Model discussion highlights

[Nick F]

20 minutes ago, joggs said:

As some have said. The arctic high may not be modelled correctly. Last couple of days it was driving to the Asian side now stays fairly central and a slight nosing towards us.........

I'm not getting carried away until we see alot more signs swing that way.

Also the experts say it's a rare set up so I suppose anything could happen.

Indeed, the U.S.A.'s NOAA  Weather Prediction Center extended forecast discussion yesterday mentioned uncertainty with pattern upstream over the USA due to how quickly upper high closes off northwest of Alaska & the historical tendency for low predictability to arise from cases when there is an influence from a closed upper high at mid-high latitudes.

So the development and movement of the arctic high formed from the closing off of the ridging north between Alaska and Siberia is likely causing some model wobbles and this greatly influences patterns in the mid-latitudes through displacement of the tropospheric Polar Vortices. One TPV looks to be displaced south across N America in the coming 7-10 days, though EC det has been flip-flopping between runs where to place the TPV trough. Yesterday's 12z had the trough over western US, today it has it over mid-to-eastern US. An this greatly affects the 500mb flow pattern downstream. There is also influences on how much ridging moves across Greenland and thus the 500mb patterns over the N Atlantic. Probably why we're seeing these swings in the models atm.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768235

[SqueakheartLW]

GFS 12z cold prediction for next week

If anyone thought the GFS Op run on the 12z was cold it has good company with several other members that are equally as cold or even colder at the 2m temp level.

P05, P11, P14 and P23 are the best ones in the bunch, especially P11 which turns very cold once again after the mild blip.

There were several more cold 2m temp runs in the collection but not as consistently cold as the above list of ensemble members.

I would say there's a quite strong signal based on 2m temps that things will be turning colder than average again from around midweek onwards. Could it be possible that a white Christmas is on the way.

How will what happens after the mild blip affect the final annual CET mean for 2022?

Recently we have been trending way below the 1991-2020 December mean CET and up to 13th December we are provisionally at 1.408C (-3.702C below the 1991-2020 December mean). This in itself is impressive considering the year we've had and despite this cold start it is still not cold enough to stop us beating the 2014 CET Annual mean record on either the legacy value 10.948C or the new 10.980C value either.

In order to avoid setting a new record we need to get a December mean of 1.052C with no other corrections upwards in any of the other daily means from 2022 to be absolutely certain to avoid setting a new record based on the lower legacy value and a December of 1.052C would see us set an annual mean of 10.947C which is 0.001C short of 2014 (legacy)

Current December 2022 rolling mean vs requirements for 2014 annual mean (legacy + new), an 11C CET year and 1991-2020 mean

Below is the current data I have for December 2022 vs the above categories.

Things are looking promising currently based on provisional CET data if you are looking to avoid setting an annual CET mean record as for up to the 13th we are currently on track at this stage in December to just avoid setting any new records. However if the model mean 2m temp mins and maxes predictions which are the average between the 00z GFS, 00z GEM and 00z ECM turn out to be correct we are about to shoot back above the requirements and put the course to beat 2014 very much back on track again and even an 11C CET mean year is looking very much back on again unless we can trend much colder again after early next week.

Using the mildest GFS 00z ensemble member P17 by 2m temp mins and maxes

If the mildest GFS 00z ensemble member P17 turns out to be correct. This one averages out at a mean of 6.353C for the entire run based on 2m temp mins and maxes for each day between 14th December and 29th December 2022.

As you can see here if we go mild as predicted around Sunday into Monday time then we generally stay mild or very mild out to the 29th then we are very much dead on course to smash the 2014 mean CET records and the 11C CET one quite easily. We could even undo the cold start to this month and end up very much around the average for December based on 1991-2020 December mean so there's still no guarantee at this stage that December 2022 will end below average but there are currently quite high odds on a significantly below average December 2022.

Using the coldest GFS 00z ensemble member P15 by 2m temp mins and maxes

If the coldest GFS 00z ensemble member P15 turns out to be correct. This one averages out at a mean of 0.559C for the entire run based on 2m temp mins and maxes for each day between 14th December and 29th December 2022.

Using this coldest GFS 00z ensemble member 15 we can see that we stay very much in the hunt to avoid setting a new annual CET mean and if things did head this way then it could go right down to the wire and could all end up hinging on what the 31st December daily mean CET value ended up as.

