Cambrian

Over the last day or two we’ve begun to see some very interesting charts in FI, but it’s worth emphasising the exceptional degree of build in heights that will “cut it” for a Scandinavian high far enough north to bring a surface easterly flow and cold into the UK and Ireland, and the kind of build that might look quite impressive on the charts but won’t cut it. All taken from yesterday’s output - First of all, builds that are not quite there, i.e. some that won’t cut it…. 12z ECM ensemble mean at day 10, good build of heights but clearly too far south. Euro high. 12z ECM op at day 10, very impressive northward push of heights… …even 12z GEFS at day 14 So don’t let the anomalies tease you! Both attain heights over southern or central Scandinavia, with the PV centred over Greenland, and even though the trough is angled favourably north-south, this setup is still not good enough, the trough is too far east in the Atlantic and at the western periphery of Europe we are again pulled into an Atlantic flow. Some that will cut it (this type of output is what I’ll be looking out for)… 12z GFS at day 16 PV centred to the northwest of Greenland, trough heading down further west into Newfoundland, very strongly anomalous heights over the far north of Scandinavia into the Barents Sea / Arctic Ocean, allowing lower heights to pool over Europe that can bring us into a long fetch easterly. Now something like that should do it nicely! However, it is an exceptional and relatively rare setup with just the right amount of PV energy needed in just the right places to pull it off. Moreover, we are in exciting times where it is possible to connect atmospheric developments over the Pacific with weather patterns on the other side (our side) of the world. The trick is to temper our expectations of these teleconnections, and their advanced interpretation in these pages, with respect to their translation to the weather in our local geographical regions, and outside our even more local windows. The level of self and mutual congratulation is at times far too high as is the level of criticism by others of those learned and brave enough to give it a go and share that with us. If there is one place on the planet where those delicate intellectual balances can be struck, it is this forum. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5016288

Over the last day or two we’ve begun to see some very interesting charts in FI, but it’s worth emphasising the exceptional degree of build in heights that will “cut it” for a Scandinavian high far enough north to bring a surface easterly flow and cold into the UK and Ireland, and the kind of build that might look quite impressive on the charts but won’t cut it. All taken from yesterday’s output - First of all, builds that are not quite there, i.e. some that won’t cut it…. 12z ECM ensemble mean at day 10, good build of heights but clearly too far south. Euro high. 12z ECM op at day 10, very impressive northward push of heights… …even 12z GEFS at day 14 So don’t let the anomalies tease you! Both attain heights over southern or central Scandinavia, with the PV centred over Greenland, and even though the trough is angled favourably north-south, this setup is still not good enough, the trough is too far east in the Atlantic and at the western periphery of Europe we are again pulled into an Atlantic flow. Some that will cut it (this type of output is what I’ll be looking out for)… 12z GFS at day 16 PV centred to the northwest of Greenland, trough heading down further west into Newfoundland, very strongly anomalous heights over the far north of Scandinavia into the Barents Sea / Arctic Ocean, allowing lower heights to pool over Europe that can bring us into a long fetch easterly. Now something like that should do it nicely! However, it is an exceptional and relatively rare setup with just the right amount of PV energy needed in just the right places to pull it off. Moreover, we are in exciting times where it is possible to connect atmospheric developments over the Pacific with weather patterns on the other side (our side) of the world. The trick is to temper our expectations of these teleconnections, and their advanced interpretation in these pages, with respect to their translation to the weather in our local geographical regions, and outside our even more local windows. The level of self and mutual congratulation is at times far too high as is the level of criticism by others of those learned and brave enough to give it a go and share that with us. If there is one place on the planet where those delicate intellectual balances can be struck, it is this forum.

