WYorksWeather

EC clusters below. Days 3-4 A deep trough through the UK on Sunday (day 3) and Monday (day 4). Days 5-7 Cluster 1 (47%) has low pressure clearing east, with a developing ridge to the west and north of the UK. Cluster 2 (29%) is similar, but the ridge to the west of the UK is flatter, and low pressure south of Greenland more vigorous. Cluster 3 (24%) has a stronger build of heights, with the Azores High combining with heights to our north to block off the Atlantic by day 7. The OP was in Cluster 1. Days 8-10 Cluster 1 (31%) has a N/S split - heights building into the south but still unsettled in the north. Cluster 2 (29%) builds a mid-Atlantic ridge and turns cool and unsettled with a trough diving down from the NW or N. Cluster 3 (27%) has high pressure building over the north and west of the UK at days 8 and 9, but then the high retrogresses into the Atlantic, so turning cooler and more unsettled. Cluster 4 (12%) is strongly anticyclonic and very warm or hot with high pressure building through the UK. The OP was in Cluster 1. Days 11-15 Cluster 1 (41%) maintains a N/S split. Cluster 2 (33%) is cool and unsettled at day 11, then reverts to a N/S split. Cluster 3 (25%) is anticyclonic and warm but not especially so, with high pressure generally centred well to the west of the UK. Summary There remains not a great deal to say about the output, to be honest. Turning cooler and more unsettled over the weekend, especially for Sunday. Beyond that, days 5-7 shows some variation in the subtleties of the setup in terms of how strong the initial ridge is. This leads to a wide range of outcomes at days 8-10, the most likely being either a N/S split, followed by some sort of high to the west (either warm-ish and settled, or cool and unsettled, depending on how far west). There is a cluster in there for a very strong build of pressure, but only with 10% support at the moment, so very unlikely to verify at the moment. Looking further ahead, the extended range is just a bit of a mess - all fairly average setups within some sort of broad westerly pattern. I'd probably ignore that period for now as there's so much earlier uncertainty, and in any case there's not much of a signal. Overall then, a fairly typical summer mix of conditions is showing for the next 10 days or so. We are in a short warmer spell for the south and east at the moment, then cooler and more unsettled for the weekend, probably some sort of ridge around the middle to late next week but nothing spectacular showing up at the moment. For the north probably a below average and wetter than average signal, for the south likely somewhat drier and warmer. This does seem to continue to conflict somewhat with the background signals - it is still possible of course that the AIFS is right with its much stronger ridge, but I always try to remain objective here. Just because the AIFS is showing what I might subjectively like to see, doesn't mean I'm going to ignore virtually the entire ECM ensemble! As always, hopefully some more runs will provide clarity on some of the details, but for the moment I'm not expecting anything too out of the ordinary over the next 10 days or so. Original post: https://community.netweather.tv/topic/101615-summer-2025-forecast-and-model-commentary/?do=findComment&comment=5314545

EC clusters below. Days 3-4 A deep trough through the UK on Sunday (day 3) and Monday (day 4). Days 5-7 Cluster 1 (47%) has low pressure clearing east, with a developing ridge to the west and north of the UK. Cluster 2 (29%) is similar, but the ridge to the west of the UK is flatter, and low pressure south of Greenland more vigorous. Cluster 3 (24%) has a stronger build of heights, with the Azores High combining with heights to our north to block off the Atlantic by day 7. The OP was in Cluster 1. Days 8-10 Cluster 1 (31%) has a N/S split - heights building into the south but still unsettled in the north. Cluster 2 (29%) builds a mid-Atlantic ridge and turns cool and unsettled with a trough diving down from the NW or N. Cluster 3 (27%) has high pressure building over the north and west of the UK at days 8 and 9, but then the high retrogresses into the Atlantic, so turning cooler and more unsettled. Cluster 4 (12%) is strongly anticyclonic and very warm or hot with high pressure building through the UK. The OP was in Cluster 1. Days 11-15 Cluster 1 (41%) maintains a N/S split. Cluster 2 (33%) is cool and unsettled at day 11, then reverts to a N/S split. Cluster 3 (25%) is anticyclonic and warm but not especially so, with high pressure generally centred well to the west of the UK. Summary There remains not a great deal to say about the output, to be honest. Turning cooler and more unsettled over the weekend, especially for Sunday. Beyond that, days 5-7 shows some variation in the subtleties of the setup in terms of how strong the initial ridge is. This leads to a wide range of outcomes at days 8-10, the most likely being either a N/S split, followed by some sort of high to the west (either warm-ish and settled, or cool and unsettled, depending on how far west). There is a cluster in there for a very strong build of pressure, but only with 10% support at the moment, so very unlikely to verify at the moment. Looking further ahead, the extended range is just a bit of a mess - all fairly average setups within some sort of broad westerly pattern. I'd probably ignore that period for now as there's so much earlier uncertainty, and in any case there's not much of a signal. Overall then, a fairly typical summer mix of conditions is showing for the next 10 days or so. We are in a short warmer spell for the south and east at the moment, then cooler and more unsettled for the weekend, probably some sort of ridge around the middle to late next week but nothing spectacular showing up at the moment. For the north probably a below average and wetter than average signal, for the south likely somewhat drier and warmer. This does seem to continue to conflict somewhat with the background signals - it is still possible of course that the AIFS is right with its much stronger ridge, but I always try to remain objective here. Just because the AIFS is showing what I might subjectively like to see, doesn't mean I'm going to ignore virtually the entire ECM ensemble! As always, hopefully some more runs will provide clarity on some of the details, but for the moment I'm not expecting anything too out of the ordinary over the next 10 days or so.

