prone to abnormally mild weather depending on the finer details of the displacement. Wave-2 type SSWs are more common during a La Niña so you never know… but at the moment and for the foreseeable future the atmospheric state is being pushed so far from La Niña-like that I can’t see typical La Niña tendencies being a useful guide for most or all of Jan 2023. Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4778009

So the very important MJO action has corrected a bit weaker during the weekend, reducing the strength of push toward a Scandinavian high - hence we’re seeing a lot of Euro highs in the modelling as the amplifying ridge becomes trapped over central parts of the continent. Still enough spread across stronger MJO outcomes that we could yet see a Scandi high emerge, but probably of the ‘saggy’ variety that requires a narrow range of orientations in order for cold continental air to reach the UK much - or at all. Failing that we have the SSW possibility but honestly, that has to be mainly wave-2 driven to capture my interest. When wave-1 dominates as modelling currently suggests, the resulting displacement type SSW tends to put notably cold weather across N America while we get at most a 10 day cold spell but are just as prone to abnormally mild weather depending on the finer details of the displacement. Wave-2 type SSWs are more common during a La Niña so you never know… but at the moment and for the foreseeable future the atmospheric state is being pushed so far from La Niña-like that I can’t see typical La Niña tendencies being a useful guide for most or all of Jan 2023.

These nudges of the polar jet further south along with reductions in zonality might yet prove to be a big deal by the early days of Jan. Scandi highs are rarely picked up on at long lead times due to models having a propensity to keep troughs too neat and tidy rather than disrupt them. The further south the storm track and slower moving the main lows, the more likely trough disruption becomes. One thing that really keeps my confidence down at the moment, though, is EC42 consistently showing a classic MJO phase 7 response halfway through Jan. It’s as if there’s something in play that can add 5-7 days to the usual 4-10 days lag time for pattern response to the MJO. What could it be…? Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4776577

The edge of the COD is +1 sigma which is significant amplitude based on composites. There’s no actual cut-off within that circle on the phase diagram.

These nudges of the polar jet further south along with reductions in zonality might yet prove to be a big deal by the early days of Jan. Scandi highs are rarely picked up on at long lead times due to models having a propensity to keep troughs too neat and tidy rather than disrupt them. The further south the storm track and slower moving the main lows, the more likely trough disruption becomes. One thing that really keeps my confidence down at the moment, though, is EC42 consistently showing a classic MJO phase 7 response halfway through Jan. It’s as if there’s something in play that can add 5-7 days to the usual 4-10 days lag time for pattern response to the MJO. What could it be…?

Telling that as we draw nearer the time, next week’s lows are being corrected to messier systems with smaller secondary lows galore. The zonal mean isn’t looking to become strong in the lower strat, just near normal. The MJO forcing has more work to do than it might have (had it been quicker) but the situation could be far worse for those seeking renewed cold weather in early Jan.

None taken, I understand the frustrations that can arise when the modelled forcing shifts significantly before our eyes. I believe the behaviour of the storm has been affected by the phase 5 MJO in a way that the models took time to fully catch onto, though most got there by Tue (GFS the glaring exception). Y’know, I’d not be surprised if the MJO-polar vortex battle played out broadly similar to how it did last January, with mid-latitude blocking early next year, though perhaps centred a bit further east this time. UK mainly settled especially in south where temps lowest, chance of sneaking in a cold E to SE flow depending on how the high orientates. Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4776148

Next weeks pattern is a very good match for zero lag MJO phase 5 composites all the way from eastern N America to East Asia. Most model runs then move the MJO into phase 6 which corresponds to below normal heights over Europe. That implies that we should see the Atlantic storm track tend to head further south as we approach the new year. So far this has only been hinted at in the modelling with slight adjustments south bringing the polar boundary down to central UK at times. Question then is whether the MJO is strong enough to force sufficient deceleration of the Atlantic westerlies for a high to establish N or NE of the UK via a stalling trough that disrupts into Europe. Again, only hints of this in the modelling. We often see westerly regimes extended until further notice by the modelling due to their relatively high stability, only for a change to manifest at the 7-10 day lead times having barely been represented beforehand. So… we’ll see - I’m not (yet, at least) convinced that a predominately westerly regime is going to lock in for much more than a week. Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4776134

None taken, I understand the frustrations that can arise when the modelled forcing shifts significantly before our eyes. I believe the behaviour of the storm has been affected by the phase 5 MJO in a way that the models took time to fully catch onto, though most got there by Tue (GFS the glaring exception). Y’know, I’d not be surprised if the MJO-polar vortex battle played out broadly similar to how it did last January, with mid-latitude blocking early next year, though perhaps centred a bit further east this time. UK mainly settled especially in south where temps lowest, chance of sneaking in a cold E to SE flow depending on how the high orientates.

It’s not the winter storm that’s changed things alone. More significant has been a slowing of the forecast MJO propagation, shifting the window of Scandi High promotion away from next week when it would have been just in time to sneak in ahead of the zonal flow working down from the stratosphere, to the first third of Jan when it looks to have far more of a battle on its hands - but could still get there.