Just for fun at this range, but interesting to see the end of the 12z GFS control run obliging our winter interests by getting the jet stream north all the way through Svalbard, a very solid build of heights and a surface high over Scandinavia, an easterly flow and a most interesting T850 chart! Support from the ensemble mean for a sustained rise in pressure and for the northward extension of heights into Scandinavia. One door closes and another one opens…. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5013532

Just for fun at this range, but interesting to see the end of the 12z GFS control run obliging our winter interests by getting the jet stream north all the way through Svalbard, a very solid build of heights and a surface high over Scandinavia, an easterly flow and a most interesting T850 chart! Support from the ensemble mean for a sustained rise in pressure and for the northward extension of heights into Scandinavia. One door closes and another one opens….

These are interesting to look at - the 0z EPS 200mb winds for the North Atlantic, synonymous with the jet stream. The animation for days 1-5 shows its reinvigoration, heading northeast out of the US, making a beeline towards the UK and Ireland. Days 6-10 shows the northern arm of the jet stream readily weakening and dipping south over the mid Atlantic before splitting (bifurcating is the word I am told!) around the southern European high pressure. To be taken with a pinch of salt for days 10-15, but at face value, some indications that the northern arm will continue to weaken, with the southern arm becoming more dominant. The trend over the US is interesting with the jet stream noticeably sinking south and strengthening towards the end of the run, which raises the eyebrows with a longer range mean. This all fits with the strong NAO+ signal in yesterday’s ECM46 weather regime forecast becoming established by the 21st, but as currently shown, it’s a relatively sharp peaking in mobility that weakens by the end of the month, with a strong and persistent blocking signal thereafter. The pressure anomalies for the first two weeks of February show high pressure steadily building north through the UK and Ireland, consistent with the weakening of the northern arm of the Atlantic jet stream shown in the EPS. A long way off, but something considerably more blocked being indicated, possibly worth looking out for more evidently coming into the range of the NWP models over the next few days. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5012968

These are interesting to look at - the 0z EPS 200mb winds for the North Atlantic, synonymous with the jet stream. The animation for days 1-5 shows its reinvigoration, heading northeast out of the US, making a beeline towards the UK and Ireland. Days 6-10 shows the northern arm of the jet stream readily weakening and dipping south over the mid Atlantic before splitting (bifurcating is the word I am told!) around the southern European high pressure. To be taken with a pinch of salt for days 10-15, but at face value, some indications that the northern arm will continue to weaken, with the southern arm becoming more dominant. The trend over the US is interesting with the jet stream noticeably sinking south and strengthening towards the end of the run, which raises the eyebrows with a longer range mean. This all fits with the strong NAO+ signal in yesterday’s ECM46 weather regime forecast becoming established by the 21st, but as currently shown, it’s a relatively sharp peaking in mobility that weakens by the end of the month, with a strong and persistent blocking signal thereafter. The pressure anomalies for the first two weeks of February show high pressure steadily building north through the UK and Ireland, consistent with the weakening of the northern arm of the Atlantic jet stream shown in the EPS. A long way off, but something considerably more blocked being indicated, possibly worth looking out for more evidently coming into the range of the NWP models over the next few days.

The 0z UKMO, is a fascinating and, I feel, quite a concise run. The pressures and heights chart shows the machinations of the PV, the Siberian lobe dropping down into Northern Europe during the first 5 days, the main lobe of the PV actually brushing our shores, before spinning back around north and bridging through Svalbard and Greenland to the regathered Canadian lobe, ultimately coalescing over northern Greenland, leaving the UK and Ireland in a westerly flow by day 6. It’s the T850s that caught my eye though. Pulses of very cold air heading south through the UK and Ireland, cold through to day 6 before it turns milder. The first warm sector gets squeezed out to our west around day 5, before the second one manages to break through. It might be well worth keeping an eye on the shape of these warm sectors and how far north they are, this could well change over the next few days, with the scope for some chilly uppers to follow from the eastern seaboard, mixing with cold air being pushed south out of Greenland. With such a lot going on with the PV, the models might well struggle with the north-south placement of the Atlantic jet stream for a fair few days. 0z ECM / GEM / GFS ops at day 7, a wide range, particularly at our end. So over the last couple of days, a more sudden curtailment of the deeper northerly flow than that being indicated previously in the models, let’s not pretend, but another good cold week in the middle of January, which we rightly saw coming for a fair few weeks. Up here, we still have sections of solid ice where normally bubbling springs froze over a full week ago, so irrespective of if and where the snow falls over the coming days, this is quite different to many of the Januarys we have seen in the last couple of decades, an interesting spell in a winter that is far from a typical winter too. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5011284

