Cambrian

Looking at the next ten days, which cover the middle third of April, t’s good to see the relevant part of the annual cycle, in the form of the springtime waning of the PV, well underway. It is happening a bit later than the last couple of years, but is striking nonetheless, here on both the 0z ECM operational run and ensemble mean, day 1 - 10 animations, the PV very much on the wane, being nibbled away by evolving mid-latitude ridges, with the core PV diminished and returning from the Siberian Arctic to the pole. This leaves us with a solidly meridional 5-wave hemispheric pattern by day 10, shown nicely on the op and mean anomalies, ridges by then through western North America, up through the eastern seaboard, Western Europe, Kazakhstan up into Siberia, and the far east of Russia. The picture for the UK and Ireland alternates from seeing a shallow ridge over the next few days, to being under the influence of an active low pressure system passing through with the trough around days 5-6, before the arrival of a better amplified ridge by day 9-10 as the more meridional pattern firms up. Day 10 provides a convenient juncture for the UK and Ireland for showing the potential for something more settled getting a bit further north to be more widely shared, op and mean again. However, it looks very much like it’s a snapshot in a slow but steady revolution of the 5-wave ridge / trough pattern around the pole, with approximately 4 days appearing to be the current phase for the ridge to trough transition, hinting at a period of drier, brighter spells of 3-4 days alternating with more changeable spells of 3-4 days. A much more normal and much more pleasant a pattern on view as spring more clearly shows its hand.

As the great man himself promised, Kathleen is being taken back home again, to be reabsorbed within the PV by day 5, here on the 0z ECM operational run. As a result, for the UK and Ireland, a vastly improving picture over the coming week as heights build northeast from the Azores, temperatures picking up by the end of the week for a while too if the T850s are anything to go by. Before exiting east, Kathleen ejects some energy westward to help the next system along, this one passing east further north through Iceland, to set up over Scandinavia from day 6 through to day 10. The bulk of Kathleen has by then rejoined the PV from the Siberian side, helping it pivot to be elongated down towards the Norwegian Sea, fuelling the dropping of the trough down through Scandinavia, bringing the UK and Ireland into a cold northerly flow for several days. After this rather dramatic phase (following three inches of snow in west Wales last week and three days of gales in the west of Ireland this week!), Kathleen looking to help ultimately deliver something for everyone - a storm, briefly something a bit warmer, followed by some clear Arctic air and the possibility of a quieter, more standard meridional pattern as we head into the second half of April. So I guess it’s only fair to leave the man have the last words: “And when the fields are soft and green I will take you to your home Kathleen”

Attempting to piece together (as we are inclined to do!), emerging trends of the outputs from different models, here the 12z GEFS and UKMO at day 4: And then, taking it through to day 7: That is one very welcome surge in heights from the southwest early next week, resulting in a 1030mb+ high pressure cell making a visit on the UKMO. A tad optimistic perhaps and probably not that long-lasting in the event, but the contrast is striking and very good to start seeing all the same. Have a good one.

Hope you all had a great Easter and that the beginning of April finds you in good health. Looking at the models over the last couple of weeks it’s easy to perceive little or no change to the pattern and no respite from low pressure for the UK and Ireland. However, there are slow but important changes consistently creeping in to the Northern Hemisphere profile in the longer ranges. They’ve been in recentruns of the EPS / ECM too, but here we’re looking at the 12z GEFS. At day 5, not much change from the here and now, PV elongated in our direction, with the trough dropping down to our west, and anchored there due to an attractive bump of unseasonably warm heights along with a surface high pressure over the central Mediterranean. Another notable bump of heights up through central North America too, into Hudson Bay, maintaining the tight Bering Sea to eastern Atlantic alignment of the PV. By day 10, we’ve lost the heights heading up through the American Midwest into central Canada, replaced by an upper level trough, allowing the PV to fill out west, so we get low heights working down into Hudson Bay. As a result the Atlantic jet stream and trough begins to relax north, so though we still have low pressure anchored to our west, it’s further northwest and nowhere near as deep, central pressure about 20mb higher. At the same time, the loss of heights over the Mediterranean should allow easier eastern transfer of frontal systems through Northern Europe, so the setup not near as stuck. The same process continues through to day 16, so much indeed that the PV is now orientated Siberia to Labrador, with a fully fledged trough pointed down into eastern Canada. A good build in heights from Eastern Europe up into northern Scandinavia puts a roof over our heads so that the draught can only get in through the window. The gradual warming out of the Atlantic trough continues with the 1015mb isobar clipping the far south of the UK, high pressure relocating to the Azores, bringing the foundation for a greater chance of some more substantial ridging northeast in time, the models at least now showing signs of beginning to search out a route out of the rut towards a slow but steady improvement by, and more especially after, mid month. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5053938

