[Model Output Discussion - Colder but how cold and for how long?]

I think the hope of wedges remaining to the north with colder air across the UK beyond the 20th can be discounted now, this is a very strong signal from the EPS for a return to milder, wetter weather. 

The Canadian PV lobe being pulled towards Siberia (likely thanks to the recent warming) is the dominating signal and will override any potential blocking. 

Late January into early February however does still hold potential for a return to more blocked conditions with this next momentum surge. 

Before all of that though - A week of cold weather across the whole of the UK with the potential for snowfall just about anywhere. 

[Model Output Discussion - Colder but how cold and for how long?]

If we look at the GFS strat forecasts I think we can find an answer to where this zonal push within the modelling is coming from.

The Canadian lobe of the sPV appears to get sucked towards Siberia by the stronger SIberian lobe, this occurs 17th-20th, around the time we're seeing modelling flatten the Atlantic pattern out and return the tropical jet northwards. This could perhaps be a response to the minor SSW we've recently seen, perhaps a case of the worst timing ever? (I did muse that a SSW minor or otherwise could potentially disrupt any cold spell a while back).

Once that transition has occurred & based on the current background forcing I see no reason why we wont see high pressure beginning to re-establish itself in a more favourable place for further cold weather outbreaks, Tamara has outlined this far better than I can in a few of her recent posts on this thread. 

[Model Output Discussion - Colder but how cold and for how long?]

Just now, Scandinavian High. said:

Is there any chance this system could move north to effect southern England.or  As the dust settled now on this.

Seems unlikely now. The vast majority of the EPS, MOGREPS and GEFS keep this system south and the envelope is shrinking rather quickly. I think for us in the south we need to start looking for smaller disturbances within the flow coming down from the north next week. 

[Model Output Discussion - Colder but how cold and for how long?]

2 minutes ago, festivalking said:

GFS 18z still making landfall on the south coast on Wednesday

Not quite.. 

[Model discussion highlights]

12 minutes ago, mountain shadow said:

So, despite favourable EAMTs, MTs, GWO orbits, MJOs and a weak PV, the long awaited cold spell is now a cold snap with snow pretty much only in Scotland and favourable high ground in the North West.

Here's hoping February delivers.

Because of a single run? I think we need a little perspective here. 

We currently have below average/cold conditions across the UK and by the end of next week we would have had (apart from a couple of days) 2 weeks of colder, blocked conditions. Many southern areas saw snow (albeit not a lot) last Monday. 

We go into next week with much colder air moving south across the UK with increasing snow risks. 

Background forcing has done it’s job, the broadscale pattern is as advertised & expected, high latitude blocking is becoming established. 

The UK is a small island though so despite everything being favourable, blocking & cold conditions in place snow can still be elusive, but I don’t think it will be. 

Next week looks very unstable and open to small-scale features developing and bringing widespread snow risks, these may not be picked up until 12hrs before they arrive.

Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5009289

[Model Output Discussion - Colder but how cold and for how long?]

12 minutes ago, mountain shadow said:

So, despite favourable EAMTs, MTs, GWO orbits, MJOs and a weak PV, the long awaited cold spell is now a cold snap with snow pretty much only in Scotland and favourable high ground in the North West.

Here's hoping February delivers.

Because of a single run? I think we need a little perspective here. 

We currently have below average/cold conditions across the UK and by the end of next week we would have had (apart from a couple of days) 2 weeks of colder, blocked conditions. Many southern areas saw snow (albeit not a lot) last Monday. 

We go into next week with much colder air moving south across the UK with increasing snow risks. 

Background forcing has done it’s job, the broadscale pattern is as advertised & expected, high latitude blocking is becoming established. 

The UK is a small island though so despite everything being favourable, blocking & cold conditions in place snow can still be elusive, but I don’t think it will be. 

Next week looks very unstable and open to small-scale features developing and bringing widespread snow risks, these may not be picked up until 12hrs before they arrive.