However P11 on GFS 12z could turn out to be correct and that one looks like an even colder ensemble member than that P15 member from the 00z and that one would make sure we don't set an annual CET mean record and would give us a shot at a possible 0C December or even possibly a slim chance of a sub zero CET December.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768360

[Singularity]

Not much of a signal from the tropics for the models to work with which doesn’t help matters. A somewhat muddled MJO influence from the Maritime Continent or thereabouts, not driving any big changes in AAM unless a positive mountain torque occurs over East Asia which seems pretty uncertain.

Any west-based negative NAO setups are pretty much happenstance arising due to the movement of the powerful Arctic High that others have flagged up.

Same goes for central or east based ones, pretty much a raffle at this stage!

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768288

[chionomaniac]

Took a break from this afternoons outputs - glad I have returned to a more positive outlook. 
Keeping an eye on that MJO, and the hints again of long term Scandi rises. MJO forecasts are nudging away from IO activity and now are focussed into activity in the western Pacific, and post day 10 Scandi height rises result. This is crucial imo to the next phase of winter. The trop PV is still massively disrupted, the Strat PV is trying to gets its act together, but no matter how hard it strengthens if the stoke is large enough into the Strat PV bike wheel, then it can be brought down. Feb 2009 is a classic example of this. 
 

So medium and long term hope, but definitely not guaranteed. 
 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768347

[bluearmy]

Clusters have two largest ones with the Arctic flow around the trough to our northeast 

infact the second one (not the one containing op and control) has the flow in faster 

Looks like quite a few runners across these clusters 

expect a flip in the opposite direction next run! 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768256

[Eagle Eye]

Well, our extended period of +VE AAM may be dropping off but I expect the +VE mountain torque to rebound us back to stronger +VE AAM values once again and so further extended blocking should be the more likely solution as long as it's got MJO backing. Beyond the first Euro/Scandi blocking as that's not really AAM backed (although a slight rebound backing lag) but more to do with the downstream effects of the EAJR that I've noted before. Hence why it isn't forecasted to be that amplified on the models compared to previous blocking as it's not a sustained EAJR signal.

Fairly substantial torque gain over the next week or so should lead to the rebound of AAM and increasing GWO strength as we move towards the circumglobal trough's with the tall gradient blocking. Possible re-enforcement of Greenland as long as the signal remains fairly sustained and with the Atlantic struggling, I would suggest that there may be another deepening of the -VE AO state as we go into late December/January with the MJO finally back towards helping us with phases 5/6/7 as a rough guide.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769090

[Eagle Eye]

If we look at the zonal wind in the tropics at the upper Troposphere level, there's a clear indication of a -VE anomaly extended over East Asia to the Pacific (120E-180) from the jet retraction that's recovering essentially now. The initial zonal wind packet causing an extension of the energy scattering across the world and that's shown in the closely linked erratic zonal wind anomaly behaviour and the retraction of the 60W-0 (Atlantic) zonal wind behaviour into a forecasted -VE effect. Eventually that should edge out across to the Eurasian area and hence the timing is important. The reason it's important is because with a relatively high gradient and despite the waning Eurasian -VE EAMT feedback, I think it should have enough to force some sort of Scandinavian blocking and so the models showing that at the moment should have the best handle on this.

Of course, the amplification is unlikely to be very strong but that doesn't mean that cold can't fall on the other side of it so a short cold snap (especially away from the south) may be the most likely solution here. How strong the amplification gets will be difficult to tell just from this but I think that as long as a high thermal gradient can be maintained then we should see it advect fairly far North so it may end up resulting in more than just a cold snap. This is shown in the quick -VE AO dip timing itself with this that's been talked about but dynamically how it'll evolve is very difficult to tell with the differences between model runs do I'll leave it up to you more knowledgeable about each model than me.

This isn't a sure-fire guarantee of cold but it really helps the cold prospects with a stalled westerly jet and despite the westerly QBO phase moving fairly slowly through Mike Ventrice's noted phase 5, the effects seem to be fairly low here when there's erratic behaviour. The erratic behaviour of the zonal wind anomalies because of the sustained +VE AAM and then the EAJR really seems out of place given the La Nina oceanic base state but the fairly sustained Nino-esque pattern is showing good signs that it may be a reccuring theme (as half mentioned earlier) until at least early January.