Hope you all had a great Easter and that the beginning of April finds you in good health. Looking at the models over the last couple of weeks it’s easy to perceive little or no change to the pattern and no respite from low pressure for the UK and Ireland. However, there are slow but important changes consistently creeping in to the Northern Hemisphere profile in the longer ranges. They’ve been in recentruns of the EPS / ECM too, but here we’re looking at the 12z GEFS. At day 5, not much change from the here and now, PV elongated in our direction, with the trough dropping down to our west, and anchored there due to an attractive bump of unseasonably warm heights along with a surface high pressure over the central Mediterranean. Another notable bump of heights up through central North America too, into Hudson Bay, maintaining the tight Bering Sea to eastern Atlantic alignment of the PV. By day 10, we’ve lost the heights heading up through the American Midwest into central Canada, replaced by an upper level trough, allowing the PV to fill out west, so we get low heights working down into Hudson Bay. As a result the Atlantic jet stream and trough begins to relax north, so though we still have low pressure anchored to our west, it’s further northwest and nowhere near as deep, central pressure about 20mb higher. At the same time, the loss of heights over the Mediterranean should allow easier eastern transfer of frontal systems through Northern Europe, so the setup not near as stuck. The same process continues through to day 16, so much indeed that the PV is now orientated Siberia to Labrador, with a fully fledged trough pointed down into eastern Canada. A good build in heights from Eastern Europe up into northern Scandinavia puts a roof over our heads so that the draught can only get in through the window. The gradual warming out of the Atlantic trough continues with the 1015mb isobar clipping the far south of the UK, high pressure relocating to the Azores, bringing the foundation for a greater chance of some more substantial ridging northeast in time, the models at least now showing signs of beginning to search out a route out of the rut towards a slow but steady improvement by, and more especially after, mid month.

Hats off to the models. They had picked up on the deepening trough in the nearby Atlantic on the run-up to Easter about a week ago. Not a great outcome in terms of the weather, but the UK and Ireland in the box seat for observing some industrious cyclogenesis over the next 4 days, here on the 0z ECM op. Interesting to see the two packets of low heights dropping out of northeast Canada, the second one really packing a punch and deepening the low pressure to around 965mb, as it spins around nearby giving us all a good wallop, centred to the northwest of Ireland by Thursday. The 0z ECM mean anomaly chart for day 5, Good Friday, shows the problem for the Easter holiday in the form of the trapped trough, held firmly in place for several days by a bank of heights pushing up over the western half of the Atlantic and the other bank stubbornly holding on over the eastern half of Europe. The western Atlantic heights look to ultimately prove the stronger, migrating north to Greenland by day 10, squeezing the trough to the north through the Norwegian Sea as the eastern European heights get slowly nudged into Russia, allowing the surface low to move through to Germany, bringing the UK and Ireland into a northeasterly flow. So looking like we’ll eventually get rid of the poorly timed unsettled spell for something a bit drier and chillier. Yes, hats off to the models but I’ll definitely be keeping my hat on. At this time of the year in particular, with the full range of weather that the spring can offer, the choice of hat is indeed a valuable indicator of how it’s going. Clearly far too early for the time being to venture to the apparel shown on the left, looks like I’ll be sticking with my reliably functional fisherman’s beanie for now. Have a great day. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5050682

Hats off to the models. They had picked up on the deepening trough in the nearby Atlantic on the run-up to Easter about a week ago. Not a great outcome in terms of the weather, but the UK and Ireland in the box seat for observing some industrious cyclogenesis over the next 4 days, here on the 0z ECM op. Interesting to see the two packets of low heights dropping out of northeast Canada, the second one really packing a punch and deepening the low pressure to around 965mb, as it spins around nearby giving us all a good wallop, centred to the northwest of Ireland by Thursday. The 0z ECM mean anomaly chart for day 5, Good Friday, shows the problem for the Easter holiday in the form of the trapped trough, held firmly in place for several days by a bank of heights pushing up over the western half of the Atlantic and the other bank stubbornly holding on over the eastern half of Europe. The western Atlantic heights look to ultimately prove the stronger, migrating north to Greenland by day 10, squeezing the trough to the north through the Norwegian Sea as the eastern European heights get slowly nudged into Russia, allowing the surface low to move through to Germany, bringing the UK and Ireland into a northeasterly flow. So looking like we’ll eventually get rid of the poorly timed unsettled spell for something a bit drier and chillier. Yes, hats off to the models but I’ll definitely be keeping my hat on. At this time of the year in particular, with the full range of weather that the spring can offer, the choice of hat is indeed a valuable indicator of how it’s going. Clearly far too early for the time being to venture to the apparel shown on the left, looks like I’ll be sticking with my reliably functional fisherman’s beanie for now. Have a great day.