[Model Output Discussion - Colder but how cold and for how long?]

Just now, Mike Poole said:

UKMO T144 rather a lone voice now, in keeping the ‘lobe separation’ this far:

Although worth noting the increase in ridging on the GFS, it's certainly a step towards the UKMO even it it hasn't quite managed to go the whole 10 yards.

[Model discussion highlights]

48 minutes ago, Tamara said:

The extended period continues to look very problematic in respect of NWP to me.

The MJO passing through the Indian Ocean has bumped up easterly inertia/trade winds and creating problems in the modelling with Pacific ridging and with the models gunning for booting up the polar jet downstream as a consequence. This at the same time providing insight into the modelling of the lobes of the polar vortex and the spectre of them fusing together.

However, westerly wind inertia remains above average across the extra tropics and this presents a question mark about the Pacific amplification and downstream polar jet being over modelled. But most especially based on the passage of tropical convection quickly set to navigate through Phases 4 & 5 imminently during the period in question when the northern arm is being programmed to flatten the pattern. As the MJO hits the borders of Phase 5 the next starting pistol is renewed to ladder up angular momentum tendency. Putting all this together,  a very squeezed timeframe for this sequence of polar jet modelling is only available in my book to complete, let alone sustain itself if it does.

The suspicion I continue to have is that these poleward eddies that the models propose to increase polar jet momentum, will get sheared off and this whilst this still leaves a flat jet stream coming from upstream, the shearing off process will cut off the supply to the polar jet in mid stream and force it to backtrack into the sub tropical jet. Much the theme of discussion of late. 

That process, leaving some of the residual ridging between Greenland & Iceland intact and/or re-programming re-emergence of wedge type ridging and subsequent disruption of Canadian lobe energy in mid Atlantic. This leaves southerly jet kayaks to resume progress and changes the extended ensemble means that propose a flat westerly pattern across NW Europe & inflating Azores High.

It will be very interesting (as I see it) to watch and monitor if this does actually transpire.

Sounds like what you’re expecting is something similar to the 00z GEM, i.e a more active jet into the Atlantic meeting residual wedges of high pressure to the north causing the jet & subsequent low pressure systems to disrupt SEwards. 

What you advertise is appearing within model outputs albeit as a lower probability outcome currently within the modelling. 

I’m not sure models will be handling this particularly well at extended ranges, resolving split flows & disrupting energy is something modelling always struggles to handle so what initially looks like a flat, zonal-esq pattern often corrects as the lead time shortens. 

No doubt that less cold air will push in as blocking wanes, but that doesn’t necessarily mean a mild zonal onslaught is on the way. 

Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5008741

[Model Output Discussion - Colder but how cold and for how long?]

48 minutes ago, Tamara said:

The extended period continues to look very problematic in respect of NWP to me.

The MJO passing through the Indian Ocean has bumped up easterly inertia/trade winds and creating problems in the modelling with Pacific ridging and with the models gunning for booting up the polar jet downstream as a consequence. This at the same time providing insight into the modelling of the lobes of the polar vortex and the spectre of them fusing together.

However, westerly wind inertia remains above average across the extra tropics and this presents a question mark about the Pacific amplification and downstream polar jet being over modelled. But most especially based on the passage of tropical convection quickly set to navigate through Phases 4 & 5 imminently during the period in question when the northern arm is being programmed to flatten the pattern. As the MJO hits the borders of Phase 5 the next starting pistol is renewed to ladder up angular momentum tendency. Putting all this together,  a very squeezed timeframe for this sequence of polar jet modelling is only available in my book to complete, let alone sustain itself if it does.

The suspicion I continue to have is that these poleward eddies that the models propose to increase polar jet momentum, will get sheared off and this whilst this still leaves a flat jet stream coming from upstream, the shearing off process will cut off the supply to the polar jet in mid stream and force it to backtrack into the sub tropical jet. Much the theme of discussion of late. 