So I think again we need to look and see why the models are showing such different outcomes at day 8. For that, we can look at day 5 and see what happens earlier on. GFS and GEM have a lot of messy lows to the south of Greenland, which aid in flattening off any developing high: UKMO and ICON keep those features a bit less relevant to the UK - they deepen a bit earlier to the west rather than pushing east: The AIFS has one much deeper low to the west, again this doesn't really affect the ridge development, so the result ends up more amplified. In general, it seems that if the lows to the south of Greenland mature and deepen earlier and further west, they promote a stronger ridge, whereas if they are weaker and progress eastwards, deepening as they approach the UK, then the ridge will be flatter. So, the question is which model is best resolving the mess to the south of Greenland and/or the strength of the developing ridge? I think once that becomes better known, the models will converge on the correct solution. Might take another day or two as we know the models often struggle with the Greenland/Iceland area. Original post: https://community.netweather.tv/topic/101615-summer-2025-forecast-and-model-commentary/?do=findComment&comment=5314468

So I think again we need to look and see why the models are showing such different outcomes at day 8. For that, we can look at day 5 and see what happens earlier on. GFS and GEM have a lot of messy lows to the south of Greenland, which aid in flattening off any developing high: UKMO and ICON keep those features a bit less relevant to the UK - they deepen a bit earlier to the west rather than pushing east: The AIFS has one much deeper low to the west, again this doesn't really affect the ridge development, so the result ends up more amplified. In general, it seems that if the lows to the south of Greenland mature and deepen earlier and further west, they promote a stronger ridge, whereas if they are weaker and progress eastwards, deepening as they approach the UK, then the ridge will be flatter. So, the question is which model is best resolving the mess to the south of Greenland and/or the strength of the developing ridge? I think once that becomes better known, the models will converge on the correct solution. Might take another day or two as we know the models often struggle with the Greenland/Iceland area.

Coldilocks Yeah not talking from a preference point of view here more from a stats perspective. There's no getting around that in terms of temperature mid-July to mid-August is a much warmer period than mid-June to mid-July even though there's more daylight hours earlier on in the season. In much the same way that late January is far colder on average than the Christmas period.

Mike Poole Yep, still not entirely sure what to make of it. The meteogram is similar in the sense that it's just very bland type weather - slightly warmer than average overall for both London and Glasgow, becoming drier in the south but still somewhat unsettled in the north. Not to say that's necessarily a bad thing depending on personal preferences - I've got no issue with slightly warmer than average and fairly dry weather even if nothing spectacular. However, at a time when signals suggest we should be starting to see something pretty spectacular gathering strength at the edges of the model output range, there's no real sign of it. So, we are still stuck with the three options: The models are wrong and will move strongly to a ridge-oriented outcome (similar to Cluster 1 in the extended which you posted) over the next few days. The background signals are missing some local factor which creates a less favourable environment for ridging close to the UK. It's just too early, and any more persistently anticyclonic spell triggered by the background signals will not kick in until August. Original post: https://community.netweather.tv/topic/101615-summer-2025-forecast-and-model-commentary/?do=findComment&comment=5314174

jonboy When I say 'evenly' I mean approximately - there are very small variations across the globe but nothing substantial, there are a variety of global monitoring stations and they're all broadly within a couple of percent in terms of the atmospheric concentrations. Remember that this even mixing has a long time to occur - the lifespan of CO2 in the atmosphere is measured in centuries, not months or years. We know that shorter-lived gases like sulphur are not evenly mixed, their lifespan is much shorter, so you can measure that e.g. a volcanic eruption in the Northern Hemisphere has less of an impact in the Southern Hemisphere in most cases.