There’s often comment on here regarding the reduced reliability of the models past a given timeframe. 5 days is often stated as the point at which models become unreliable. Studying the ECMWF web pages, it would appear to be a good deal more nuanced than this. They publish charts showing the “Continuous ranked probability skill scores (CRPSS) of forecasts of upper-air parameters by TIGGE centres”, with the following notes about the score being used: The Continuous Ranked Probability Skill Score (CRPSS) is a measure of how good forecasts are in matching observed outcomes. Where: CRPSS = 1 the forecast has perfect skill compared to climatology - forecast beneficial; CRPSS = 0 the forecast has no skill compared to climatology) - forecast has no benefit over climatology; CRPSS = a negative value the forecast is less accurate than climatology - forecast misleading. CRPSS is evaluated by calculating the function CRPSS = 1 − CRPSforecast / CRPSclimat where: Continuous Ranked Probability Score for the forecast (CRPSforecast) is calculated comparing the Cumulative Distribution Functions (CDF) for the forecast against observations (or analyses) over a given period. Continuous Ranked Probability Score for climatology (CRPSclimat) is calculated comparing the Cumulative Distribution Functions (CDF) for the forecast against climatology over the same period. Hersbach, H., 2000: Decomposition of the continuous ranked probability score for ensemble prediction systems. Wea. Forecasting, 15, 559-570. Here’s the link to the paper that ECMWF cite: https://journals.ametsoc.org/view/journals/wefo/15/5/1520-0434_2000_015_0559_dotcrp_2_0_co_2.xml From the cited paper, excerpted: Instead of two options (event occurs or does not occur), the range of the parameter of interest is divided into more classes. In addition, the RPS contains a sense of distance of how far the forecast was found from reality. For a deterministic forecast for instance, the RPS is proportional to the number of classes by which the forecast missed the verifying analysis. Although the choice and number of classes may be prescribed by the specific application, the exact value of RPS will depend on this choice. It is possible to take the limit of an infinite number of classes, each with zero width. This leads to the concept of the continuous ranked probability score (CRPS)…This CRPS has several appealing properties. First of all, it is sensitive to the entire permissible range of the parameter of interest. Second, its definition does not require…the introduction of a number of predefined classes, on which results may depend….Finally, for a deterministic forecast, the CRPS is equal to the mean absolute error (MAE) and, therefore, has a clear interpretation. This is useful because: 1. Rather than the (subjective) right or wrong, we have a graded measure of the extent of deviation from the realised outcome. 2. Any score above zero demonstrates some forecast skill over the assumption of climatology. Here are the CRPSS charts for T850 and 500hPa height for the Northern Hemisphere incorporating data for the winter just ended. At day 5, ECM attains a score of 0.62 for T850 and a score of 0.72 for 500hPa geopotential. The score for heights is higher at day 6 than that for T850s at day 5. The timeframe above the 0.5 CRPSS level (where the forecast matches the observed outcome in more instances than it doesn’t, compared to climatology) corresponds to between day 6 and 7 for T850 and between day 7 and 8 for heights. At day 10, the score is 0.28 for T850 and 0.33 for 500hPa height. Both those scores continue to offer reasonable “value” at day 10, though to be fair, at a level less than half the value offered at day 5. By day 15, the score is around 0.12 for T850 and near 0.15 for 500hPa height, much lower, but still above zero so exhibiting some skill when compared to just assuming climatological norms. The ECMWF appears to put some store in the 0.25 or 25% level, as this is what they use to illustrate the improvement in skill over the last 25 years. This graph is great. It shows the lead time in days before the score drops to 25% and is for T850. The model has the same level of skill at day 10 in 2024 than it did at 5.5 days in 1998. Noticeable too that winter T850s appear to be handled better than summer ones, probably due to the winter PV, with typically fewer meridional episodes and less dramatic fluctuations in the winter as a result. In summary, in our parlance, the models are more “reliable” for longer for predicting heights than they are T850s. For heights, these are typically of sound merit all the way out towards forecast day 8. The reliability drops off steadily from day 0, there is no sudden drop at any point and the forecasts maintain some predictive value when compared with climatological expectations all the way through to day 15, which is presumably why this timeframe is catered for in the outputs. Lastly, and perhaps most excitingly, the models appear to be better now at day 10 than they were 25 years ago at day 6. This all fits very nicely with the general narrative surrounding their use here in the model thread. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5049286