This Winter just seems to be going against the normal for the ENSO state we're in and we've already seen that so early on in Winter. It's very interesting to me how the packets of air are behaving quite unlike you would expect and.it shows how helpful an oceanic-atmospheric 'disconnect' can potentially be given +VE torque anomalies.

Don't tell anyone but I've spent the last 40 minutes of my lesson writing this out on my phone whilst not really thinking a lot so this has been quite a fun lesson but I don't know whether what I've written here makes sense because often things make sense only to me because I wrote it. So yeah, don't tell me teachers I'm doing something a lot more interesting than the subject is to me at least.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769207

[bluearmy]

A word of caution re the extended ens output 

the upper strat vortex is really ramping up in week 2. I’m not sure how well the gefs (and geps) will cope with this in the context of the current disconnect that exists with the trop.  The ens model has less levels in the strat and may not deal with this scenario too well. The eps are better resolution high up but even they may struggle.  They usually do ahead of  a reversal. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769186

[Mike Poole]

Lots of comments on how well one particular model is or isn’t doing.  But the reality is that they are all rolling out each operational run from within a very large envelope of uncertainty, and will all therefore blow hot and cold (pretty literally for the UK!) from run to run until the uncertainty gets resolved.  So the fact it was the GEM that looked promising this evening is irrelevant.  GFS only seems more erratic because it runs 4 times a day!

Why the uncertainty?  Well, it isn’t because it is a rare setup per se, because the models have no knowledge of history at all, they just get our best knowledge of the T0 conditions, and simulate the laws of physics (or their approximations to them) onwards from there, whether it be 10 days in the case of ECM, or 9 months in the case of CFS.  But there must be something about this setup, my guess it is part lack of good data over the arctic region where important stuff is happening, and part due to a really convoluted set up with the jet stream (compared to zonal) which is difficult for the models to resolve.  

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769479

[Nick F]

12z EPS day 10 h500 mean looks pretty amplified with the upper flow, over the N America and N Atlantic by Xmas Day.

If that -ve height / trough anomaly west of Iberia cuts-off eventually, as would be expected with a highly amped flow as indicated above, it could lead to pressure rises over the top close our N and NW, pretty much what the week 2 progression to week 3 suggests on the updated EC weeklies:

So even though we may endure a mild week in the south next week, could turn cold for all Xmas or Boxing Day onwards ...

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769795

[Nick F]

13 minutes ago, Benny123 said:

That’s a big temperature contrast upstream..

They are getting excited over on the eastern US weather forums, while we get a little downbeat over our prospects next week, as that cold there comes with a possible big east coast Nor'easter end of next week, which will probably amp ridging downstream over the far N Atlantic which may benefit us from Xmas Day onwards.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769804

[Singularity]

This weekend, just a couple of days from now, a little low by Iceland will interact with the trough from the Atlantic and either:

- Be absorbed into the circulation of the bigger low as that races past to its southeast, the whole lot then heading to Scandinavia by Wed 

or

- Avoid being absorbed and then develop into an intense low that holds up the trough across the UK for a day or two while maturing to the N or NW of the UK

That 2nd scenario has suddenly become the majority vote of the deterministic runs this evening. It leaves less room for a ridge to our west, allowing the trough to the west of that to cut across and send over a low or two to  visit our shores.

At such short lead time this would usually be final, but that little low looks very borderline to me, its future highly sensitive to fine details. Hence we still see a fair few colder outcomes in the ensemble sets even as the mean has shifted milder. 

So who knows, things might look markedly different yet again tomorrow. Or they might not, in which case I’ll frankly enjoy the reduced heating bills for the 4-5 days or however long the milder interlude lasts (I’m really not convinced by runs that keep it going 7+ days).

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769844

[MattH]

Morning...

A troublesome time to be a forecaster, especially for next week, but just quoting the following from the post a few days ahead and, overall, this remains true, currently and looking ahead...

"The 'problem' that NWP will have looking ahead is that we continue to be within a hemispheric regime that is not 'normal'. I used the words 'usual' and 'unusual' the other day in a post, the usual setup clearly being a robust W'ly regime, with connected strat and trop, the unusual setup being what we have had now and, at the moment, will continue to have looking ahead."