That process, leaving some of the residual ridging between Greenland & Iceland intact and/or re-programming re-emergence of wedge type ridging and subsequent disruption of Canadian lobe energy in mid Atlantic. This leaves southerly jet kayaks to resume progress and changes the extended ensemble means that propose a flat westerly pattern across NW Europe & inflating Azores High.

It will be very interesting (as I see it) to watch and monitor if this does actually transpire.

Sounds like what you’re expecting is something similar to the 00z GEM, i.e a more active jet into the Atlantic meeting residual wedges of high pressure to the north causing the jet & subsequent low pressure systems to disrupt SEwards. 

What you advertise is appearing within model outputs albeit as a lower probability outcome currently within the modelling. 

I’m not sure models will be handling this particularly well at extended ranges, resolving split flows & disrupting energy is something modelling always struggles to handle so what initially looks like a flat, zonal-esq pattern often corrects as the lead time shortens. 

No doubt that less cold air will push in as blocking wanes, but that doesn’t necessarily mean a mild zonal onslaught is on the way. 

[Model Output Discussion - Colder but how cold and for how long?]

4 minutes ago, mountain shadow said:

There is no blocking at days 9/10 though.

Pretty much all input I've seen shows the cold being swept away next weekend. 

It will take a marked change in output to change that now.

There was no blocking at day 5 either on a few recent GFS runs.. that's sortof my point. We've just seen a huge shift in the broadscale pattern from the 18z, there's no saying that wont happen again as we approach the current day 10. Wedges are extremely difficult for models to resolve and get a handle on. 

[Model Output Discussion - Colder but how cold and for how long?]

If the models aren't getting blocking right at days 3/4, why do people expect them to be right at days 9/10? 

[Model Output Discussion - Colder but how cold and for how long?]

Just now, Allseasons-Si said:

Because the cold pool to our north is intensifying on each run= deflecting them further south

i said the other night that this trough to our N/NE is the main player here

I meant I can't work out why the Met would have the system tracking south as the lowest probability.

[Model Output Discussion - Colder but how cold and for how long?]

The GEFS mean is also an improvement. Not quite as large a shift as the det but certainly a shift.

[Model Output Discussion - Colder but how cold and for how long?]

1 minute ago, Richard Fisher said:

??? The low will head back north in the next few days. The pros at the Met said there was only a low probability of the low staying in France.

Unfortunately since then it's continued to track southwards across all modelling & ensemble suites. the Met said it was a lower probability (still cannot work out why, these systems generally do trend south) but todays model trends have placed that in the "highest probability" risk. 

[Model Output Discussion - Colder but how cold and for how long?]

This is a substantial shift in modelling from the 18z so I'd very much like to wait until the ensembles before judging but this extends the blocking far further than the last few runs have managed to. 

In terms of snow detail, honestly couldn't care less at this stage, that will chop, change & new features will pop up seemingly from no where. The low in the middle of next week was always expected to go into France and miss the UK, been saying this for days now.

[Model Output Discussion - Colder but how cold and for how long?]

Erm..

Okay GFS 18z.. wow? 

[Model Output Discussion - Colder but how cold and for how long?]

The GFS det whilst not technically an outlier is completely out of wack with the vast majority of the GEFS.. as per usual really.

Many of the ensembles opt for a more southerly tracking low next week & keeps the colder air on place across the south.

[Model Output Discussion - Colder but how cold and for how long?]

UKMO very very good and probably the hallmark for what we want to look for. GFS is an improvement but looks like it's turning rather messy now. 

Colder 850hPa's continue to shift southwards.

[Met Office and BBC Weekly/Monthly Outlooks]

7 minutes ago, Derecho said:

Strangely that forecast has now vanished....

Not for me it hasn’t.. 

[Model Output Discussion - Colder but how cold and for how long?]