There’s often comment on here regarding the reduced reliability of the models past a given timeframe. 5 days is often stated as the point at which models become unreliable. Studying the ECMWF web pages, it would appear to be a good deal more nuanced than this. They publish charts showing the “Continuous ranked probability skill scores (CRPSS) of forecasts of upper-air parameters by TIGGE centres”, with the following notes about the score being used: The Continuous Ranked Probability Skill Score (CRPSS) is a measure of how good forecasts are in matching observed outcomes. Where: CRPSS = 1 the forecast has perfect skill compared to climatology - forecast beneficial; CRPSS = 0 the forecast has no skill compared to climatology) - forecast has no benefit over climatology; CRPSS = a negative value the forecast is less accurate than climatology - forecast misleading. CRPSS is evaluated by calculating the function CRPSS = 1 − CRPSforecast / CRPSclimat where: Continuous Ranked Probability Score for the forecast (CRPSforecast) is calculated comparing the Cumulative Distribution Functions (CDF) for the forecast against observations (or analyses) over a given period. Continuous Ranked Probability Score for climatology (CRPSclimat) is calculated comparing the Cumulative Distribution Functions (CDF) for the forecast against climatology over the same period. Hersbach, H., 2000: Decomposition of the continuous ranked probability score for ensemble prediction systems. Wea. Forecasting, 15, 559-570. Here’s the link to the paper that ECMWF cite: https://journals.ametsoc.org/view/journals/wefo/15/5/1520-0434_2000_015_0559_dotcrp_2_0_co_2.xml From the cited paper, excerpted: Instead of two options (event occurs or does not occur), the range of the parameter of interest is divided into more classes. In addition, the RPS contains a sense of distance of how far the forecast was found from reality. For a deterministic forecast for instance, the RPS is proportional to the number of classes by which the forecast missed the verifying analysis. Although the choice and number of classes may be prescribed by the specific application, the exact value of RPS will depend on this choice. It is possible to take the limit of an infinite number of classes, each with zero width. This leads to the concept of the continuous ranked probability score (CRPS)…This CRPS has several appealing properties. First of all, it is sensitive to the entire permissible range of the parameter of interest. Second, its definition does not require…the introduction of a number of predefined classes, on which results may depend….Finally, for a deterministic forecast, the CRPS is equal to the mean absolute error (MAE) and, therefore, has a clear interpretation. This is useful because: 1. Rather than the (subjective) right or wrong, we have a graded measure of the extent of deviation from the realised outcome. 2. Any score above zero demonstrates some forecast skill over the assumption of climatology. Here are the CRPSS charts for T850 and 500hPa height for the Northern Hemisphere incorporating data for the winter just ended. At day 5, ECM attains a score of 0.62 for T850 and a score of 0.72 for 500hPa geopotential. The score for heights is higher at day 6 than that for T850s at day 5. The timeframe above the 0.5 CRPSS level (where the forecast matches the observed outcome in more instances than it doesn’t, compared to climatology) corresponds to between day 6 and 7 for T850 and between day 7 and 8 for heights. At day 10, the score is 0.28 for T850 and 0.33 for 500hPa height. Both those scores continue to offer reasonable “value” at day 10, though to be fair, at a level less than half the value offered at day 5. By day 15, the score is around 0.12 for T850 and near 0.15 for 500hPa height, much lower, but still above zero so exhibiting some skill when compared to just assuming climatological norms. The ECMWF appears to put some store in the 0.25 or 25% level, as this is what they use to illustrate the improvement in skill over the last 25 years. This graph is great. It shows the lead time in days before the score drops to 25% and is for T850. The model has the same level of skill at day 10 in 2024 than it did at 5.5 days in 1998. Noticeable too that winter T850s appear to be handled better than summer ones, probably due to the winter PV, with typically fewer meridional episodes and less dramatic fluctuations in the winter as a result. In summary, in our parlance, the models are more “reliable” for longer for predicting heights than they are T850s. For heights, these are typically of sound merit all the way out towards forecast day 8. The reliability drops off steadily from day 0, there is no sudden drop at any point and the forecasts maintain some predictive value when compared with climatological expectations all the way through to day 15, which is presumably why this timeframe is catered for in the outputs. Lastly, and perhaps most excitingly, the models appear to be better now at day 10 than they were 25 years ago at day 6. This all fits very nicely with the general narrative surrounding their use here in the model thread.