Even I, 24 hours or so back, thought we were on to something when both the GEFS and the EPS, for example, generally flatlined on the colder N'ly flow for next week, but, as we have seen many times over the years whether deterministic models or ensembles, they can suddenly flip when the overall evolution is as complex and problematic as it currently is. I won't bark on about AAM this time as, overall, there is no real change there apart from to say that some recovery in AAM is still expected, linked to the movement of the MJO, so that's a positive (no pun intended!) looking ahead.

However...The problem with the run up to Christmas is as I mentioned the other day, the block of late is breaking down and we have a number of conflicting regimes trying to play out. Once again we can put the strat influence to one side, for now, so forget about that, but I am wondering whether it is the MJO, short-term that is actually creating some of the problems next week, in amongst what remains a highly disorganised tropospheric pattern. As I mentioned the other day the recent fall in AAM, if this were a robust W'ly regime with a strat and trop connection then we would be writing off any sig cold for a good while, but that isn't the case.

However, the MJO has been firmly over the Indian Ocean and while you can't take one variable at face value, some of the composite anomalies, especially the often correct regression plots do show a similar pattern to what the models are toying with next week, especially regarding the European high pressure.

We can see the -ve OLR signature over the I/O through the 10th to the 14th (MJO phases 2-3) and the regression plots do show a pattern that is dominated by the european high and low pressure to the NW. Obviously what we have to do, as a forecaster, is add the extra variables in play to adjust that 'black and white' output and suggest that the low pressure to the NW is obviously not likely to be that significant due to the on-going amplification to the troposphere etc. 

 

As many have said on here in recent days it is the European high that is one of the key problems as this essentially stops the pattern from progressing eastwards and clearly stops the low pressure next week from trending eastwards. The 00Z GEM, of course, continues to highlight how if that does happen we end up with a far quicker and more significant way to cold weather once again. The evolution into next week, therefore, is very uncertain indeed, simply because of the all the conflicting signals that are trying to dominate, now that the original ingredients for a prolonged block are waning and there is, IMO, still the chance that the GEM, for example could come off, but, as the UKMO shows, there remains a worst case scenario as well with a SW'ly flow if the european heights dominate.

If we look at this from a model perspective and just taking N England from the 00Z EPS as an example, we can see how compared with yesterday's 12Z ECMWF it is a colder run, but the spread in the box and whiskers plots for the middle of next week is huge(!) and, again, this highlights well how uncertain the forecast is for next week.

Where we go from there remains problematic through to the end of the year, but with the MJO still set to be on the move and with the AAM set to recover once again, with no strat and trop connection then continue to expect some further 'fun and games' from NWP looking ahead. AAM, from the latest EC Monthly does fall into January and *IF* the strat and trop do connect then this could well be the first signs of the +ve NAO development that was always expected for the middle and latter half of the winter without an SSW, for example. It is unrealistic to think that the strat and trop won't connect at some point, but note the predicted recovery through the rest of December, January, meteorologically speaking, is a very long way off especially under the currecnt synoptics and situation.

Cheers, Matt.

 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770187

[nick sussex]

Although the ECM doesn’t really get there in the end I’m highlighting some early differences which either put you in the game for some interest or drag out the saga and make it a long way back for cold and snow .

The day 5 shows marked differences upstream between the UKMO and the rest . Broadly speaking the UKMO is different upstream  at the key time and because of that it can’t close off the high and downstream the jet impacts are clear . 

So day 5 UKMO

You’ll see further east pretty similar to the rest but then day 6 you’ll see the effect of that failure to close off the high .

Now the ECM day 5 .

Now day 6 on the UKMO

Day 6 on the ECM 

The upstream set up effects the jet track to the ne and we can see here the impacts .

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770134

[carinthian]

Christmas Evening  850mb temp chart from GFS. Extreme in the making. Parts of Northern Scotland forecast -11c at surface by 18z and +11c on south coast of England at same time. Fair to say based on that chart mild Southern England and S Wales. Below freezing for Scotland and NI. Potential snow zone for Northern England and N Wales ?. Well who would want to make an advanced forecast based on that output ? Crazy chart. Just have an update from the team over here. Mild for much of Euroland over the Festive period to start with then indications of a Scandinavian High to form by New Year to bring cold back further south.

