Model output discussion - mid-winter

[andymusic]

1 minute ago, northwestsnow said:

Precip charts show the whole of the NW of England gets plastered-

Not sure i believe GFS precip charts tho !

gonna have to wait till we get closer for a better look on euro4 etc, but the set up looks rife for a good pasting

[markw2680]

41 minutes ago, MattStoke said:

December had uppers at -8 to -10 widely. Colder than anything showing next week.

Not trying to put a downer on things. Just cannot see how it is looking like the coldest NW’ly in years when only a few weeks ago we had a colder NW’l. It’s as if December just gone has been forgotten already.

True and the one in December gave us just short of a foot of snow, I can’t see this been anywhere near that tbh. Let’s see if the upgrades continue in the 18z

[ACCOUNT DELETED]

3 minutes ago, Bring Back1962-63 said:

A FEW OF THE UNCERTAINTIES AND ISSUES IN RELATION TO MODEL OUTPUT GOING FORWARD

It is a whole week since I last posted on the model thread and I ended that one by saying that I was working on an exciting new project. Well, I can now tell you about it. Earlier this evening, I opened a new specialist thread, entitled “Learning About Teleconnection Science and Background Signals”. This post will be rather shorter than my normal meanderings as I’m pretty exhausted after launching the specialist thread and it’s my 4th post of the day - I have been working on the other 3 for several days which are all on the new thread. I have already posted this one there too but with a slightly different introduction. My introductory post there explains why I did it and what’s it all about. Here is the link to the new thread:

 https://www.netweather.tv/forum/topic/89161-learning-about-teleconnection-science-and-background-signals/

Alternatively, you can just click on the chart below:

I am very much one of the learners but I’m hoping that the specialist thread will evolve into something that is not only for learners but also to cater for the more technical aspects of the subject. It has only been running for an hour or so and it’s obviously very early days but for those interested, please take a look. I have spoken to one or two other members and I know that there will be a few posts going on there during the next few days, so the message is “please keep checking in”.

Right, back to this post.  Although I am a passionate coldie and love the white fluffy stuff like the majority of members on this weather forum, I have been endeavouring to manage my expectations in recent weeks. I have been learning a little more about some of the processes that are largely responsible for what delivers the broader weather patterns to us. This is the background signals and the teleconnections. I still have an awful lot to learn but from what I now understand, I will now attempt to provide my take on what is likely to drive the model output going forward through January and into February.

Much of this winter so far has been a case of “almost but not quite”. We started with many brief northerlies almost once a week but these were always quickly replaced by less cold conditions. Several of these Arctic outbreaks were slightly more potent and many (but by no means all) parts of the country saw some significant snowfall. Even these cold spells did not last long. A typical La Nina winter is often front-loaded, starting off with frequent HLB and we did indeed, more or less, see some of that. Then this would normally give way to the seasonal strengthening of the PV and the jet stream with flatter patterns and generally much more unsettled and milder conditions but with brief colder interludes also possible through mid-winter. Well, it hasn’t really been a typical La Nina. It hasn’t been the strongest of them but it has often still had enough oomph to make an impact. The more unusual aspect was that it has been largely east Pacific based with the colder currents upwelling there but with less cold currents further west. In fact with the generally well above average SSTs over the tropics and much of the rest of the oceans (especially in the Arctic), the surface currents in the central and west Pacific have really been “less warm” than recently rather than colder than average.