The 0z UKMO, is a fascinating and, I feel, quite a concise run. The pressures and heights chart shows the machinations of the PV, the Siberian lobe dropping down into Northern Europe during the first 5 days, the main lobe of the PV actually brushing our shores, before spinning back around north and bridging through Svalbard and Greenland to the regathered Canadian lobe, ultimately coalescing over northern Greenland, leaving the UK and Ireland in a westerly flow by day 6. It’s the T850s that caught my eye though. Pulses of very cold air heading south through the UK and Ireland, cold through to day 6 before it turns milder. The first warm sector gets squeezed out to our west around day 5, before the second one manages to break through. It might be well worth keeping an eye on the shape of these warm sectors and how far north they are, this could well change over the next few days, with the scope for some chilly uppers to follow from the eastern seaboard, mixing with cold air being pushed south out of Greenland. With such a lot going on with the PV, the models might well struggle with the north-south placement of the Atlantic jet stream for a fair few days. 0z ECM / GEM / GFS ops at day 7, a wide range, particularly at our end. So over the last couple of days, a more sudden curtailment of the deeper northerly flow than that being indicated previously in the models, let’s not pretend, but another good cold week in the middle of January, which we rightly saw coming for a fair few weeks. Up here, we still have sections of solid ice where normally bubbling springs froze over a full week ago, so irrespective of if and where the snow falls over the coming days, this is quite different to many of the Januarys we have seen in the last couple of decades, an interesting spell in a winter that is far from a typical winter too.

Well, if an Atlantic trough on its way in to engaging with very cold air over the UK and Ireland, increasingly disrupting and now pointing southeastward towards northwest Spain, riding a jet stream that is being progressively modelled further and further south with each passing day and seemingly getting splattered against an atmospheric wall on approach… …aren’t all things to tick your exciting winter synoptics box, then I’m afraid that the 12z GEFS at day 7 might not be for you.

6z ICON is neat for showing the temperature changes over the weekend, during the next 72h, hour by hour, reflected in freezing levels and the rain / snow boundaries Get the cold in… Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5008662

6z ICON is neat for showing the temperature changes over the weekend, during the next 72h, hour by hour, reflected in freezing levels and the rain / snow boundaries Get the cold in…

0z EPS for days 7, 8 and 9. In broad terms, day 7 shows the battleground setup, with a northwesterly upper flow, sending cold air into the UK and Ireland. Day 8 shows the flow turn more westerly with low heights in the mid Atlantic, looking like the Atlantic has broken through. But by day 9, the gap is reinstated, with mid Atlantic heights pointing up to Greenland. Compare this with the day 10 chart from yesterday. Markedly less of a mid-Atlantic gap, much less amplification yesterday, replaced now at day 9 by the beginnings of a good looking ridge up to southern Greenland pumped up by cleaner core low heights to the west of Newfoundland, the low getting hindered in its attempt to move east out of North America. Yesterday’s day 10 / today’s day 9. The models are struggling big time. The Scandinavian trough is still very much in attendance and looking at that, a reasonable observation would be that it only needs just a little more amplification in the Atlantic heights to altogether prevent any phasing of the troughs associated with two lobes of the PV and keep us very much under the influence of the cold trough to the northeast. Next few days model watching for me reminds me of getting on the train - when the guy over the tannoy says - watch the gap! Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5008372