C

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770429

[MattH]

Just a quick one on the long-term, as now all monthly data has been reviewed it is clear that both MSLP and 500mb height anomalies are potentially set to become more focused (in a +ve way) to the N and NE of the British Isles through late Dec and into early Jan, potentially over Scandinavia. As we all know the Scandinavia block is often very difficult to achieve, especially in a W'ly, winter regime, but obviously that isn't the case.

Some further analysis of this has lead to an interesting outcome when using some GWO composite anomalies. The key here, could well be the MJO and whether it keeps progressing eastwards into the Maritimes and the W Pacific, through phases 4-5-6-7 over the next 2 weeks. The primary caveat here is that it needs to do this, but IF it does, then, in theory this would bring about the predicted rise in AAM and the GWO. As a prediction, were the MJO to progress eastwards then another GWO rise through phases 4-5-6 would be possible, as I've suggested below.

Now, using December 500mb height composite anoms for phases 4 and 5 of the GWO, we get the following...

Then if we take a look at the 500mb height anoms from the overnight EC46 through the 26th Dec to 2nd Jan time period and that is a pretty darn close match...

Obviously, as usual, this is all subjective and potentially all hinging on the movement and track of the MJO looking ahead, but it is always good to try and find some science behind the models and to why they suggest one thing and not another. Clearly, a Scandinavian high doesn't always bring cold, but that kind of pattern through late Dec and into Jan would certainly increase the risk!

Keep a close eye on the MJO looking ahead, along with EPS, GEFS 500mb height anoms through the 10-15 day period for something similar.

Cheers, Matt.

 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770531

[Singularity]

2 hours ago, carinthian said:

Just have an update from the team over here. Mild for much of Euroland over the Festive period to start with then indications of a Scandinavian High to form by New Year to bring cold back further south.

C

You've essentially described the typical response to the MJO traversing phases 5-7 in Dec.

That's what EPS currently indicate when allowing for some interference from the Indian Ocean subduing the mean:

On which basis, a mild festive period then colder by New Year via Scandinavian high pressure development is highly plausible.

The UK will be in the uncertain zone on the NW periphery of where anomalously high heights are encouraged over Europe by MJO phase 5. Usually it turns out mild for most of the nation but this time there will be an exceptionally strong Arctic high complicating matters & keeping cold chances a bit higher for northern parts especially.

The fine balance of the situation is well demonstrated when considering the 00z GEM versus 00z ECM, UKM, & GFS. Unlike those three, it didn't develop the low merging into the northern flank of the trough this weekend into the dominant feature by Monday. Just by leaving a bit more room for high pressure to the W & NW of the UK next Tue-Wed, the path of the next approaching Atlantic low was diverted fully south of the UK.

It's of course unlikely that GEM will trump the other three at such short lead time but on the other hand, those three are showing an extreme scenario with how that low behaves (it becomes impressively deep for such a compact system), so I don't think it's as unlikely as would normally be the case.

Even so, here in the far south of England, I'm bracing for a notably wet week to come (potentially near a whole Dec's worth of rainfall!) with temps mild then either still mild or near average through to at least Friday, which is more or less the current ensemble consensus.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770527

[chionomaniac]

1 hour ago, MattH said:

Just a quick one on the long-term, as now all monthly data has been reviewed it is clear that both MSLP and 500mb height anomalies are potentially set to become more focused (in a +ve way) to the N and NE of the British Isles through late Dec and into early Jan, potentially over Scandinavia. As we all know the Scandinavia block is often very difficult to achieve, especially in a W'ly, winter regime, but obviously that isn't the case.

Some further analysis of this has lead to an interesting outcome when using some GWO composite anomalies. The key here, could well be the MJO and whether it keeps progressing eastwards into the Maritimes and the W Pacific, through phases 4-5-6-7 over the next 2 weeks. The primary caveat here is that it needs to do this, but IF it does, then, in theory this would bring about the predicted rise in AAM and the GWO. As a prediction, were the MJO to progress eastwards then another GWO rise through phases 4-5-6 would be possible, as I've suggested below.