Here at day 5 on the 12z GEFS, as we’ve seen several times during the last few months, the hemispheric profile achieves a striking north-south orientation of heights, the PV with its core over Siberia coordinating a Scandinavian trough, which combined with cross-polar heights at the merging of Alaskan and Atlantic ridges, delivers us a handsome northerly flow. But as during the winter, it’s brief. The pattern has no hold. By day 10, the PV relaxes, the Siberian core leaks out west again towards northern Greenland, so the pattern falls lateral again and the reinforcement of the Greenland trough drops a low pressure southeast out of Greenland. The Alaskan heights are squeezed out, and there is a complete reversal over the Atlantic - with a 40+ dam drop in heights over 5 days to the west of the UK and Ireland, we’ve swiftly lost the Atlantic ridge and we’re back into an Atlantic cyclonic pattern heading into Easter. And as during the winter, once it’s set up, this pattern is not brief. By day 15, there are heights to the north but the PV is lateral, and still very much on our side of the pole, preventing a full build, so instead, we get a southerly tracking jet stream brewing up systems carrying a lot of moisture running up from the southwest, through the corridor paradoxically being held in place by the northern heights. It’s as we were then, seemingly too far south for the northern heights to bring us blocked cold, and too far north for the southern heights to send us a proper block, so we get a fleeting Atlantic ridge that can’t form a robust block with the heights to the north, because they themselves lack sufficient traction, followed by the PV and shallow heights to their south then going lateral and serving us up another extended run off the Atlantic. Fair enough to point out that is what we would expect for a temperate Atlantic climate but the unrelenting nature of never more than a few days with any other option on the table, at the moment at least, seems quite grimly exceptional. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5049100

Here at day 5 on the 12z GEFS, as we’ve seen several times during the last few months, the hemispheric profile achieves a striking north-south orientation of heights, the PV with its core over Siberia coordinating a Scandinavian trough, which combined with cross-polar heights at the merging of Alaskan and Atlantic ridges, delivers us a handsome northerly flow. But as during the winter, it’s brief. The pattern has no hold. By day 10, the PV relaxes, the Siberian core leaks out west again towards northern Greenland, so the pattern falls lateral again and the reinforcement of the Greenland trough drops a low pressure southeast out of Greenland. The Alaskan heights are squeezed out, and there is a complete reversal over the Atlantic - with a 40+ dam drop in heights over 5 days to the west of the UK and Ireland, we’ve swiftly lost the Atlantic ridge and we’re back into an Atlantic cyclonic pattern heading into Easter. And as during the winter, once it’s set up, this pattern is not brief. By day 15, there are heights to the north but the PV is lateral, and still very much on our side of the pole, preventing a full build, so instead, we get a southerly tracking jet stream brewing up systems carrying a lot of moisture running up from the southwest, through the corridor paradoxically being held in place by the northern heights. It’s as we were then, seemingly too far south for the northern heights to bring us blocked cold, and too far north for the southern heights to send us a proper block, so we get a fleeting Atlantic ridge that can’t form a robust block with the heights to the north, because they themselves lack sufficient traction, followed by the PV and shallow heights to their south then going lateral and serving us up another extended run off the Atlantic. Fair enough to point out that is what we would expect for a temperate Atlantic climate but the unrelenting nature of never more than a few days with any other option on the table, at the moment at least, seems quite grimly exceptional.

Things shaping up a little bit better now. Successive waves of building heights around the Azores are gaining a modicum of momentum in the models. As I alluded to a few days ago, there’s a practice run this coming weekend, here on the 0z EPS at day 4. It’s a shallow ridge, giving just a bit of a break before the flow from the southwest resumes, but a pleasant Sunday afternoon over England in particular with temperatures up to 15 degrees on the op. What is interesting from there is that though the Atlantic trough does make very slow eastward headway to be hanging down through the UK and Ireland by day 8, it weakens considerably against the European heights. This weakening of the trough means that the next wave of Azores heights is able to line up a better effort towards the weekend after next as the trough gets shunted east and squeezed out. This build looks to be getting its act together by around day 10. The 0z EPS / ECM mean shows a solid build in heights nicely oriented northeastwards in the direction of the UK and Ireland. So slowly gaining Azores momentum. What we’d perhaps expect this time of year. It’s not looking like sustained dry spells by any means, more like 2-3 day spells of drier, more settled weather between slow moving troughs, but I’m alright with it, as long as we get the breaks. When it rains, I can wear a coat. There’s always a lot of interest in the potential for wintry weather and at the other extreme, summer heatwaves. But spring, with its better light and resurgence of nature, though often under-rated, can be a lovely time of the year. To make the most of it, I’ll probably be posting a good deal less, so have a good few weeks.

Next weekend looking quite reasonable on the 12z GFS op as some decent heights encourage a ridge of high pressure to nose up from the south in between the two low pressure systems on Saturday. This establishes a gentle southwesterly flow travelling up all the way from the Azores for all of the UK and Ireland by Sunday. Largely dry away from the west of Scotland bar a light shower or two. Temperatures responding - into double figures generally by the afternoon. Warm for many parts of England, 14-16 degrees, some 5-7 degrees above average for the time of year on the anomaly chart. Sweaters optional.

