[Model discussion highlights]

2 hours ago, Nick F said:

Sometimes I think some are putting too much emphasis on tropical forcing / teleconnections to predict what will happen over our part of the world and not looking what's going on with the atmosphere on a less global scale, i.e. stubborn East European cold trough to the east and how this is causing the jet to buckle upstream to form blocking over Iceland well into next week. Was not long ago that some were telling us the blocking would be temporary and would quickly collapse to allow resumption of Atlantic zonal conveyor of lows, based on MJO / AAM forecasts. Clearly something else going on and more 'local'. Not saying that tropical forcing isn't having some say in the current patterns, but perhaps less influential than we think?

My own take, given last night, and following up from a suggestion last week, explained how this weeks mid latitude Atlantic ridging was anticipated. I then expressed doubts about that regime's longevity at the expense of greater sub tropical flow. Tropical forcing now shows clearly increasing amplitude heading eastwards across I/O and renewed WWB's reversing the lull in momentum this week which led to the mid Atlantic ridge. This is important, because angular momentum tendency soaring heading into December, tied into seasonal wavelength changes and increasingly unstable tropopause boundary is ripe for high impact weather across Europe into December.That mid latitude ridging is set to become a road block to the polar jet as it becomes a higher latitude feature with undercutting jet flow.

The expectation of renewed sub tropical flow was sooner than modelled up to today. What has been surprising today has been the even greater speed of return of that sub tropical flow. The suggestion of that stark thermal boundary in last nights post c/o a growing -ve NAO  profile is being brought forward and provides great interest (of different kinds) for UK/NW Europe and for me down here in SW Europe.

Confidence of undercut of blocking is exponentially increasing - though still a lot of detail to sort out. As stated just now, December has potential for high impact weather as cold air advection meets warm moist sub tropical Atlantic air.  For all those of you in UK/NW Europe, I would look in the modelling in the extended period for lows to keep sliding along this boundary and bumping into increasingly cold air feeding in from the north and east.

Original post: https://community.netweather.tv/topic/99428-model-output-discussion-mid-autumn/?do=findComment&comment=4956486

[Model Output Discussion - mid Autumn]

2 hours ago, Nick F said:

Sometimes I think some are putting too much emphasis on tropical forcing / teleconnections to predict what will happen over our part of the world and not looking what's going on with the atmosphere on a less global scale, i.e. stubborn East European cold trough to the east and how this is causing the jet to buckle upstream to form blocking over Iceland well into next week. Was not long ago that some were telling us the blocking would be temporary and would quickly collapse to allow resumption of Atlantic zonal conveyor of lows, based on MJO / AAM forecasts. Clearly something else going on and more 'local'. Not saying that tropical forcing isn't having some say in the current patterns, but perhaps less influential than we think?

My own take, given last night, and following up from a suggestion last week, explained how this weeks mid latitude Atlantic ridging was anticipated. I then expressed doubts about that regime's longevity at the expense of greater sub tropical flow. Tropical forcing now shows clearly increasing amplitude heading eastwards across I/O and renewed WWB's reversing the lull in momentum this week which led to the mid Atlantic ridge. This is important, because angular momentum tendency soaring heading into December, tied into seasonal wavelength changes and increasingly unstable tropopause boundary is ripe for high impact weather across Europe into December.That mid latitude ridging is set to become a road block to the polar jet as it becomes a higher latitude feature with undercutting jet flow.

The expectation of renewed sub tropical flow was sooner than modelled up to today. What has been surprising today has been the even greater speed of return of that sub tropical flow. The suggestion of that stark thermal boundary in last nights post c/o a growing -ve NAO  profile is being brought forward and provides great interest (of different kinds) for UK/NW Europe and for me down here in SW Europe.

Confidence of undercut of blocking is exponentially increasing - though still a lot of detail to sort out. As stated just now, December has potential for high impact weather as cold air advection meets warm moist sub tropical Atlantic air.  For all those of you in UK/NW Europe, I would look in the modelling in the extended period for lows to keep sliding along this boundary and bumping into increasingly cold air feeding in from the north and east.

[Model discussion highlights]

On 14/11/2023 at 12:22, Tamara said:

A GWO Phase 4, currently in progress, back through to Phase 8 would drive a retrogressive signal for the amplification in the Atlantic, and with the polar jet then returning as AAM tendency falls back again in tandem with westerly inertia being scrubbed from the Pacific as tropical convection returns to the Indian Ocean.

And so this suggested progression has verified, reflecting the accuracy of NWP at a distance spotting the momentum transport drop-off & gunning for retrogression of the downstream pattern to an Atlantic ridge.

GWO Phase 8

C/O scrubbing of westerly inertia in Pacific in tandem with tropical MJO signal returning to the I/O. Tendency in angular momentum in both topics & extra tropics on the slide to underpin the GWO progression.

At this stage of verification of these anticipated sequences, it is my want to start to question the longevity of an Atlantic ridge signal and the retreat of angular momentum tendency that creates the upstream retrogression vacuum that is filled accordingly. NWP wants to sustain a repeated sequencing throughout the medium term but based on new momentum building into December would expect some sub tropical jet energy to return and amplified Atlantic ridge to be ultimately adjusted eastwards. The question then is the ratio of split energy that heads underneath the ridging according to how much renewed momentum propagates through from the tropics to extra tropics through the first half of December.

This anticipated renewed AAM uptick,occurring as seasonal wavelength changes are kicking in, in turn will influence the relationship between the troposphere & stratosphere and the latitude of renewed blocking structures. My main interest is when, and to what extent a defined sub tropical jet ultimately returns dictating a potentially noteworthy thermal boundary. It also has different implications here in Portugal to that of UK where members will no doubt watch this sequence closely for opposite reasons......