Next weeks pattern is a very good match for zero lag MJO phase 5 composites all the way from eastern N America to East Asia. Most model runs then move the MJO into phase 6 which corresponds to below normal heights over Europe. That implies that we should see the Atlantic storm track tend to head further south as we approach the new year. So far this has only been hinted at in the modelling with slight adjustments south bringing the polar boundary down to central UK at times. Question then is whether the MJO is strong enough to force sufficient deceleration of the Atlantic westerlies for a high to establish N or NE of the UK via a stalling trough that disrupts into Europe. Again, only hints of this in the modelling. We often see westerly regimes extended until further notice by the modelling due to their relatively high stability, only for a change to manifest at the 7-10 day lead times having barely been represented beforehand. So… we’ll see - I’m not (yet, at least) convinced that a predominately westerly regime is going to lock in for much more than a week.

I thought you had this down yesterday but it seems not - zonality is west-east movement of large scale weather systems, not the wind. What you’ve described first there is a cold zonal flow. I know it’s not the clearest of distinctions.

So most of the det runs have now clustered into a scenario where the big US storm throws a lot of vorticity across the N Atlantic, perhaps due to influence from the polar vortex in the lower stratosphere (or maybe not and purely driven by the storm’s exceptional energy, considering @bluearmy’s handy update, though that might be purely off GFS…?). A plausible scenario which, if it manifests, leaves us in a more classic position next week compared to resent weeks, looking for signs of Pacific MJO influence to amplify the Atlantic-Europe pattern form a relatively flat state (might not be as flat as the 12z runs had it - excessive flattening is a common bias when a signal for reduced planetary wave amplification emerges at the 7-10 day lead times). Extended EPS and especially the EC42 ensemble version suggest just that with signs of height rises N or NE of the UK. GFS continues to bang the ‘stalling storm leading to Greenland high’ drum… but with an increasingly large muffler applied. Now seems very odds against it having the right idea there.

3 separate sources of uncertainty are interacting to keep us on our toes. 1st what goes on in just a few days time - how much of a secondary low develops on the western flank of the main Atlantic low. Significant differences evident as soon as +72 hours between GFS on the left and ECM on the right. The latter outcome allows most or all positive vorticity (rising air, cyclonic spin) to clear E or ENE during the weekend, whereas the former leaves more behind to the west or southwest. Even within these two scenarios - but especially the 2nd one - there are a range of outcomes for positioning of lows and resulting flow and temps across the UK. Then, the way the intense storm over the US behaves as it matures and decays, which others have discussed already this morning. GFS continues to be the only real advocate for it stalling with a dispersing warm sector becoming the seed for a blocking high over Greenland. EPS aren't as disinterested in the 00z set as they have been previously, mind. Finally, whether the MJO drives sufficient amplification to the N. Atlantic pattern for high pressure to establish in the Iceland-Scandinavia area by NYD. The outcome of the 1st source determines how mild conditions in the UK, especially the south, are during the early stages of next week. That of the 2nd (the intense US storm aftermath) dictates whether we then see a significant and sustained push of cold air south toward the UK (which either battles with pre-existing southerlies or sweeps in easily, depending on the outcome of the 1st), or a transient mid-Atlantic ridge with low pressure establishing more generally on the Atlantic side of the Arctic. Then, if the MJO drives sufficient amplification of the pattern, we see build of high pressure in the Iceland-Scandinavia region that either ends up more toward Greenland due to a pre-existing blocking high there (so extending the high-latitude blocking spell), or stays put as a blockade battling with low pressure areas near or over Greenland while others head for SW Europe. Otherwise, we either see blocking across Greenland collapse or retrograde to N. America, or establishment of a 'traditional' zonal flow regime across the N. Atlantic and N. Europe, bringing changeable, mainly mild conditions. You see, that's a lot of possible combinations for the broad scale setup - let alone the finer details! Currently, the MJO-driven Iceland-Scandinavia high scenario with no Greenland high has surprisingly little representation in the various ensemble sets. That tells me that the models are leaning heavily on the outcome of the intense US storm for producing sufficient amplification in the N. Atlantic sector. The MJO signal is currently being diluted too much to noticeably influence what they show around NYD. Unfortunately there's no way to be sure whether that's because the MJO won't be strong enough, or because of over-suppression by the models. Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4774621