We are ten days away from the spring equinox. Looking at the 0z ECM ensemble mean for day 10 shows a much depleted PV. If we imagine a line running west-east across the Arctic dividing the hemisphere between the Greenwich meridian half and the International Date Line half, a continuation of 90 degrees west over the pole to 90 degrees east in longitude, we see that the PV is entirely on our side of the hemisphere. An Alaskan ridge extends all the way through to the far east of Russia so the core PV is being squeezed to the Greenland-Svalbard-Kara Sea region. The core cold sits further west over northern Greenland. There are two main zones of polar troughing being orchestrated, also shown nicely on the 0z EPS chart, one running south through the Norwegian Sea, and a really handsome one, with an easy run south for the cold air over the continuous ice / land mass of the Canadian Arctic down into the eastern United States, much easier than over the long sea track down into Western Europe. As a result, the T850 -4 degree isotherm and 540 dam heights contour runs from Washington DC at 39 degrees North to just north of the central belt in Scotland at 57 degrees North. Nevertheless, in this same broad setup, that is maintained for a fair few days thereafter, especially when the trough sets in just that little bit further east over Scandinavia, there are possibilities for incursion of cold air into Western Europe too, as here on the 0z ECM control at days 11 (a clean northerly shot there) and day 13/14 (a bit more messily cyclonic but still cold). I’m enjoying the longer and brighter days at the moment and it’s interesting to see some warmer southerly episodes cropping up in the ops. However, I really like the ECM for the way that it typically keeps the control (in effect the post day 10 op) synoptically coupled to the mean, even at the longer ranges - it’s better than the other models in this regard - so for me, with this polar setup, and in many ways, typical for the time of year with waning PV and marked long wave troughs, some colder weather is still very much a distinct possibility for the last third of the month. Indeed, in keeping with the above, it would be little surprise to see stark and sudden transitions between warm and cold spells over the next few weeks. Happy Mother’s Day. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5046768

We are ten days away from the spring equinox. Looking at the 0z ECM ensemble mean for day 10 shows a much depleted PV. If we imagine a line running west-east across the Arctic dividing the hemisphere between the Greenwich meridian half and the International Date Line half, a continuation of 90 degrees west over the pole to 90 degrees east in longitude, we see that the PV is entirely on our side of the hemisphere. An Alaskan ridge extends all the way through to the far east of Russia so the core PV is being squeezed to the Greenland-Svalbard-Kara Sea region. The core cold sits further west over northern Greenland. There are two main zones of polar troughing being orchestrated, also shown nicely on the 0z EPS chart, one running south through the Norwegian Sea, and a really handsome one, with an easy run south for the cold air over the continuous ice / land mass of the Canadian Arctic down into the eastern United States, much easier than over the long sea track down into Western Europe. As a result, the T850 -4 degree isotherm and 540 dam heights contour runs from Washington DC at 39 degrees North to just north of the central belt in Scotland at 57 degrees North. Nevertheless, in this same broad setup, that is maintained for a fair few days thereafter, especially when the trough sets in just that little bit further east over Scandinavia, there are possibilities for incursion of cold air into Western Europe too, as here on the 0z ECM control at days 11 (a clean northerly shot there) and day 13/14 (a bit more messily cyclonic but still cold). I’m enjoying the longer and brighter days at the moment and it’s interesting to see some warmer southerly episodes cropping up in the ops. However, I really like the ECM for the way that it typically keeps the control (in effect the post day 10 op) synoptically coupled to the mean, even at the longer ranges - it’s better than the other models in this regard - so for me, with this polar setup, and in many ways, typical for the time of year with waning PV and marked long wave troughs, some colder weather is still very much a distinct possibility for the last third of the month. Indeed, in keeping with the above, it would be little surprise to see stark and sudden transitions between warm and cold spells over the next few weeks. Happy Mother’s Day.

The afternoons and early evenings are giving better and better value as spring gets underway. Temperatures very reasonable for central and southern England this afternoon in the southeasterly draw off the near continent, ahead of the approaching low, here on the 0z ECM op, better indeed than for most of France and Iberia, where temperatures are subdued to say the least… …and getting pegged back generally for the UK and Ireland by tomorrow as the low pressure passing through to the south slowly fills over southern UK and Ireland but pulls us around into an easterly flow. A slow but steady improvement during the week, as wafts of heights from the south, combine with the longer days and a southerly flow to deliver the vast majority double figure afternoon temperatures by Thursday, as a more organised belt of showery rain approaches from the west. Southeast Spain up into the twenties. In keeping with what we often see early on in the season, the gradual upward trend in afternoon temperatures is not emulated by the early morning temperatures, with frosts likely for many midweek. The Friday feature looks potentially interesting. Good levels of precipitable water down the spine so with this marked diurnal heating, scope for some early spring convection to enhance the belt of showers as it passes through. All very fitting for the second week of March, which will do nicely. Have a good one.