 

Original post: https://community.netweather.tv/topic/99428-model-output-discussion-mid-autumn/?do=findComment&comment=4955572

[Model Output Discussion - mid Autumn]

On 14/11/2023 at 12:22, Tamara said:

A GWO Phase 4, currently in progress, back through to Phase 8 would drive a retrogressive signal for the amplification in the Atlantic, and with the polar jet then returning as AAM tendency falls back again in tandem with westerly inertia being scrubbed from the Pacific as tropical convection returns to the Indian Ocean.

And so this suggested progression has verified, reflecting the accuracy of NWP at a distance spotting the momentum transport drop-off & gunning for retrogression of the downstream pattern to an Atlantic ridge.

GWO Phase 8

C/O scrubbing of westerly inertia in Pacific in tandem with tropical MJO signal returning to the I/O. Tendency in angular momentum in both topics & extra tropics on the slide to underpin the GWO progression.

At this stage of verification of these anticipated sequences, it is my want to start to question the longevity of an Atlantic ridge signal and the retreat of angular momentum tendency that creates the upstream retrogression vacuum that is filled accordingly. NWP wants to sustain a repeated sequencing throughout the medium term but based on new momentum building into December would expect some sub tropical jet energy to return and amplified Atlantic ridge to be ultimately adjusted eastwards. The question then is the ratio of split energy that heads underneath the ridging according to how much renewed momentum propagates through from the tropics to extra tropics through the first half of December.

This anticipated renewed AAM uptick,occurring as seasonal wavelength changes are kicking in, in turn will influence the relationship between the troposphere & stratosphere and the latitude of renewed blocking structures. My main interest is when, and to what extent a defined sub tropical jet ultimately returns dictating a potentially noteworthy thermal boundary. It also has different implications here in Portugal to that of UK where members will no doubt watch this sequence closely for opposite reasons......

 

[Model Output Discussion - mid Autumn]

4 minutes ago, MattH said:

As ever need some caution of taking the MJO phases and multiplying them forward to get an amplified ridge in 10 to 14 days, or potential blocking in December, when that simply isn't the expected route of travel in terms of the N Hem pattern and particularly the N Atlantic.

This signal for amplification within the week of the 20th to the 24th has, essentially 'come from nowhere'. One could argue that the trop remains disconnected from the strat, but for how long and if you do use the MJO and VP200 anomalies as a forecasting tool, within a Nino November, with the key areas of forcing within phases 8-1, the outcome is far from amplification in the N Atlantic.

The only data I can see, to hand, that connects these uncertain dots is a rise in the GWO, in November, which given the current and significant WWB through the dateline could well be driving this amplification but, again, I wouldn't want to put money on it. Nov GWO phase 5 leads to this anom pattern...

Could this be one of "those" scenarios whereby we are seeing influences from the unknown brought about the wild SST anomalies, globally? It is very difficult to tell. The weather will do whatever it wants but as usual, following the medium and long-term almost daily, but particularly weekly, this signal for an amplified N Atlantic goes against the grain for all forecasts for the second half of November in recent weeks.

An interesting one but I wouldn't hold your hopes up for charts like the GFS is currently showing with heights right up into Greenland and a proper -ve NAO trend, I have a feeling this signal will be watered down but we watch with interest...

Matt.

A good summary from Matt. I would very much agree with most of this. The extra tropical evolution is indeed rather uncertain. A GWO Phase 4, currently in progress, back through to Phase 8 would drive a retrogressive signal for the amplification in the Atlantic, and with the polar jet then returning as AAM tendency falls back again in tandem with westerly inertia being scrubbed from the Pacific as tropical convection returns to the Indian Ocean.

The upcoming tropical convection wave through December needs to be watched in respect of its eastward progression and subsequent interaction with potential propagation into the extra tropics, and in turn, how this evolves the relationship between the troposphere & stratosphere.

 

 

[Model Output Discussion - mid Autumn]

15 hours ago, Metwatch said:

In the more closer time frame there is an interesting phenomenon, which although it affects us less, I still think it's important to some extent. That is the 'Summer of San Martin.'

Essentially it's a few day period in mid November, starting around the 11th where conditions settle down for Iberia as high pressure builds more compared to October / early November, and it often happens most years; the data has been analysed to prove it during this period in the year. It's a sort of Indian summer if you like.

We see it this year with the ECM height anomaly. Heights in Spain (sort of Euro heights if they move more north in some years) usually keep the UK under a zonal flow, so it remains unsettled and it can be on the mild side. Spain is often under high pressure a lot of the time as it is on the border between the Hadley and Ferrel cell with descending air and close to the Azores high, but even so this seems to stand out more. Mid November 2015 was the biggest example of the quirk.

Not sure if it's the MJO, GWO or AAM etc that causes this, probably a multitude of teleconnections working together in the background which coincide themselves for mid November. I am curious if @Tamara who is in southern Portugal would notice this perhaps where it's called 'Verão de São Martinho.'

If you would like to read more about it, an article below:

Know everything about the summer of San Martín | Network Meteorology

WWW.METEOROLOGIAENRED.COM

---

In this article we tell you all the secrets, curiosities and we explain why the summer of San Martín is formed. Come in and find out.

Thanks for posting that link - its a very interesting background to the phenomenon

The ECM in particular advertises a classic Verão de São Martinho for this part of the southern Europe from the weekend and extends it well through next week. The rains have been welcome easing the drought (though the Algarve & Alentejo need a lot more rain). But some late autumn sun & temperatures into the low 20s will be much appreciated

 

 

[Model discussion highlights]

A lot of talk continues about tropical forcing in isolation of the relationship between both the tropics and the extra tropics (as measured by the Global Wind Oscillation).  I auto-repeat so often that total global wind-flow has to be considered if one is to try to gauge how effectively (or otherwise) potential jet stream pattern changing catalysts might propagate through from the tropics to the extra tropics.