3 separate sources of uncertainty are interacting to keep us on our toes. 1st what goes on in just a few days time - how much of a secondary low develops on the western flank of the main Atlantic low. Significant differences evident as soon as +72 hours between GFS on the left and ECM on the right. The latter outcome allows most or all positive vorticity (rising air, cyclonic spin) to clear E or ENE during the weekend, whereas the former leaves more behind to the west or southwest. Even within these two scenarios - but especially the 2nd one - there are a range of outcomes for positioning of lows and resulting flow and temps across the UK. Then, the way the intense storm over the US behaves as it matures and decays, which others have discussed already this morning. GFS continues to be the only real advocate for it stalling with a dispersing warm sector becoming the seed for a blocking high over Greenland. EPS aren't as disinterested in the 00z set as they have been previously, mind. Finally, whether the MJO drives sufficient amplification to the N. Atlantic pattern for high pressure to establish in the Iceland-Scandinavia area by NYD. The outcome of the 1st source determines how mild conditions in the UK, especially the south, are during the early stages of next week. That of the 2nd (the intense US storm aftermath) dictates whether we then see a significant and sustained push of cold air south toward the UK (which either battles with pre-existing southerlies or sweeps in easily, depending on the outcome of the 1st), or a transient mid-Atlantic ridge with low pressure establishing more generally on the Atlantic side of the Arctic. Then, if the MJO drives sufficient amplification of the pattern, we see build of high pressure in the Iceland-Scandinavia region that either ends up more toward Greenland due to a pre-existing blocking high there (so extending the high-latitude blocking spell), or stays put as a blockade battling with low pressure areas near or over Greenland while others head for SW Europe. Otherwise, we either see blocking across Greenland collapse or retrograde to N. America, or establishment of a 'traditional' zonal flow regime across the N. Atlantic and N. Europe, bringing changeable, mainly mild conditions. You see, that's a lot of possible combinations for the broad scale setup - let alone the finer details! Currently, the MJO-driven Iceland-Scandinavia high scenario with no Greenland high has surprisingly little representation in the various ensemble sets. That tells me that the models are leaning heavily on the outcome of the intense US storm for producing sufficient amplification in the N. Atlantic sector. The MJO signal is currently being diluted too much to noticeably influence what they show around NYD. Unfortunately there's no way to be sure whether that's because the MJO won't be strong enough, or because of over-suppression by the models.

Much flatter GEM 00z which would actually have meant a notably colder outcome for much of the UK had it not developed an extra low just east of Iceland. The polar jet rapidly undercuts the high over Greenland on this run which gives it less time to gather strength beforehand. Results from a different handling of the intense storm over NE N. America to the GFS and UKM 12z runs. Could be right but odds-against at face value. GFS 12z is showing us a halfway house of sorts where a bit more vorticity is left behind to our west but not so much as to become the dominant player for our weather. UKM 12z again leaves a truckload to our west. Uncertainty remains very high!

Two separate stories unfolding at the moment. First one to do with how the Atlantic trough shapes up late this week, which is where the ECM 00z made a notable jump over to GFS with consolidation of most of the positive vorticity into one strong system that moves east, as opposed to two competing systems with the western one later developing into a broad low to our west in the style of the UKM 00z. Second to do with the MJO signal which has been trending stronger in the modelling. GFS has been responding strongly to that, predicting a rapid response leading to a cut-off high attempt in the vicinity of Iceland. It will be interesting to see how the ensembles behave in the coming few days, because the implication of an active MJO crossing phase 6 is that the high should not sink southeast across the UK. That is, providing the MJO does actually have sufficient amplitude - this underpins the chances, not guarantees, of cold weather that Catacol so brilliantly overviewed earlier today. The first story might yet prove to be one that follows the UKM/GEM 00z pathway but it's very feasible to get a high north of the UK from that position too - the key difference being much higher UK temps until any cold feed from the E or NE establishes (again, if the MJO forcing proves sufficient). The Scandinavian High chances will hit peak as we approach NYD. The GFS 06z is a bit west-shifted compared to the composite but I don't think it's an outlandish run given the signals at hand. Should the MJO forcing prove sub-par, we probably would end up with some manner of high pressure across the UK after all, making the current ensemble signal look rather skilful. In fact, it would be a similar sequence of events to what set up an unusually dry January this year, except the zonal flow looks considerably weaker this time around so there's more chance of the high being in a chilly rather than mild position relative to the UK. Original post: https://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4773981

Two separate stories unfolding at the moment. First one to do with how the Atlantic trough shapes up late this week, which is where the ECM 00z made a notable jump over to GFS with consolidation of most of the positive vorticity into one strong system that moves east, as opposed to two competing systems with the western one later developing into a broad low to our west in the style of the UKM 00z. Second to do with the MJO signal which has been trending stronger in the modelling. GFS has been responding strongly to that, predicting a rapid response leading to a cut-off high attempt in the vicinity of Iceland. It will be interesting to see how the ensembles behave in the coming few days, because the implication of an active MJO crossing phase 6 is that the high should not sink southeast across the UK. That is, providing the MJO does actually have sufficient amplitude - this underpins the chances, not guarantees, of cold weather that Catacol so brilliantly overviewed earlier today. The first story might yet prove to be one that follows the UKM/GEM 00z pathway but it's very feasible to get a high north of the UK from that position too - the key difference being much higher UK temps until any cold feed from the E or NE establishes (again, if the MJO forcing proves sufficient). The Scandinavian High chances will hit peak as we approach NYD. The GFS 06z is a bit west-shifted compared to the composite but I don't think it's an outlandish run given the signals at hand. Should the MJO forcing prove sub-par, we probably would end up with some manner of high pressure across the UK after all, making the current ensemble signal look rather skilful. In fact, it would be a similar sequence of events to what set up an unusually dry January this year, except the zonal flow looks considerably weaker this time around so there's more chance of the high being in a chilly rather than mild position relative to the UK.