Now, using December 500mb height composite anoms for phases 4 and 5 of the GWO, we get the following...

Then if we take a look at the 500mb height anoms from the overnight EC46 through the 26th Dec to 2nd Jan time period and that is a pretty darn close match...

Obviously, as usual, this is all subjective and potentially all hinging on the movement and track of the MJO looking ahead, but it is always good to try and find some science behind the models and to why they suggest one thing and not another. Clearly, a Scandinavian high doesn't always bring cold, but that kind of pattern through late Dec and into Jan would certainly increase the risk!

Keep a close eye on the MJO looking ahead, along with EPS, GEFS 500mb height anoms through the 10-15 day period for something similar.

Cheers, Matt.

 

100% Matt. We have been teased over the last week or so with suggestions that the western Pacific will become active, but in reality all we see is a picture of confusion from the IO to the western PAC. The height anomalies to our NE been showing too for post Christmas - but are these aligned to the MJO uncertain- probably. If we can get a Scandi high then this would be promising both in a direct way and also indirect through Strat feedback disturbances. Winter is very much at a T junction. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770604

[Uncertainty]

Both the GEFS and GEPS have really flipped their projections over  between runs, day 11 below but the big change is much earlier with a big change in the amplification off the eastern seaboard and corresponding height rises over Greenland.

GEFS 6z

GEFS12z

Perhaps a slight southerly mean flow there. More hp influence and less westerly.

GEPS 0z had Weak heights over Greenland and a weak southwesterly for us.

GEPS 12z

Note the cut off high over Greenland. A bit of a mess over us but that’s because the forecast for the behaviour of the weakening  Atlantic  low to the sw of us is just as, if not more uncertain than the Greenland regime to the north! 

I think this shift is notable and unusually large. The UKMO, whilst not ideal within the timeframe of the run, has really significant amplification taking place over Greenland and I’d have loved to see it’s day 8-10 charts. 
 

It’s also far more palatable than the hideous runs it has churned out of late, 0z today a case in point.

I wouldn’t be surprised if the ec op churned out a really amplified run, given the evidence from the other two ens sets tonight. 
 

The one thing missing for coldies is the euro heights remain stubborn even on the above. It’s been said enough about shifting those but the profile to the N has improved considerably tonight so far.

Here’s how the GEPS mean ends, plenty to consider then tonight as we wait for the eps to add their two penneth.

 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770768

[damianslaw]

Are the models coming to a consensus how the run in to christmas may play out, possibly... let's see what ECM shows.

Looking at the GFS and UKMO this afternoon, both show the longwave atlantic trough positioning itself over and to the west of the UK, pulling in a mild flow, very mild initially, with heights building to the SW, these factors alone suggest no easy route to cold, however, importantly we have strong heights to the NW, and an amplified flow over the atlantic, the jet is sluggish and more southerly positioned as well - there is a signal for a battleline set up around the 23rd and into christmas Eve with a secondary low moving in from the SW, because it is on a more southerly track it has the affect of kicking Iberian heights west and this allows it to phase somhat but also possibly disrupt and slide - and push eastwards as we approach christmas. A northerly would then ensure. All very complicated, and we are talking a week away.

In the meantime, a cold Saturday, becoming mild/very mild through Sunday with risk of snow in the north before rain - the first decent amount for many in 3 weeks, will feel and seem odd. Early-mid next week stays relatively mild and unsettled, with limited frost risk, also more wind, again something we haven't experienced for weeks. 

The period from the 23rd continues to look more interesting and possibly wintry especially in the north. Watch this space.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770725

[Nick F]

A look at the 500mb  and 250mb patterns today and you can see that blocking over Alaska and NE Canada / Labrador with the polar front jet displaced at quite a southerly latitude across the Pacific, N America and N Atlantic as a result.

However, roll on into early next week and we can see the blocks have lifted poleward over Baffin / Greenland and also over Alaska side while the jet stream has correspondingly lifted north over N America and the N Atlantic. This allows Tm air to move in across much of the UK next week, though Scotland will flirt with colder air just to the north. 