A long period of -ve PDO influence has kept reign on the extent that the current El Nino has impacted the atmospheric circulation. There is however no doubt that a tropical convection standing wave signal is rooted in the Pacific, redolent of El Nino, and which has continued to account for an active sub tropical jetstream penetrating across the Atlantic and associated widespread wind and rain high impacts across the greater parts of both north western, central and southern parts of Europe..

However, strongest rises in angular momentum tendency occur when MJO activity creates WWB in the Indian Ocean and these actually intensify as they propagate across the region, without erratic stumbles, to the area of greatest tropical convergence according to the ENSO standing wave. in simple terms, this refers currently to the Pacific. At present there is effectively a cut off along the tropics with WWB in the tropics isolated in the Pacific themselves.  Eastward propagation from upstream is truncated by convection suppression in the Eastern Indian Ocean and lack of widespread presence of eastward progressing WWB across the whole tropics mutes the most robust route to catalyst for downstream amplified poleward pattern change in the extra tropics..  Ergo, the extra tropics are not being subject to the greater potential for rises in angular momentum tendency as the strength of the WWB's in the Pacific might suggest in themselves.

A lot of the MET's on twitter (or X as it is now called) are highly MJO centric and without considering this relationship of both the tropics & extra tropical relationship as one integral forcing - so there is error risk in the calculations (on top of certain bias confirmation processes according to the various weather preferences of where one resides).

This is just one reason, on top of the effects of superimposed climate forcing, why I am more and more sceptical of the reliability of composite forecasting in this way with the over manipulation of them, and indeed over reliance on them.

Perming MJO composites based on a given phase blended with a given ENSO state is prone to error, because as this post tries to explain, the relationship between the tropics and extra tropics can be disconnected (to various degrees) from that base state. Much better to diagnose, by looking at both MJO & GWO budgets & checking the relationship of wind-flow propagation between them.

For some time now, the extra tropical GWO has been neutral or even weakly La Nin-esque negative - an evidential measure of the fact that angular momentum propagation from tropics to extra tropics is muted, as detailed in this post by explanation, and despite the distinct tropical convection signal in the Pacific. The GWO is a calculated measure of wind-flow propagation in both the tropics AND extra tropics, so more attention needs to be paid to its phase state evolution than just the MJO in isolation.

In synoptic terms, the fractured relationship between the tropics & extra tropics means that assumptions made about amplification pathways to influence the high latitudes and the tropopause layers just ahead of the incumbent stratospheric vortex winter, should not be prematurely & automatically expected to create conditions this thread in UK craves for. 

Much can change of course with time, but a greater rossby wave propagation harmony & maximised poleward propagation from tropics > extra tropics > polar field will lead to to this and its worth cautioning that this is not yet in complete harmony. Better to view the situation from a neutral position to observe this.

 

Original post: https://community.netweather.tv/topic/99428-model-output-discussion-mid-autumn/?do=findComment&comment=4949175

[Model Output Discussion - mid Autumn]

A lot of talk continues about tropical forcing in isolation of the relationship between both the tropics and the extra tropics (as measured by the Global Wind Oscillation).  I auto-repeat so often that total global wind-flow has to be considered if one is to try to gauge how effectively (or otherwise) potential jet stream pattern changing catalysts might propagate through from the tropics to the extra tropics.

A long period of -ve PDO influence has kept reign on the extent that the current El Nino has impacted the atmospheric circulation. There is however no doubt that a tropical convection standing wave signal is rooted in the Pacific, redolent of El Nino, and which has continued to account for an active sub tropical jetstream penetrating across the Atlantic and associated widespread wind and rain high impacts across the greater parts of both north western, central and southern parts of Europe..

However, strongest rises in angular momentum tendency occur when MJO activity creates WWB in the Indian Ocean and these actually intensify as they propagate across the region, without erratic stumbles, to the area of greatest tropical convergence according to the ENSO standing wave. in simple terms, this refers currently to the Pacific. At present there is effectively a cut off along the tropics with WWB in the tropics isolated in the Pacific themselves.  Eastward propagation from upstream is truncated by convection suppression in the Eastern Indian Ocean and lack of widespread presence of eastward progressing WWB across the whole tropics mutes the most robust route to catalyst for downstream amplified poleward pattern change in the extra tropics..  Ergo, the extra tropics are not being subject to the greater potential for rises in angular momentum tendency as the strength of the WWB's in the Pacific might suggest in themselves.

A lot of the MET's on twitter (or X as it is now called) are highly MJO centric and without considering this relationship of both the tropics & extra tropical relationship as one integral forcing - so there is error risk in the calculations (on top of certain bias confirmation processes according to the various weather preferences of where one resides).

This is just one reason, on top of the effects of superimposed climate forcing, why I am more and more sceptical of the reliability of composite forecasting in this way with the over manipulation of them, and indeed over reliance on them.

Perming MJO composites based on a given phase blended with a given ENSO state is prone to error, because as this post tries to explain, the relationship between the tropics and extra tropics can be disconnected (to various degrees) from that base state. Much better to diagnose, by looking at both MJO & GWO budgets & checking the relationship of wind-flow propagation between them.

For some time now, the extra tropical GWO has been neutral or even weakly La Nin-esque negative - an evidential measure of the fact that angular momentum propagation from tropics to extra tropics is muted, as detailed in this post by explanation, and despite the distinct tropical convection signal in the Pacific. The GWO is a calculated measure of wind-flow propagation in both the tropics AND extra tropics, so more attention needs to be paid to its phase state evolution than just the MJO in isolation.