12 minutes ago, Tamara said:

References, as ever, crop up to x+y= responses to MJO phases in isolation. Again, to keep reiterating, tropical AND extra tropical wind-flow has to be assessed in respect of synoptic responses. There is no linear single interaction/response between the tropics and extra tropics without looking at directional flux of propagation of wind-flow as well, at this time of year, considering the interactions between the troposphere as a whole and the polar field.

Eastward moving tropical convection waves (MJO) at high amplitude and set against very high +ve inertia within the aggregate global wind-flow budget is going to retain greater amplification tendency than a situation where angular momentum and angular momentum tendency are both suppressed. In this sense, and taking the present scenario where the MJO is moving through the so called "La Nina response phases" of the Maritime continent, it is not as simple as equating this to increased polar jet+ Atlantic westerly forcing = mobile +NAO pattern and breakdown of blocking patterns.

The more extended period is fraught with so many pitfalls for NWP that I would genuinely stop trying to second guess the outlook for the last third of January. It makes sense that the Canadian lobe of the polar vortex will send energy towards the Atlantic but based on both the modelling within the polar field, which is highly unstable, and so much latent +AAM inertia within the aggregate wind-flow budget and then in addition to all that  - another very sizeable push of inertia and momentum between the tropics and extra tropics during the period under consideration;  there are so many hurdles for NWP to handle that extended ensemble data is next to rudderless at this time.

This assessment is intended to be neutral in respect of the overall diagnostic - but it happens to be one that continues to keep cold blocked patterns in the ascendancy for N/NW Europe and tendency for the white water rafting within the sub tropical jet stream to be adjusted upwards within modelling suites day on dy. That doesn't inextricably mean another 1947 redux, but probabilities do suggest that attempted Atlantic breakdowns will be attritional and prone to be shorter lived, if they occur, rather than any wholesale mobile pattern change.

Increased westerly mobility has higher probability for more southern parts of Europe - overall.

Mentioning 1947 is going to cause a riot.. 

[Model Output Discussion - Colder but how cold and for how long?]

The 20th-25th January still looks like the broader period for less cold weather to begin moving in across the UK, albeit with a lot of uncertainty. This in itself could bring significant snowfall risks but before that, cold next week with the risk of snow for some.

Ensemble spread for the above mentioned period is large with some members keeping the cold until later in that period, it may take a few attempts but there's certainly a signal there now for less cold air to push into the UK as blocking to the NW begins to relax. 

[Model Output Discussion - Colder but how cold and for how long?]

I'm not expecting a change the Met extended today but possibly tomorrow. The extended forecast is written from a longer, more detailed piece of internal guidance that is typically written on Monday & Thursday nights. If there's no change today it's tomorrows update that could be an important one & begin to suggest a change in the outlook. 

Currently though, things are progressing largely as expected. Anything beyond next week remains open to doubt & significant changes within outputs, only takes a small wedge to completely change expectations across the UK. 

[Model Output Discussion - Colder but how cold and for how long?]

1 minute ago, jmp223 said:

No offence but that looks like a three year old just took a crayon to the map.

 

 

 

Ha, yes I appreciate it's not the cleanest of charts to read. Essentially each red line shows where an ECM ensemble member has the warm front.

[Model Output Discussion - Colder but how cold and for how long?]

Just now, That ECM said:

how far south has the gold line moved in 24/48 hours please?

Here's yesterdays 12z run and todays 00z run.

[Model Output Discussion - Colder but how cold and for how long?]

The low goes south, the south stays cold but we miss out on a potentially significant snowfall event. The 00z spaghetti frontal plot shows a substantial shift south on the last two runs but with still a large amount of spread across France & the English channel, the exact track of this low still hasn't been resolved within the modelling.

Going forward I suspect models are being too progressive with removing the cold. Wedges will be key for cold remaining in situ across the UK and there's certainly some support within the ensembles for cold weather to hang on, more so across northern counties. 

A very difficult and volatile forecasting period coming up. Don't be surprised to see modelling little by little extending the cold..