Could be quite a wet period next week, particularly western hills and coasts and also southern counties of England. Been dry so far this month, aside occasional localised snowfalls, so perhaps be good to top up water storage levels which are still below average in parts of the UK, despite the wet autumn. Models indicating an atmospheric river taking aim at UK which can be traced back to the Caribbean transporting moist Tm air and feeding the rain machine:

Despite the zonal nature of the jet through much of next week, signs are that the upper flow amplifies upstream, as Alaskan turned arctic high and Greenland ridge combine to force a TPV south across the USA around Christmas time, this in turn amplifies a trough downstream over N Atlantic to the west of Europe - which eventually looks to cut off. 

>>>>

So could get interesting after Christmas, with high pressure building over or close to the north of UK, as that low cuts off towards SW Europe / Morocco. We are seeing this manifest on 18z GFS, with high pressure building to our NW initially before building east to the north of the UK, though the 18z doesn't drop that low south enough to get the cold south across all but Scotland. Need to flatten that damn Euro ridge!!!

Into early January and we may have the MJO on our side again. For now RMM plots have MJO in COD so one would assume having little impact on the patterns, but it is still an area of enhanced convection at the mo over the Maritime Continent and is expected to drift towards the Western Pacific later this month looking at the VP200 maps below. Indeed today's EC and yesterday's weekly / extended MJO points to moving into phase 7.

This could promote blocking to continue or re-emerge to our N and NW in early-mid January perhaps. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4771195

[Mike Poole]

UKMO the pick this morning, with a route to cold, and a good cross polar blocking pattern as early as T168.  

On the models generally in the short term, I wish we could get rid of the mess of lows slowly drifting our way in the Atlantic that are messing up an otherwise decent hemispheric pattern, and, of course, pumping up the euro high.  The two go together, the angle of the WAA from this train of lows directed to just north of east is really unhelpful.  

Noting it is 17th December, we are approaching the crunch time for this winter in my view.  If you read my comments in the early winter forecast thread, I was expecting both from the seasonal models, and from expected La Niña winter, that in the absence of a SSW, a zonal +NAO pattern would take hold from mid-winter as the strat and trop vortexes couple.  I don’t think that is imminent, and won’t happen with a dirty great arctic high floating about, but the clock is ticking now - we have about a month, I would suggest.  

So, in the short term, we can hope that one of the more decent evolutions (and there are some) shown by the models verifies to give some snow potential, even if it is marginal for some (by some I mean the south ), and in the medium term, we can hope for a renewed burst of amplification due to a AAM surge as the MJO makes its way into the favourable phases.  

We do need to keep our eye on setups that will disturb the strat vortex, so I can see why there has been much talk of a scandi high in here, even though it has only fleetingly appeared in the outer reaches of the modelling.  I still think a SSW is odds against, but there is a chance if blocking can set up more favourably in the next month.  On a positive, if the blocks set up favourably, the cold should be readily available to tap into.  

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4771375

[Uncertainty]

Clusters day 8-10

One of those where the algorithm assigning +NAO to everything doesn’t tell the whole story. All about that low and the manner in which it is squeezed between the euro blob and the Atlantic ridge.

Clusters beyond day 10: +NAO options fade away, lots of blocking and mid Atlantic ridges. None of them scream deep cold and snow but the lack of zonality is notable.

The GEFS, predictably, lose the stronger blocking signal earlier on from the 12z but do end on an interesting note

After the low clears, The GEPS remain really keen on an MAR

There’s also some developing ural ridging - would love to see a link up to that and indeed for the Ural / Scandi blocking  to become the focus for early January. 
Those new to this game need to know this: If we don’t get a block developing over this region in the next few weeks  the stratospheric vortex aloft will, most likely, overpower any meaningful attempt at blocking. This also makes future attempts at Scandi blocking harder and thus a +NAO late winter period becomes harder to shift. 
 

Personally, given the pacific configuration, I remain fairly convinced we will see an SSW this winter. But if this mjo cycle fails to deliver the goods it could well be into mid February that this manifests thus we would be looking for a March 13 or Feb 18 bookend to winter.

All in all, and with Christmas snow still plausible for many in the N, a fascinating outlook and potentially a defining one for the months ahead, as is often the case at this crucial time of year.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4771465

[Mucka]

Synopsis for UK Christmas period weather.

I am using GFS 06z 168h through 192h as an example as it most clearly demonstrates how low pressure is likely to force the pattern over Christmas for the UK although obviously this is not set in stone. 