In synoptic terms, the fractured relationship between the tropics & extra tropics means that assumptions made about amplification pathways to influence the high latitudes and the tropopause layers just ahead of the incumbent stratospheric vortex winter, should not be prematurely & automatically expected to create conditions this thread in UK craves for. 

Much can change of course with time, but a greater rossby wave propagation harmony & maximised poleward propagation from tropics > extra tropics > polar field will lead to to this and its worth cautioning that this is not yet in complete harmony. Better to view the situation from a neutral position to observe this.

 

[Storm Ciarán]

The Channel Islands & NW France clearly took the brunt of last nights storm. It could be deemed ironic that areas very much further away from the centre of Ciaran have seen higher impact weather conditions than across much of the inland parts of southern England that the storm has tracked with the lowest pressure.

Very strong winds have been recorded all the way down to the north of Portugal, though clearly not on anything the same level as the Channel Islands & NW France.   The very powerful jet streak has brought polar maritime air all the way south east & the Serra de Estrala mountain range has snow above 1400 metres

At the much lower altitudes the wind has been in evidence - gusting up to 110km on coasts, mainly again, in more northern parts of the country.

https://fb.watch/o3mx2rTAQu/

Though its been very gusty even here locally this morning when blustery showers move through.

This very powerful jet pattern arcing SE out of Canada & Newfoundland has sustained for over a fortnight, with Ciaran being the biggest peak (though another deep low to come this weekend) & there are only now signs of a flatter pattern coming, though still mobile & quite unsettled over more North West parts of Europe.

[Storm Ciarán]

This storm has been presenting as, and modelled as, an (especially intense) RACY jet straddling low with a diffluent trough & occlusion quickly rather than the Shapiro-Keyser variety with the split jet & fractured cold front that can creates sting jets. The powerful thermal gradient  pulls the occluded front back around the western flank of the low pressure system, as it switches to the polar side of the jet, which creates the impression of the 'scorpion tail' feature of a sting jet. However with an occlusion indicated and no fractured cold front present on the modelling then it is has not indicated in modelling as a Shapiro-Keyser system.

It is not out of the question that there are characteristics of this system that are not being picked up in its evolution - based on its sheer power and the intensity of the thermal gradient along an immensely powerful jet streak.. Clearly this is a nowcasting situation and nothing can be ruled out in respect of the speed of occlusion which may present modelling problems and error, & which could determine how the dry air intrusion into the storm manifests itself in real time vs the modelling. 

The coming hours are going to e very interesting  (and potentially worrying) in respect of the accuracy of modelling and IF the dynamics of this dangerous storm evolve in an unexpected way to that suggested....

[Storm Ciarán]

2 minutes ago, Fiona Robertson said:

I asked some questions about this in a post on the models thread. No answers, so I'm assuming we'recurrently within the territory of "Dunno".

In very basic terms, the dynamics of this storm are quite different to the 1987 storm. Track & depth have obvious similarities, but unlike 1987 this storm does not possess the dynamics to create a sting jet.

The rapid cyclonic deepening of this storm is created by the low crossing a stark thermal boundary of the jetstream and creating a vortex funneling effect with the very rapid deepening. But there is no forcing suggested of pulling air down from the mid troposphere to the surface and fracturing the frontal system as air pressure rises extremely rapidly in the immediate wake of the low (a sting jet) All the forcing from this storm comes from cold conveyors of air c/o the mixing of the very different airmasses from where the low crossed the jetstream from the tropical to polar side.

The low then starts to fill again as the effects of the crossing of the thermal boundary are fully mixed out & oceanic fuelling is cut off.

[Storm Ciarán]

There is no confidence at all that can be drawn from such a microscale area of the channel and the immediate coast where the tiniest adjustment makes such a stark differences of outcome. The latest Arpege & Arome have repeated their same track almost to a tee from this morning and the coast from the Isle of Wight eastwards still comes under gusts up to 80mph and 55mph to 65mph are found a bit further inland The GFS is fractionally further east of that and the 12z ICON is the most south & east of all. Representing the 'best case scenario'

But that 'best case scenario' which still has fairly strong winds for the far SE is, for now at least, far from guaranteed.

At the same time, parts of Western Cornwall are still at risk from the very strong winds that come around the south west flank of the low as it heads east/northeastwards across central southern parts.

There is some broad agreement that greater parts of inland most southern parts are, relatively speaking, less at risk compared to this time yesterday - but even then there is another good 24h to 30hrs modelling to come bearing in mind the most sensitive fine lines involved to rule out as much of inland southern parts as possible.

The highest level of confidence, unfortunately, is for the channel itself and its islands as well as NW France where there is no bandwidth of variation to escape the strongest winds.

[Model Output Discussion - mid Autumn]

12 minutes ago, SLEETY said:

Hoping the storm upgrades over the next 36 hours, not often we see these type of storms in the UK. Don't understand why people in a weather forum would won't  a very interesting weather event to downgrade. 

There comes a point where weather enthusiasm & common sense for safety come together. On that basis the guilt trip card cannot be endlessly played. People on weather forums still live in houses, have daily routines and need to get from A to B in safety. Its all well and good watching a storm on Youtube, or marvelling at the power of nature on a synoptic chart on a phone or computer - but the actual reality of the danger of how that power manifests itself in ones back yard is another altogether.

This has been a year of especially anomalously high impact conditions of various types all round the globe with climate induced stresses that have already breached global forecasting predictions for medium/longer term. Traditional (and even less traditional pattern analysis methods such as I try to study & employ) are being severely tested by such superimposed forcing that none of us, irrespective of skill level, understand. That lack of understanding adds to the vulnerability of increasing unknowns. It needs respect at the very least for the ever increasing threats posed

The coming week is going to see very, very high impact weather across Western Europe. Consensus is growing that the channel ( & islands) and Northern & Western France are virtually certain not to escape the worst of the midweek storm (and with another to follow next weekend). Less certain is southern counties of England & Wales for inclusion in those highest impacts - the closest to the coast, the higher the risk. If I was still living in my previous location, for example, I would be keeping a very very close eye on the microscale forecasting of this one. Its all too close to call. And to be very honest I would not be excited about it.