This east position can produce a rather different outcome to that associated with a more normal La Nina. Much would depend on the extent of “angular momentum” (AAM) and how this impacts on the GWO (global wind oscillation) phases. It is this part where I’m still learning some of the basics from several of our teleconnection specialists. I’m confident that they will explain if, where and how I have gone wrong (that’s where the “learning area” in the specialist thread should prove its worth). From what I understand (and this is really in far too simplistic terms), we saw an uptick in AAM in late December – hence the “initial” suggestion (with all the usual caveats) that we would see a build of heights to our north-east. A positive AAM tendency with its tropical convection forcing creates waves or ripples that generate frictional torque (stretching of the atmosphere with any movement one way having to be corrected the other way to cancel out what would otherwise be a vacuum which is unsustainable). These waves then ripple out and start to generate ridges and troughs in the atmosphere. These are most prominent initially in the Pacific. Then this ridging has knock on effects downstream setting up further ridges and troughs. As this movement impacts on mountainous areas like the Rockies it creates further stretching known as “mountain torque”. It is this that can strongly impact on the jet stream, causing it to take on different trajectories and to buckle and meander. With higher AAM, this often eventually favours ridging in the central Atlantic and this can also favour greater HLB.

The GWO is an indicator of how the AAM tendency is likely to impact. I believe that phase 4 is the best position for the most favourable blocked patterns, whereas phase 2 is usually associated with little or no HLB but with rather more MLB possible (last winter was quite an extreme example of this). This can allow very modest and only temporary amplification. Shortly before Christmas, the AAM tendency nose-dived and that was why some of our teleconnection specialists started to say the easterly would barely materialise. The blocking simply would not be sustainable and would, in due course, be pushed away eastwards. The models eventually locked on to this background signal. Then, there were some better signs going forward. It seemed that the AAM would recover and the GWO would enter phase 4 down the line. Hence some rather more optimistic tones from the likes of GP. Unfortunately, the AAM has remained stubbornly low and, I believe, the latest forecasts do not indicate an early change to this. So, the GWO may well remain in phase 2 for a while. That would be back to a rather flatter pattern. It does not mean that all is lost. We would need a “bounce” in AAM tendency but we may have to wait until the east Pacific La Nina weakens a little more.

I haven’t mentioned the MJO but that too is largely influenced by the primary drivers. The MJO can assist and compliment the AAM when it’s in the right phases. There has been some discrepancies over the MJO output in recent weeks. The GFS currently show it at decent amplitude in phase 5. The problem is that a negative AAM is likely to cause the MJO to die a death again as it reaches the key phases of 7, 8 and 1. That would be into early February anyway, so there is still just about time for a sudden bounce in AAM but , as I said this may not be very soon (if at all). Then if we get an SSW (sudden stratospheric warming)  and this does propagate down to the surface, it seems that we still need the AAM to play ball with the tropospheric profile in order to assist with HLB to release the cold from the Arctic (and in the correct alignment to deliver that cold to western Europe and the UK). I have learnt about some of this in very recent weeks and I can assure all of the coldies that I do not wish to deliver any bad news. I may still be a novice regards to the AAM/GWO etc and I’m still learning but I do believe that I’m broadly correct in what I've just said.

Having got all the potentially bad news out of the way, things could actually be a lot worse. This winter has had a habit of managing quite a few colder interludes and several snowfalls. In fact every time the models have switched their mid-term output to a more zonal, unsettled and milder pattern, when we get to zero in on that pattern change it has mostly turned out to be only a brief period of milder and unsettled weather. So this is how we coldies should deal with this. Accept that the specialists will indicate to us as soon as they see any improvements to the longer term picture (and I will post on that very quickly if or when that happens). Meanwhile, most of the rest of us can focus on the short to medium term, which is what the model thread is all about – dissecting the more “reliable” period. There definitely seems to be some rather better news to report here, although there are considerable uncertainties as well. 

Firstly, I want to have a close look at the jet stream. To give an idea of the variety of outcomes, I’ll do what I did once before Christmas, have a look at the latest GEFS panels from the 12z output:

                     T+6                                                T+120                                         T+180                                          T+240                                          T+360

               