0z EPS for days 7, 8 and 9. In broad terms, day 7 shows the battleground setup, with a northwesterly upper flow, sending cold air into the UK and Ireland. Day 8 shows the flow turn more westerly with low heights in the mid Atlantic, looking like the Atlantic has broken through. But by day 9, the gap is reinstated, with mid Atlantic heights pointing up to Greenland. Compare this with the day 10 chart from yesterday. Markedly less of a mid-Atlantic gap, much less amplification yesterday, replaced now at day 9 by the beginnings of a good looking ridge up to southern Greenland pumped up by cleaner core low heights to the west of Newfoundland, the low getting hindered in its attempt to move east out of North America. Yesterday’s day 10 / today’s day 9. The models are struggling big time. The Scandinavian trough is still very much in attendance and looking at that, a reasonable observation would be that it only needs just a little more amplification in the Atlantic heights to altogether prevent any phasing of the troughs associated with two lobes of the PV and keep us very much under the influence of the cold trough to the northeast. Next few days model watching for me reminds me of getting on the train - when the guy over the tannoy says - watch the gap!

Fitting with the 18z GFS op - some interesting changes in the 12z EPS for the low over the Azores at day 6. Compared with yesterday’s 12z for day 7, the low heights now much more elongated and becoming disrupted at the base of the cold trough. To the west, higher heights in the gap from Greenland through Newfoundland, the gap keeping the lobes of the PV apart. Closer to home, lower heights through the UK and Ireland from the Scandinavian trough. The ensemble means, GEFS and EPS, are repeatedly toning down the Atlantic as the days approach between day 10 and day 6, adding to the notion of a greater depth and longevity to the cold spell than might first meet the eye. It would be no surprise to see this trend working through to the ops more consistently in the coming days.

Regarding longevity of the cold spell, here are the 12z GEFS charts for 18 January, a week today, from Monday at day 10, Wednesday at day 8, Thursday at day 7. The Atlantic progressively modelled to be more sealed off with each passing day, by decent looking Atlantic ridging leading north to stronger heights over the Canadian Arctic. And the icing on the cake, heights of 525 dam over northeast Scotland from the Scandinavian trough three days ago, now 516 dam, so lower heights and deeper cold entrained in the trough, aligned southwestward through the UK and Ireland. We couldn’t ask for better charts to sum up decidedly cold and decidedly unsettled. Nicely reflected in the corresponding mean T850 anomalies too - the means even more solidly indicating cold for the reasonably foreseeable, the ensemble showing once more that we should take the GFS op beyond day 4-5 with about as much salt as we’ll need for the roads!

It looks like the ECM is drawing up even more fierce battle lines for the middle of next week. If we compare yesterday’s 12z ensemble mean for day 8, with today’s for day 7, the low to the southwest is markedly deeper on today’s run, though clearly stressed and stretched west-east. The 0 degree isotherm is further north. However, this is countered by the low heights associated with the Scandinavian trough being lower over the northeast of Scotland and the -8 degree T850 being further south over Scotland too. Milder to the south, colder to the north, a deeper low, steeper pressure and temperature gradients across the UK and Ireland - where gets the snow is just one part of the equation, nowhere near being resolved. The likelihood of it being highly disruptive is the other, and that has markedly increased today. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5005929

It looks like the ECM is drawing up even more fierce battle lines for the middle of next week. If we compare yesterday’s 12z ensemble mean for day 8, with today’s for day 7, the low to the southwest is markedly deeper on today’s run, though clearly stressed and stretched west-east. The 0 degree isotherm is further north. However, this is countered by the low heights associated with the Scandinavian trough being lower over the northeast of Scotland and the -8 degree T850 being further south over Scotland too. Milder to the south, colder to the north, a deeper low, steeper pressure and temperature gradients across the UK and Ireland - where gets the snow is just one part of the equation, nowhere near being resolved. The likelihood of it being highly disruptive is the other, and that has markedly increased today.