raz.org.rain Yep, seasonal lag is a much bigger thing than most people appreciate. We've seen a lot of July records lately which distorts perceptions a bit, but only about 20 years ago we saw over 38C on 10th August 2003, which was then the all-time record high. Translate that to the present day and 40C could be possible. Historically we've seen other records from August as well - the 1990 record was set in August, as was its predecessor in 1911. This attitude of writing the season off early makes even less sense in summer than it does in the winter - in winter the slight decline in cold potential after about the first or second week in February is often enough to scupper snow chances, so there is an element of 'roll on spring' in many years from about mid-Feb. But in summer we have a lot more margin for error these days, such that very warm weather is more frequent even going into September. So overall, I'd say the chance of a serious record-contender heatwave disappears after mid-August. But a more ordinary heatwave (say a few days of high 20s / low 30s) is very possible right through late August and into the first half of September. These days even a remote chance probably as late as the last week of September and first week of October, given we saw 29.9C in October 2011.

Quick update on where we are with the SSTs around the UK. Still broadly a very warm setup for most of the waters close to the UK, both in relative and absolute terms. Quite warm in the Mediterranean but the exceptional anomalies showing a week or two ago have dissipated for now. Definitely plenty here to enhance the likelihood of heatwaves, due to reduced moderation of any imported hot air from the south, and more generally to keep temperatures on the above average side even with fairly unspectacular synoptics. In short, even if we keep a fairly typical outlook in terms of synoptics, I'd still expect temperatures for most areas to remain above average given this latent heat around our shores.

jonboy CO2 is evenly spread as it's a well-mixed greenhouse gas. Things like aerosols (e.g. sulphur) are less evenly spread as they have a lifespan of days to years rather than centuries in the atmosphere.

Mike Poole Yep, still not entirely sure what to make of it. The meteogram is similar in the sense that it's just very bland type weather - slightly warmer than average overall for both London and Glasgow, becoming drier in the south but still somewhat unsettled in the north. Not to say that's necessarily a bad thing depending on personal preferences - I've got no issue with slightly warmer than average and fairly dry weather even if nothing spectacular. However, at a time when signals suggest we should be starting to see something pretty spectacular gathering strength at the edges of the model output range, there's no real sign of it. So, we are still stuck with the three options: The models are wrong and will move strongly to a ridge-oriented outcome (similar to Cluster 1 in the extended which you posted) over the next few days. The background signals are missing some local factor which creates a less favourable environment for ridging close to the UK. It's just too early, and any more persistently anticyclonic spell triggered by the background signals will not kick in until August.