Based on the macro scale setup as it is evolving heading towards and into winter, the trend of greater & greater superimposed impacts (on top of standard synoptic forcing as previously seen) will continue and as things stand intensify even further. Each year becomes more of a worry & concern, as well as a fascination of the power of Mother Nature.

 

[SE, London & EA - Weather Discussion]

33 minutes ago, Daniel* said:

Yes I’m in Winchelsea what a wet night 50mm+ since midnight. Bounced it down. Serious flooding issues if this continues, Hastings is flooding. @Tamara thought you might be interested.

 

I live in Portugal now, but those pictures are not a surprise. Not helped at all by the ineptitude of the water company.

The present pattern is a more prolonged version of what is fairly common mid to late autumn conditions along those channel coasts. The coming week is also looking very cyclonic.

Here in Portugal the drought has broken, except for parts of the Algarve & Alentejo, with a fair amount of rain over the past two weeks. The hottest first half of October is heading towards one of the wettest second halves, more especially though for the north of the country.

[Model discussion highlights]

Uncertainties, in general abound. Not least, the superimposed forcing exerted from excess stratospheric water vapour following Tonga continuing to add such uncertainty to controlling influences through late autumn into winter patterns

What is known is that Pacific feedbacks have changed considerably over the mid autumn period with the strongly -ve PDO reversing towards a neutral position. In tandem with continued tropical convection standing wave signals close to the dateline. This means less Nina-esque resistance to the downstream pattern, so a likelihood of amplified Atlantic troughs & secondary features running around it, continues to consolidate further heading into late autumn. Where blocking structures set up based on this highly meridional jet stream configuration will no doubt be of much interest as winter approaches.

Overall the background as described maintains an unsettled, at times very unsettled, theme with little to shift the anchor of the upper trough away from its present position. Modelling in extended periods has been trying to point to a flatter more traditional autumn pattern but there is a tendency for this to come unstuck in closer time periods. This accounts for some of the longer term erratic signals in ensemble products.

Based on the PDO switch lifting its suppression on global windflow propagation of AAM anomalies between tropics & extra tropics, and the citing of tropical convection feedbacks in the Pacific, the sense is that angular momentum is on a more linear rising trend heading towards the back end of autumn and this contributes to the interesting implications for the longer term. Irrespective of personal preferences & purely from a synoptic diagnostic point of view.  

Babet, Aline & today newly named Bernard, clearly evident from this satellite imagery heading towards the Algarve here in Portugal).........

.....have been features of this looped very southern tracked branch of the jet stream accelerating SE out across the Atlantic from Newfoundland bringing high impact conditions to the Iberian continent - and then as very well known, being sucked northwards in this loop towards NW Europe, and impacting UK especially, closest to the anchor of the upper trough.. 

Bernard is set to repeat his predecessors, moving quickly northwards across central Spain tonight, through France tomorrow, and being incorporated within the gyre of the upper trough close to UK tomorrow

Long term: How longer term developments evolve in the polar stratosphere are going to determine how this persistent split energy pattern plays out, especially as seasonal wavelength changes under this particular background of macro scale tropospheric pattern, as described above, are not conducive to an organised vortex. 

 

 

 

 

 

 

Original post: https://community.netweather.tv/topic/99428-model-output-discussion-mid-autumn/?do=findComment&comment=4941435

[Model Output Discussion - mid Autumn]

Uncertainties, in general abound. Not least, the superimposed forcing exerted from excess stratospheric water vapour following Tonga continuing to add such uncertainty to controlling influences through late autumn into winter patterns

What is known is that Pacific feedbacks have changed considerably over the mid autumn period with the strongly -ve PDO reversing towards a neutral position. In tandem with continued tropical convection standing wave signals close to the dateline. This means less Nina-esque resistance to the downstream pattern, so a likelihood of amplified Atlantic troughs & secondary features running around it, continues to consolidate further heading into late autumn. Where blocking structures set up based on this highly meridional jet stream configuration will no doubt be of much interest as winter approaches.

Overall the background as described maintains an unsettled, at times very unsettled, theme with little to shift the anchor of the upper trough away from its present position. Modelling in extended periods has been trying to point to a flatter more traditional autumn pattern but there is a tendency for this to come unstuck in closer time periods. This accounts for some of the longer term erratic signals in ensemble products.

Based on the PDO switch lifting its suppression on global windflow propagation of AAM anomalies between tropics & extra tropics, and the citing of tropical convection feedbacks in the Pacific, the sense is that angular momentum is on a more linear rising trend heading towards the back end of autumn and this contributes to the interesting implications for the longer term. Irrespective of personal preferences & purely from a synoptic diagnostic point of view.  

Babet, Aline & today newly named Bernard, clearly evident from this satellite imagery heading towards the Algarve here in Portugal).........

.....have been features of this looped very southern tracked branch of the jet stream accelerating SE out across the Atlantic from Newfoundland bringing high impact conditions to the Iberian continent - and then as very well known, being sucked northwards in this loop towards NW Europe, and impacting UK especially, closest to the anchor of the upper trough.. 

Bernard is set to repeat his predecessors, moving quickly northwards across central Spain tonight, through France tomorrow, and being incorporated within the gyre of the upper trough close to UK tomorrow

Long term: How longer term developments evolve in the polar stratosphere are going to determine how this persistent split energy pattern plays out, especially as seasonal wavelength changes under this particular background of macro scale tropospheric pattern, as described above, are not conducive to an organised vortex. 