Some indications emerging now of how the aftermath of the SSW might play out in terms of the Northern Hemisphere profile over the next couple of weeks. The 0z ECM ensemble mean / EPS charts show low pressure centred near the Low Countries at day 4. The Scandinavian heights are being squeezed northwest towards Greenland. Strong heights in this region appear to be a feature of the next couple of weeks. They are being held there at day 7 by a westward extension of the Siberian core of the PV to the north of Greenland, which reinvigorates the Greenland trough, bringing the western fringe of Europe to the eastern periphery of an Atlantic cyclonic influence. By day 10, this westward extension of the PV has developed further, forming a major trough down through eastern North America, a very cold spell looking to develop there, peaking at around day 12, while Europe remains quiet with temperatures near average. Indications of a change for Europe by day 15 though, as heights rise through the Atlantic and a Scandinavian trough becomes established, along with a surface low over the northern Mediterranean… …bringing signs, albeit at that distant range, of scope for a northerly surface flow, picked up on the EPS MSLP charts too, flowing from the Siberian Arctic, through the Norwegian Sea, to Western Europe. From what has proved at times to be a quite exasperating winter, though the Atlantic might at times dabble with us, it looks like we’ll move towards and pass the spring equinox with a largely blocked setup and one more tease for prospects of cold weather, this one perhaps being directed rather than being diverted by events in the stratosphere. Relatively quiet it almost certainly will be. As for something notably cold later in the month, well maybe, but let’s not hold our breath! Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5045896

Some indications emerging now of how the aftermath of the SSW might play out in terms of the Northern Hemisphere profile over the next couple of weeks. The 0z ECM ensemble mean / EPS charts show low pressure centred near the Low Countries at day 4. The Scandinavian heights are being squeezed northwest towards Greenland. Strong heights in this region appear to be a feature of the next couple of weeks. They are being held there at day 7 by a westward extension of the Siberian core of the PV to the north of Greenland, which reinvigorates the Greenland trough, bringing the western fringe of Europe to the eastern periphery of an Atlantic cyclonic influence. By day 10, this westward extension of the PV has developed further, forming a major trough down through eastern North America, a very cold spell looking to develop there, peaking at around day 12, while Europe remains quiet with temperatures near average. Indications of a change for Europe by day 15 though, as heights rise through the Atlantic and a Scandinavian trough becomes established, along with a surface low over the northern Mediterranean… …bringing signs, albeit at that distant range, of scope for a northerly surface flow, picked up on the EPS MSLP charts too, flowing from the Siberian Arctic, through the Norwegian Sea, to Western Europe. From what has proved at times to be a quite exasperating winter, though the Atlantic might at times dabble with us, it looks like we’ll move towards and pass the spring equinox with a largely blocked setup and one more tease for prospects of cold weather, this one perhaps being directed rather than being diverted by events in the stratosphere. Relatively quiet it almost certainly will be. As for something notably cold later in the month, well maybe, but let’s not hold our breath!

Once the low detaches from the trough and trundles by to the south, the 12z ECM op sets up a distinctly cold looking high pressure over the UK and Ireland by day 7. Animations here through to day 8. The high pressure squeezes around the northwestern flank of the departing low, bringing cold air with it from Scandinavia and delivering some sharp looking uppers. Could be a bit crisp that.

ECMWF coolly trying to pass this one off as the 0z ensemble pressure plot for the next 10 days. They’ve even gone to the lengths of publishing accompanying charts that show an Atlantic trough being disrupted as it gets forced around the southern flank of a Scandinavian high, making most inroads around day 4, before being broken up, the cut-off low heading into northern France and the remaining stem of the trough getting pushed back west, at least for a while. But those computers have been caught red-handed in what is clearly a poorly disguised analysis of the number of posts on the model thread, starting heading into December, the phrenetic activity peaking around Christmas and the New Year, then falling away steadily, each month is craftily concealed as a day, the current quieter March to April period corresponding conveniently with the squeeze from the trough. Then we’re in to their current prediction for the next few months, featuring a resurgence between Easter and June, peaking with interest in potential heatwaves between June and August, before falling away again towards the autumn. Evidently modelling more than just the weather, but we’re on to them - you’re fooling no-one here, Reading.

12z GEFS and ECM op - models beginning to look a bit crabby as early as day 4.

12z GEM op is very interesting indeed. No mucking around with that shortwave around Iceland, so has a cleaner route to cold, the low going under the high into France at day 8. Then the bonus of moving it through to Poland by day 10, reinforcing the feed of cold air from the northeast into the UK and Ireland, with further signs of Atlantic system disruption. Nice clean route, which is what we need this time of the year.