12z ECM ensemble mean for days 6-10 paints the picture of a classical battleground setup. The challenge from the southwest is repelled by means of the low over the Azores ultimately being absorbed into the base of the Scandinavian PV driven trough, with the mild air making little progress - mean, anomalies and mean T850s. In the same way that the pole has relinquished one chunk of the PV to Siberia and ultimately Europe, there are signs by day 10 that the Canadian lobe is getting pulled southeastward through Newfoundland, the build of heights over the Pacific side of the Arctic maintaining the anomalous forcing of the Northern Hemisphere winter cold towards the North Atlantic and Northern Europe, with a further battle being set up for the ensuing days for the UK and Ireland. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=5004008

12z ECM ensemble mean for days 6-10 paints the picture of a classical battleground setup. The challenge from the southwest is repelled by means of the low over the Azores ultimately being absorbed into the base of the Scandinavian PV driven trough, with the mild air making little progress - mean, anomalies and mean T850s. In the same way that the pole has relinquished one chunk of the PV to Siberia and ultimately Europe, there are signs by day 10 that the Canadian lobe is getting pulled southeastward through Newfoundland, the build of heights over the Pacific side of the Arctic maintaining the anomalous forcing of the Northern Hemisphere winter cold towards the North Atlantic and Northern Europe, with a further battle being set up for the ensuing days for the UK and Ireland.

The ECM ensemble mean has firmed up again on the 12z. Charts for the 16th at days 10, 9 and 8. Better and better amplification up into Greenland and the PV lobe progressively further south over Scandinavia. Growing and better defined role for the Arctic high too. The beauty of how these models work towards the solution.

Looking at the ensemble means is always very valuable. Identifying the similarities and assessing the differences between the leading models allows the establishment of a baseline of expectation based on a more centred range of the wider ensemble envelope, which can’t be a bad thing to take into account, irrespective of any divergence in the longer term. The later divergence is also of interest for the same reasons. The models will inevitably diverge over the longer timeframes, but again there will be commonalities in their evolutions, which are worth noting. There are also of course the changes that occur between runs of the same model, which are useful to observe for the purpose of identifying trends. In addition, over time, with experience, we develop an understanding of the leanings and biases of particular models in certain situations. This is a great example of why it’s very worthwhile keeping an eye on the ensemble means. 12z GEFS yesterday for day 13 vs 12z GEFS today for day 12. Yesterday’s loss of Canadian Arctic heights and merged PV - dropped in favour of today’s maintenance of heights, Greenland ridge and well-separated lobes of the PV, and as a result, downstream a deeper Northern European trough, its core low heights held back further west. A very different perspective may be derived for the potential longevity of the forthcoming cold spell. Over the next few days, keeping an eye on the development of these aspects will inform our expectations for the Northern Hemisphere profile that will underpin our setup for week 2. It doesn’t matter even if the means for later in week 2 are indeed way off the mark right now. Studying the way that they gradually converge towards a consensus first with each other and then with the operational runs is valuable not only for the enjoyment of it, but also both as a learning exercise and to hone our interpretation skills, particularly for this specific type of scenario, and of course, precisely because the models are struggling with it in the longer timeframes. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=5000019

Looking at the ensemble means is always very valuable. Identifying the similarities and assessing the differences between the leading models allows the establishment of a baseline of expectation based on a more centred range of the wider ensemble envelope, which can’t be a bad thing to take into account, irrespective of any divergence in the longer term. The later divergence is also of interest for the same reasons. The models will inevitably diverge over the longer timeframes, but again there will be commonalities in their evolutions, which are worth noting. There are also of course the changes that occur between runs of the same model, which are useful to observe for the purpose of identifying trends. In addition, over time, with experience, we develop an understanding of the leanings and biases of particular models in certain situations. This is a great example of why it’s very worthwhile keeping an eye on the ensemble means. 12z GEFS yesterday for day 13 vs 12z GEFS today for day 12. Yesterday’s loss of Canadian Arctic heights and merged PV - dropped in favour of today’s maintenance of heights, Greenland ridge and well-separated lobes of the PV, and as a result, downstream a deeper Northern European trough, its core low heights held back further west. A very different perspective may be derived for the potential longevity of the forthcoming cold spell. Over the next few days, keeping an eye on the development of these aspects will inform our expectations for the Northern Hemisphere profile that will underpin our setup for week 2. It doesn’t matter even if the means for later in week 2 are indeed way off the mark right now. Studying the way that they gradually converge towards a consensus first with each other and then with the operational runs is valuable not only for the enjoyment of it, but also both as a learning exercise and to hone our interpretation skills, particularly for this specific type of scenario, and of course, precisely because the models are struggling with it in the longer timeframes.