 

 

 

 

 

 

[Model discussion highlights]

40 minutes ago, Mcconnor8 said:

GFS and GEM ensembles are almost a 50/50 split between their Operational and the ECM operational so basically could go either way at this point.

There are only two 12z GEM ensembles similar to ECM at day 6. Out of 20 members - so very little agreement with that model. The GEFS ensembles have trended more towards extending the warmth into next week for the UK since this weekend.

A big jump in angular momentum tendency has been underway and with a signal for quite organised convection at, and just east of the dateline in the Pacific. The lagged effect of all this is to split upstream jet energy with the troughs in the Atlantic later this week (which are further complicated in respect of phasing c/o of ex tropical storms). The numerical models have been having some difficulty with this. The ECM wanting the phasing to be furthest north (attendant with returning energy to the northern branch of the jetstream) while the other models have phasing further south in the Atlantic with more of a sub tropical jetstream coming out of the southern US states from upstream in the Pacific.

At some stage a return to greater polar flow is likely and cooler air across NW Europe as the WWB and attendant tropical convection in the central/eastern Pacific wanes, but the timing of this still being resolved by the models. ECM reflects AAM tendency falling sooner rather than later with Atlantic ridging building in behind a more progressive eastward moving trough to the north of the UK. At the other end of the spectrum is the GEM with its ensembles. With greater southern stream flow and phasing of the trough equally not as far north in the Atlantic, this means greater disruption of the trough and underpins the downstream European ridging longer and associated warm air advection from Iberia. GFS12z operational fairly similar to this idea as GEM with its ensembles something of a blend.

Meantime here in Portugal, records go on breaking after a record breaking August and September for heat. My own region recorded a staggering 38.5C over the weekend and the coming week sees afternoon values between 32C and 35C. With 37C possible again at the end of the week. Records also tumbling across Spain and France and to come, also across central Europe.

These values across Portugal are for Saturday 30th Sept.

Boa noite e todos

Original post: https://community.netweather.tv/topic/99056-model-output-discussion-mid-summer-onwards/?do=findComment&comment=4931368

[Model Output Discussion - Mid Summer Onwards]

40 minutes ago, Mcconnor8 said:

GFS and GEM ensembles are almost a 50/50 split between their Operational and the ECM operational so basically could go either way at this point.

There are only two 12z GEM ensembles similar to ECM at day 6. Out of 20 members - so very little agreement with that model. The GEFS ensembles have trended more towards extending the warmth into next week for the UK since this weekend.

A big jump in angular momentum tendency has been underway and with a signal for quite organised convection at, and just east of the dateline in the Pacific. The lagged effect of all this is to split upstream jet energy with the troughs in the Atlantic later this week (which are further complicated in respect of phasing c/o of ex tropical storms). The numerical models have been having some difficulty with this. The ECM wanting the phasing to be furthest north (attendant with returning energy to the northern branch of the jetstream) while the other models have phasing further south in the Atlantic with more of a sub tropical jetstream coming out of the southern US states from upstream in the Pacific.

At some stage a return to greater polar flow is likely and cooler air across NW Europe as the WWB and attendant tropical convection in the central/eastern Pacific wanes, but the timing of this still being resolved by the models. ECM reflects AAM tendency falling sooner rather than later with Atlantic ridging building in behind a more progressive eastward moving trough to the north of the UK. At the other end of the spectrum is the GEM with its ensembles. With greater southern stream flow and phasing of the trough equally not as far north in the Atlantic, this means greater disruption of the trough and underpins the downstream European ridging longer and associated warm air advection from Iberia. GFS12z operational fairly similar to this idea as GEM with its ensembles something of a blend.

Meantime here in Portugal, records go on breaking after a record breaking August and September for heat. My own region recorded a staggering 38.5C over the weekend and the coming week sees afternoon values between 32C and 35C. With 37C possible again at the end of the week. Records also tumbling across Spain and France and to come, also across central Europe.

These values across Portugal are for Saturday 30th Sept.

Boa noite e todos

[Model discussion highlights]

6 hours ago, Sun Chaser said:

GFS consistently throwing some nice warm temperatures in the hat for the end of September now. With this ridge to the east and a trough to the west setup the models seem to be sniffing out, summer might not be done with us just yet

Is mid twenties possible in the final week of the month?

Boa tarde

Its almost certain to be over 30C down here in Portugal to see out the month & I think, yes. the mid 20s is certainly possible to bookend a quite remarkable month for the UK

Usual emphasis on some of this thread on MJO composites alone, but without considering the extra tropical GWO circulation (which is also fully alight with tropical activity) then the full story is not told.

Something of a repeat of early September looks to be in the offing, albeit with an increasing temperature gradient across N Europe this time, which is likely to hinder the full omega block pattern that lead to the longevity of that amazing heatwave.

Notwithstanding that, a similarity still revolves around another boost to angular momentum tendency as further momentum transport propagates from the tropics c/o convection returning into the Pacific & imprinting the Nino standing wave which continues to expand westwards across the ENSO region. This, taking the Global Wind Oscillation out of its doldrums and creating a more harmonious wind-flow signal with the MJO in the tropics that likely means a stronger signal than represented by the MJO alone. However, it is also the movement of ex tropical Nigel that looks to coincide within the framework of this rising AAM tendency. Creating some further extra amplification on top of the tropical>extra tropical wave-breaking to offset a more volatile jet stream than was present earlier in the month.

The net result should be enough to amplify the Atlantic profile sufficiently to take Nigel far enough west (!) of North Western Europe  - sharpening the downstream ridge extending from the Azores and pulling up air from my part of the world that is set to embark on a late heat up from next weekend. Mainland continental Europe looks to benefit from this first of all. How much of the UK might be influenced from the advection of this heat is uncertain, but latest NWP trends seem to be adjusting away from the flatter pattern advertised yesterday.