The most populated of the 3 clusters from the 0z ECM ensemble at day 10, which gathers 28 of the 51 members, is a NAO- scenario. The representative member is 14, showing the UK and Ireland in a flow off the continent, chilly at the surface no doubt, but not cold air at T850. From there, the low to the southwest runs along the southern flank of the high pressure before fizzling out. The high then relaxes south before the next Atlantic low brings us into a flow from the southwest by day 15. Cluster 2 has 13 members and is a more blocked scenario at day 10. The representative member is 47, which has the UK in a dry, cold, easterly flow from Russia. Again, the high sinks south by day 15, low surface temperatures probably but the cold air is pushed to the south over Italy. Nice March weather that one. Cluster 3 has 10 members and though designated as a blocking scenario, it doesn’t look very blocked from the perspective of the UK and Ireland (nor for the rest of Europe for that matter), the representative member is 49, with an Atlantic westerly flow presiding. By day 15, the pattern is cyclonic with some hints of a chillier polar maritime influence eventually on the way from the northwest. So, all in all, presently looking like a strong signal for an easterly flow but only roughly 1 in 4 chance of a cold easterly for a short while around day 10, before the Scandinavian high sinks south, perhaps giving us a few days of a pleasant anticyclonic influence, but more likely the Atlantic pattern reasserting itself by day 15. The ensemble mean captures it nicely, with a southeasterly to easterly flow at day 10, rather cold for a few days, before the next Atlantic trough approaches bringing us into a flow from the west by day 15, with temperatures recovering to near normal. By then, we’ll be two days away from the spring equinox, and just a few days from the Easter break. ‘Nuff said. Have a lovely Sunday.

A big battle ensuing on the 0z ECM op through to day 10 with possibly the best example of Atlantic trough disruption that we have seen through these colder months. The first cut-off is rescued at day 2 as it joins the Greenland trough, but the reformed trough is sent sharply into reverse, pushed west, through to day 4 or 5, as heights build through Scandinavia, on their migration west to Iceland. The second cut-off is paid in instalments between day 6 and 8, and this is firmly forced southeast around the southern flank of the heights. The steady reappearance of some very cold air over Scandinavia, extending southwestwards is intriguing to say the least. The ensemble mean shows a similar trend, ending up with positive heights extending from Scandinavia deep into Canada. The very strong heights anomalies (in black) over Scandinavia during the next few days is a particularly notable signal. It’s looking like developing into proper Northern blocking across the entire Atlantic sector. Some interesting discussions about the value of ensemble mean charts. I agree that they’re not offering a likely solution in themselves, but then neither for that matter, does the op or any of the individual ensemble members past day 6 or 7. The clusters are useful, but there we are still working with a representation of the cluster and at a relative probability in terms of the whole ensemble that can at times be quite modest. What the ensemble mean does give is an idea of the general direction of travel of the whole pack for that particular timeframe. When, like with the above, this is in broad agreement with the op, this provides support for the general evolution being modelled, in this case a quite striking trough disruption at the hands of impressive and sustained heights to the north, which is undeniably of value.

The end to another week marked by another cold trough dropping down from the north over the UK and Ireland, 12z GFS op, 0-78h. These are the 500mb temperature and heights charts. Nice to look at. Last week’s colder snap flattered to deceive in many ways. The impressive looking 528 dam thicknesses only delivered for the tops. I must admit that I saw more scope in it for snow than transpired. This one will perhaps prove more interesting in the event as there is a low pressure system with its own circulation that forms at the base of the trough. The trajectory of arrival is more sharply NW-SE so there is slightly colder air wrapped up in it, with heights as low as 512-520 gpm, and there is also more organised precipitation. This time there is a keen ridge building behind the low, which serves to cut it off from the trough at least for a while, until it merges west with the base of the next incarnation of the Greenland longwave. The models have struggled all week with the rate at which the low gets warmed out, with a trend over the last few days toward keeping it that bit colder for longer. @Ali1977 has been picking up on this firming up over the last few days. In any case, there’s certainly plenty of weather around on Friday into Saturday in particular, at 36-78h, as the low pivots its way through Ireland and the UK and spins out a secondary system to France. It’s made more interesting by the system arriving during nighttime hours and the relatively slack circulation as it grinds to a halt possibly aiding some evaporative cooling at times with the heavy and sustained precipitation, so maybe the first couple of days of spring this year might well have a decidedly wintry feel. Very likely my last post of the winter! I have greatly enjoyed it, though it seems that the models we study are possibly on as steep a learning curve as we are. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5042832