After a milder and wet spell there will be return to colder conditions from the N, how far S this colder air gets is not yet known but it seems there will be N/S split somewhere over the UK

The first image numbers the main drivers of the pattern that will affect the UK over the Christmas period for the casual model watcher and attempts to explain what to look out for and what the consequences for UK weather will be.

The 2nd and 3rd show a 24h progression of the pattern to visualise the explanations. (Click on image to enlarge)

1.

The Arctic trough that will dominate our Weather patter leading into Christmas, the further S we can get this then the further S we should drag cold Arctic air and the more amplified the Atlantic pattern should become sharpening up any cold feed.

2.

Low Pressure systems moving W/E will merge with the Arctic trough somewhere over or just W of the UK bringing spells of wet weather from the S

The timing and tracks of these systems will dictate how much cold air the upper trough feeds over the UK, the less they develop and further S they track the further S cold air will likely get.

Where the boundary layer between mild Atlantic and cold arctic air masses meet will likely give snow even to low levels. At present this looks like being Scotland and perhaps N England though adjustments S or N are likely.

3.

Atlantic low and ridging toward Greenland.

So far it looks like a vigorous LP system will attempt to sweep in between Xmas and New Year and depending on angular momentum WAA and a ridge will be thrown up ahead of it.

The stronger and more N that ridge gets the longer any cold air over the UK will remain. 

Currently it looks like any ridge will quickly be toppled and flatter more mobile pattern exist post Xmas but overall the output has been strengthening this feature.

If there were large enough changes in our favour then we could get the ridge to topple toward Scandi and get some undercut with a potential cold pattern later emerging.

If it were strong enough to resist and not be toppled then we would see the Atlantic undercut and continuation of any cold pattern post Christmas though this currently seems over optimstic.

4.

This possibly a developing feature through boxing day and into Christmas.

A secondary low splits off from the main low heading E possibly splitting energy SW/ E.

If this does become a feature then it can work for us or against us.

If it splits far enough W then it may head SE and become a cut off low and that will allow any northerly feed to continue longer as the rest of the energy pushes through into Scandi

It could also have consequences for the upsteam pattern depending on its behaviour or it could return NW and where and how it phased with the trough could have consequences for the UK weather, maybe widespread snow to rain event but total speculation at this stage and the feature may not even come about at all..

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4771722

[Cambrian]

It’s difficult not to be very cheered by the trend across the ensembles as we look forward to the Christmas weekend, getting within range now for meaningfully assessing the potential for further wintry weather.

Looking at the 6z GFS ensemble mean at day 8: 

 

Critically, those aren’t Iberian heights any more. They’ve been shunted east to the central Mediterranean and that’s allowed the long wave trough to extend all the way south to the Azores. Lower heights and cold air pushing south over the UK and Ireland. Indeed, there’s a lot of cold air stacked up to our north, eager to spill south. Notice the mean T850 as low as -8 degrees over the north of Scotland, all of us below zero. The point is these are means, so the -4 degree isotherm running across the middle of the UK and Ireland is a good signal for cold air to be reaching all parts at times. It certainly doesn’t look mild. 

Low pressure to our southwest, with a shortwave trough extending northeast towards southwest England. We can draw the front on that imaginary fax chart in our heads with ease! A southern battleground event a reasonable possibility from the feel of this chart. And looking upstream to the west - as the pattern moves west-east, there is plenty of cold air yet to feed in from this broad trough. 

The GFS is not on its own either - here are the 0z ECM ensemble mean charts for Christmas night, which are very similar. 

 

0z GEM for Christmas Day? I rest my case!

 

I see little reason for despondency at this stage. Far from it. Though in the event we are not going to get exactly what these mean charts are showing, with them showing this at day 8, and in such good consensus, we are likely to get something very much like it. And if that’s the case, this is clearly a very good setup to be viewing in the run-up to Christmas.

The combination of an unstable northerly flow, with fronts moving south embedded in cold air, and low pressure to our southwest feeding up moisture into that cold air, means that we’re all in the game for some seasonal weather over the festive period. 

To be honest, I’m seriously trying to suppress my glee  because there are few things more special in our weather than the prospect of snow over Christmas and right now at least it looks like we’re all in with a fair chance this year. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4771840