Boa sorte.

 

Original post: https://community.netweather.tv/topic/99056-model-output-discussion-mid-summer-onwards/?do=findComment&comment=4924706

[Model Output Discussion - Mid Summer Onwards]

6 hours ago, Sun Chaser said:

GFS consistently throwing some nice warm temperatures in the hat for the end of September now. With this ridge to the east and a trough to the west setup the models seem to be sniffing out, summer might not be done with us just yet

Is mid twenties possible in the final week of the month?

Boa tarde

Its almost certain to be over 30C down here in Portugal to see out the month & I think, yes. the mid 20s is certainly possible to bookend a quite remarkable month for the UK

Usual emphasis on some of this thread on MJO composites alone, but without considering the extra tropical GWO circulation (which is also fully alight with tropical activity) then the full story is not told.

Something of a repeat of early September looks to be in the offing, albeit with an increasing temperature gradient across N Europe this time, which is likely to hinder the full omega block pattern that lead to the longevity of that amazing heatwave.

Notwithstanding that, a similarity still revolves around another boost to angular momentum tendency as further momentum transport propagates from the tropics c/o convection returning into the Pacific & imprinting the Nino standing wave which continues to expand westwards across the ENSO region. This, taking the Global Wind Oscillation out of its doldrums and creating a more harmonious wind-flow signal with the MJO in the tropics that likely means a stronger signal than represented by the MJO alone. However, it is also the movement of ex tropical Nigel that looks to coincide within the framework of this rising AAM tendency. Creating some further extra amplification on top of the tropical>extra tropical wave-breaking to offset a more volatile jet stream than was present earlier in the month.

The net result should be enough to amplify the Atlantic profile sufficiently to take Nigel far enough west (!) of North Western Europe  - sharpening the downstream ridge extending from the Azores and pulling up air from my part of the world that is set to embark on a late heat up from next weekend. Mainland continental Europe looks to benefit from this first of all. How much of the UK might be influenced from the advection of this heat is uncertain, but latest NWP trends seem to be adjusting away from the flatter pattern advertised yesterday.

Boa sorte.

 

[Model discussion highlights]

On 23/08/2023 at 17:54, Tamara said:

In the scenario of Equatorial Atlantic developed tropical storms, and based on the evolution of the macro scale pattern though September to a trough/ridge structure in the Atlantic & European sector accordingly - these could serve one possible outcome where they bolster the amplification of upper troughs to the west in the Atlantic to push extra enhanced warm air advection draw ahead of them and sharpen the downstream ridge further. Not uncommon at all for the time of year under this type of macro pattern.

As outlined in a previous post - this is exactly what has happened.

The very active season in the Equatorial Atlantic is set to sustain the amplified trough/ridge pattern for longer than would have otherwise been the case - based on the upstream momentum that initiated the pattern now reversing - such reverse which normally would mean a quicker pattern change and returning Atlantic/Azores ridge and downstream trough.

Wind shear is greater than normally found in active tropical seasons with weaker trade winds c/o a much more El Nino profile over the Pacific & this is acting to equally slow the retrograde movement of the omega block across NW Europe and allows the slack sub tropical low pressure of ex Franklin to prevail west of my locale in the Portuguese side of the Iberian peninsula and sustain the heat advection. This, until the ex tropical activity is ultimately absorbed by the blocking which then finally splits as retrograde motion creates a fall of pressure in its wake  - and allows troughs across Greenland & Iceland to drop further south. With very slow attrition, this dislodges the heat dome SE'wards from NW Europe. However, timing of this (and as many have remarked) pushed further into the following week.

That leaves a good full 7 days from start to finish for potential record breaking heat values to be recorded as @Man With Beardhas suggested

With the Southern European heat dome of summer displaced so far northwards and the deep instability replacing it across the Mediterranean which impacted Spain and especially the Madrid region at the weekend,and even more serious unfolding this week across Greece - the high impacts of superimposed forcing on macro scale weather patterns, which in turn creates micro scale high impacts, is too stark to ignore.

As posted during July & August, there has to be caution over events to come this autumn with so much unprecedented SST heat in both the Mediterranean and also parts of the N Atlantic and how increasing thermal boundaries are going to force the jet stream development of low pressure systems, including further potential activity from the remainder of the Equatorial Atlantic season, as the polar field embarks on its seasonal cooling descent. 

Ciao - boa tarde

 

Original post: https://community.netweather.tv/topic/99056-model-output-discussion-mid-summer-onwards/?do=findComment&comment=4917074

[Model Output Discussion - Mid Summer Onwards]

On 23/08/2023 at 17:54, Tamara said:

In the scenario of Equatorial Atlantic developed tropical storms, and based on the evolution of the macro scale pattern though September to a trough/ridge structure in the Atlantic & European sector accordingly - these could serve one possible outcome where they bolster the amplification of upper troughs to the west in the Atlantic to push extra enhanced warm air advection draw ahead of them and sharpen the downstream ridge further. Not uncommon at all for the time of year under this type of macro pattern.

As outlined in a previous post - this is exactly what has happened.

The very active season in the Equatorial Atlantic is set to sustain the amplified trough/ridge pattern for longer than would have otherwise been the case - based on the upstream momentum that initiated the pattern now reversing - such reverse which normally would mean a quicker pattern change and returning Atlantic/Azores ridge and downstream trough.