Comparing the 0z runs, it’s interesting that the GEPS is again keeping the Arctic Canada heights, extending into southern Greenland, all the way through to day 14, while the GFS is moving the tropospheric PV there. Conversely, the GEPS has two lobes to the PV linked through Greenland to Svalbard. GEPS / GFS Huge differences downstream over the Atlantic as a result of course, with the GFS pepping up the jet stream and blowing through the Scandinavian trough. But notice the upper trough through Iberia on the GEPS and the lack of any heights on the GFS, and the southeastward dive of the contours through or to the west of Iberia on both models. This suggests that a good possibility of a southerly tracking jet stream is well and truly there in both scenarios. It is clearly the Arctic that is in enormous flux in the models and it could well be that these two solutions are close to representing opposite ends of the range of likely location of the PV. I have concerns about the EPS due to the rate of change in the mean pattern between days 12 and 14 looking far too rapid. An abrupt unidirectional collapsing of the pattern like that would be surprising. A GEM / GFS halfway house might position the weakened PV to the north of Greenland just our side of the pole with plenty of scope for development of meridional and blocking patterns later in the month, as indicated in yesterday’s EC46 weather regimes, following a brief NAO+ signal for a few days from day 14. So some “tethered speculation” - blocked now, the retrogression, then cold, a relatively brief relaxation to something a bit more mobile, which could be battleground and snowy, then back to blocked, the jet stream quietening down again, leading to good scope for a further phase of retrogression and potentially further plunges of Arctic air. We’re certainly finding more and more that there’s nothing ordinary about this January! Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4999301

Comparing the 0z runs, it’s interesting that the GEPS is again keeping the Arctic Canada heights, extending into southern Greenland, all the way through to day 14, while the GFS is moving the tropospheric PV there. Conversely, the GEPS has two lobes to the PV linked through Greenland to Svalbard. GEPS / GFS Huge differences downstream over the Atlantic as a result of course, with the GFS pepping up the jet stream and blowing through the Scandinavian trough. But notice the upper trough through Iberia on the GEPS and the lack of any heights on the GFS, and the southeastward dive of the contours through or to the west of Iberia on both models. This suggests that a good possibility of a southerly tracking jet stream is well and truly there in both scenarios. It is clearly the Arctic that is in enormous flux in the models and it could well be that these two solutions are close to representing opposite ends of the range of likely location of the PV. I have concerns about the EPS due to the rate of change in the mean pattern between days 12 and 14 looking far too rapid. An abrupt unidirectional collapsing of the pattern like that would be surprising. A GEM / GFS halfway house might position the weakened PV to the north of Greenland just our side of the pole with plenty of scope for development of meridional and blocking patterns later in the month, as indicated in yesterday’s EC46 weather regimes, following a brief NAO+ signal for a few days from day 14. So some “tethered speculation” - blocked now, the retrogression, then cold, a relatively brief relaxation to something a bit more mobile, which could be battleground and snowy, then back to blocked, the jet stream quietening down again, leading to good scope for a further phase of retrogression and potentially further plunges of Arctic air. We’re certainly finding more and more that there’s nothing ordinary about this January!

6z GFS op at day 10 shows the freezing level at 0m and sub-528 dam thickness throughout the UK and Ireland. A rare sight. Parts of Scotland are at sub-510 dam. Looks a bit nippy.