danm Was out earlier so just a very quick late update - focusing on days 8-10 and days 11-15. Days 8-10 Cluster 1 (41%) has a build of pressure over the south of the UK, mixed conditions further north. Could be very warm or hot in the south at times. Cluster 2 (39%) is a variation on a theme with a stronger initial build of pressure at day 9, but flattened off a bit by day 10. Cluster 3 (20%) is weakest with the build of pressure, but still turns more settled for the southern half of the UK. The OP was in Cluster 1. Days 11-15 Cluster 1 (55%) has high pressure in control but mainly centred to the west of the UK. Should be fairly dry and warm in the west of the UK, but for the east potentially cooler with the risk of showers. Definitely no very warm weather or heatwaves. Cluster 2 (45%) maintains a flat westerly pattern. A transient ridge at day 11 is followed by a more unsettled setup at days 13 and 15 with low pressure moving in from the west. Summary Within the reliable timeframe, a marginal heatwave (perhaps just about meeting the thresholds) over the next few days for some areas in the south or east could occur, before a more unsettled period over the weekend into early next week. In the longer range, a fairly typical outlook from tonight's clusters. At days 8-10 the ensemble is going for a pressure rise, but it's a fairly 'standard' one with heights only really building with any conviction for the south of the UK, where admittedly it may be very warm to hot for a time. In the extended range, it's broadly a similar picture. Two fairly even clusters, one with high pressure slightly to the west which would be warm for many but certainly no real chance of a heatwave, and one with a flat westerly pattern. So, where does this leave things? There's no obvious sign of the sort of major response you would expect to the MJO/AAM in this output. That leaves one of three possibilities: The signals are leading the models, and the ECM and other models will gradually 'catch-up' and the ridge at days 8-10 will increase in forecasted strength as the timeframe gets closer. The models are picking up a local factor that will prevent the expected response to the MJO/AAM from occurring, and they are right in showing a weaker ridge than we'd otherwise expect from the signals. It is too early to see a major response to the signals, and we will see a stronger ridge emerge for the period at the edge or just outside of current NWP horizons, so not until early August. The short version: the models are wrong, the signals are wrong (or correct, but irrelevant to the UK), or it's too early. Overall, over the last 3-4 days we've seen all sorts of chaos in the model output, models picking up and then dropping an extended -NAO signal, then a stronger ridge signal last night, now a slight step away from it again. A real case of more runs needed I think this evening - these clusters have ended up asking a lot more questions than they have answered! Original post: https://community.netweather.tv/topic/101615-summer-2025-forecast-and-model-commentary/?do=findComment&comment=5314070

Sky Full I suppose the counter-argument would be that if you break the world into individual chunks that are roughly UK-sized, then the same would apply. For example, you could divide the U.S. by state and make the same argument - reducing emissions in Texas, when these are only a small percentage of the global total, is not going to make a major difference by itself. And in some senses that is equally valid, we know that in the U.S. for example, a state like California is likely doing a better job on emissions reduction than Texas is, given different policies. And most other large countries have that sort of federal system where different states or regions can pursue slightly different policies.

Derecho Interesting, perhaps my 18.4C guess still in with a chance, which would be a bit of a surprise.

danm Was out earlier so just a very quick late update - focusing on days 8-10 and days 11-15. Days 8-10 Cluster 1 (41%) has a build of pressure over the south of the UK, mixed conditions further north. Could be very warm or hot in the south at times. Cluster 2 (39%) is a variation on a theme with a stronger initial build of pressure at day 9, but flattened off a bit by day 10. Cluster 3 (20%) is weakest with the build of pressure, but still turns more settled for the southern half of the UK. The OP was in Cluster 1. Days 11-15 Cluster 1 (55%) has high pressure in control but mainly centred to the west of the UK. Should be fairly dry and warm in the west of the UK, but for the east potentially cooler with the risk of showers. Definitely no very warm weather or heatwaves. Cluster 2 (45%) maintains a flat westerly pattern. A transient ridge at day 11 is followed by a more unsettled setup at days 13 and 15 with low pressure moving in from the west. Summary Within the reliable timeframe, a marginal heatwave (perhaps just about meeting the thresholds) over the next few days for some areas in the south or east could occur, before a more unsettled period over the weekend into early next week. In the longer range, a fairly typical outlook from tonight's clusters. At days 8-10 the ensemble is going for a pressure rise, but it's a fairly 'standard' one with heights only really building with any conviction for the south of the UK, where admittedly it may be very warm to hot for a time. In the extended range, it's broadly a similar picture. Two fairly even clusters, one with high pressure slightly to the west which would be warm for many but certainly no real chance of a heatwave, and one with a flat westerly pattern. So, where does this leave things? There's no obvious sign of the sort of major response you would expect to the MJO/AAM in this output. That leaves one of three possibilities: The signals are leading the models, and the ECM and other models will gradually 'catch-up' and the ridge at days 8-10 will increase in forecasted strength as the timeframe gets closer. The models are picking up a local factor that will prevent the expected response to the MJO/AAM from occurring, and they are right in showing a weaker ridge than we'd otherwise expect from the signals. It is too early to see a major response to the signals, and we will see a stronger ridge emerge for the period at the edge or just outside of current NWP horizons, so not until early August. The short version: the models are wrong, the signals are wrong (or correct, but irrelevant to the UK), or it's too early. Overall, over the last 3-4 days we've seen all sorts of chaos in the model output, models picking up and then dropping an extended -NAO signal, then a stronger ridge signal last night, now a slight step away from it again. A real case of more runs needed I think this evening - these clusters have ended up asking a lot more questions than they have answered!