Wind shear is greater than normally found in active tropical seasons with weaker trade winds c/o a much more El Nino profile over the Pacific & this is acting to equally slow the retrograde movement of the omega block across NW Europe and allows the slack sub tropical low pressure of ex Franklin to prevail west of my locale in the Portuguese side of the Iberian peninsula and sustain the heat advection. This, until the ex tropical activity is ultimately absorbed by the blocking which then finally splits as retrograde motion creates a fall of pressure in its wake  - and allows troughs across Greenland & Iceland to drop further south. With very slow attrition, this dislodges the heat dome SE'wards from NW Europe. However, timing of this (and as many have remarked) pushed further into the following week.

That leaves a good full 7 days from start to finish for potential record breaking heat values to be recorded as @Man With Beardhas suggested

With the Southern European heat dome of summer displaced so far northwards and the deep instability replacing it across the Mediterranean which impacted Spain and especially the Madrid region at the weekend,and even more serious unfolding this week across Greece - the high impacts of superimposed forcing on macro scale weather patterns, which in turn creates micro scale high impacts, is too stark to ignore.

As posted during July & August, there has to be caution over events to come this autumn with so much unprecedented SST heat in both the Mediterranean and also parts of the N Atlantic and how increasing thermal boundaries are going to force the jet stream development of low pressure systems, including further potential activity from the remainder of the Equatorial Atlantic season, as the polar field embarks on its seasonal cooling descent. 

Ciao - boa tarde

 

[Autumn 2023]

6 hours ago, Sunny76 said:

We just had a thunderstorm move through where I am in Greece this morning. Was baking yesterday at 30-31c, but today it’s looking like 24-25c max with cloudy conditions. Last night was very hot at 25c.

Time for massage and spa I think today.

Enjoy the lovely 29-30c heat back in Blighty.

 

I think you should be ok. September often does have a couple of unsettled days with storms in the med areas. I usually go in September, and we’ve never had a full week of sunny days. But, we always avoid July and August as it’s usually too hot, even for a heat lover like myself lol.

The current weather, in terms of temperature, is comfortable and very welcome after weeks of heat & blazing sun. Its very different living permanently here than on holiday and relying on the blue skies & sun sitting in bars & restaurants and on beaches and by pools each day.

Some rain would be welcome, but very happy & grateful to have missed the damaging downpours, floods & hail damage across parts of the Iberian continent this weekend.

The weekend was under the influence of a cut off low & associated upper cold pool creating deep instability across the continents of both Portugal and Spain, triggering some very potent thunderstorms and a few downbursts like this one in the north of Portugal 

 

Pinhão, Douro. Possível Downbrust! Via WhatsApp | By Meteo Trás os...

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Good satellite shots of the cut off low which this week phases with Franklin & helps sustain the heat advection northwards to UK and the rest of NW Europe

 

Across the border in Spain, historic flooding occurred in Madrid.

Today is 23C cloudy, breezy and again dry IMBY as a band of rain brushes Lisbon and up the Atlantic coast just to my west.  These patterns are typically hit & miss.  With the sub tropical influence of Franklin taking over in the Atlantic, southerly flow will continue and temperatures edging back up between 28C to 32C and a few more showers around, but not the intensity of this weekend.

 

 

[Model Output Discussion - Mid Summer Onwards]

With the Atlantic ridging, the cooler air has advected south this weekend to my part of the world & providing beautiful breezy and pleasantly warm conditions (28C) to replace the uncomfortable heat.  My Madeira banana tree has benefitted from the daily blue skies, dawn to dusk sun, and plenty of irrigation. Even producing tiny banana fruit

Looks like some split jet stream energy late this coming week/next weekend which may provide some showers down here with a cut-off low remnants & at the same time a watch for the beginning of ridging for NW Europe - a process to replace unsettled conditions that should slowly evolve through the fist half of September in accordance with +ve AAM wave eddies propagating from the tropics to extra tropics.

A good bank holiday to all in the UK

[Model discussion highlights]

2 hours ago, MattH said:

All eyes on the Pacific in approx a week's time and then a close eye on FT and MT tendencies within the opening 7 to 10 days of September and then potential changes towards mid-month, as a rough guide.

Hello Matt.

Already consistent upward trend in frictional torque at 30N (wind-flow additions according to jet eddy oscillations running parallel to their direction of travel within the tropics). These (westerly wind burst) wind-flow additions are set to change pressure differentials between tropics and sub tropics c/o the beginnings of the movement of the VP200 tropical convection anomaly to the Pacific - which should propagate +AAM anomalies (+ve AAM tendency) further towards the extra tropics in the time period you speak of. 

This changing dynamic has an ultimate effect on the strength of sub tropical ridging in the Atlantic. The Azores High in the Atlantic sector weakens according to a rising FT propagating from the tropics to the extra tropics. (The converse trend effect on the AZH also holds true)

Subsequent tendency in Mountain Torques in the extra tropics is influenced by direction of travel of frictional torque tendency in the tropics. So the lifespan of such a torque (3 to 4 weeks) and its lagged effect in the extra tropics does point to falling pressure to the west and south west of Europe heading towards the second/third week of September as the Azores High weakens & downstream ridging evolves accordingly. 

This analysis of the dynamics of the atmospheric momentum changes that occur through interplay of tropical convection eddy anomalies propagating over time to the extra tropics fits well with the chronology of pattern change you outline and discuss over the end of summer/start of autumn period.

In the scenario of Equatorial Atlantic developed tropical storms, and based on the evolution of the macro scale pattern though September to a trough/ridge structure in the Atlantic & European sector accordingly - these could serve one possible outcome where they bolster the amplification of upper troughs to the west in the Atlantic to push extra enhanced warm air advection draw ahead of them and sharpen the downstream ridge further. Not uncommon at all for the time of year under this type of macro pattern.

 

Original post: https://community.netweather.tv/topic/99056-model-output-discussion-mid-summer-onwards/?do=findComment&comment=4910076