The panels only show the Atlantic and Europe view, so I start off with the current GFS Northern hemisphere view. We're just on the cold side of the meandering jet with the loop passing north to south. By T+120 (day 5), the panels are pretty consistent in showing a much stronger jet heading more or less from west to east either straight towards the UK or very slightly further south. This would confirm the generally west-north-westerly pattern at that stage with some Polar Maritime air in the mix. By T+180 (day 7/8) most of the ensemble members now show the jet taking on a much more north of west tilt with almost all of the members between west-north-west/east/south-east or a more direct north-west/south-east trajectory. Most of the members show the jet slightly further south but generally still moving through the UK. This confirms that there will be considerably more Polar Maritime air in the mix. By T+240 (day 10), the trajectory has generally changed to more of a north to south trajectory. The jet has weakened to at least some extent but far more so with a few members. It is either passing straight through us or just to the west of us. This will generally put us under an Arctic air stream and hence the northerly. We can have a sneak preview of the unreliable period of T+360 (day 15). All of a sudden we see a huge range of outcomes which really proves that we should not look beyond day 10. The majority still show some sort of rather cold to colder pattern but some have us on the warm side of the jet and there is a wide range of strengths too. Remember, by this time we would need some changes feeding through from the AAM/GWO to have a chance of any more sustainable cold (but this would be too early any). So the best we can probably hope for are a few days of cold northerlies with some good snow chances for at least some of us. Yet again, we would be making the best out of what could have been a pretty dire pattern for coldies - in fact little evidence of anything mild around the corner (other than perhaps for one or two days prior to the northerly). 

If you wish to view any of the panel charts more closely, you can do this by using the links below and then just click on any of the charts to enlarge them: 

http://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=3&ech=120

http://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=3&ech=180

http://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=3&ech=240

http://www.meteociel.fr/cartes_obs/gens_panel.php?modele=0&mode=3&ech=360

I've had a good look at the jet stream profiles on the other models and there is reasonable agreement right up to day 10. Several models show the jet pushing slightly further south.

I just want to have a very quick look at northern hemisphere temperatures. I do not have time for one of my Arctic or EurAsia temperature and snow cover analyses but I will update these next week ahead of the anticipated northerly snap. I will not do a cross-model analyses at this stage either. In my last report (on page 112 of the Dec 30th model thread) I pointed out that the current severe cold in North America would ease considerably going into next week with much of the cold at last transferring back to the Asian continent with some getting closer to Europe.

So firstly several 850 charts again from the 12z output:

850 Temperatures:                      GFS T+6                                 GFS T+120                             GFS T+240

                                                        

The GFS does continue to transfer the cold across to Asia and Russia. By day 10 there are some much lower 850s in northern Asia, Russia and Siberia and rather lower temps in Scandinavia too. The 850s over us do look rather marginal suggesting it might be mostly snow over higher ground during the northerly unless we manage another slider or transitional snow when less cold air moves in later on.

 850 Temperatures:                     ECM T+0                                 ECM T+120                            ECM T+240

                                                        

The ECM charts are not programmed to register below -28c (while GFS go down as far as -40c). If anything, ECM transfer even more of the cold to Asia but also rather more into Europe by day 10 and also slightly less marginal 850s across the UK. I'm certain that many posters will follow these developments very closely as the northerly gets into the day 5 range.  

2M Surface Temperatures:         GFS T+6                                  GFS T+120                            GFS T+240

                                                          

Much the same transfer applies to the surface cold. The GFS shows that North America takes a little longer to give up some of the deeper cold but by day 10 the USA is considerably warmer. Meanwhile Northern Asia, Siberia and much of Russia have developed their deepest cold of the winter with an extensive area of sub -28s to sub -40s. By day 10, Scandinavia is very cold and most of Europe is close to freezing and the UK is under 0c to +4c temperatures. Remember, these charts are for 1300, so very close to the maximums (everything else remaining equal). We must give credit where it is due. I pointed out a week ago that the GFS was showing this cold transition in its FI charts and now it has moved into the more reliable period. So, they seem to have at least nailed this change and quite well ahead of the other models, which are now all following suit. Let's just confirm that with several other models' charts.

2M Surface Temperatures:    GEFS mean T+240                        GEM T+240                         NAVGEM T+180

                                                         

I've only used the Meteoceil Northern Hemisphere charts and several models, like the ECM, are not shown there. The GEFS mean is interesting at T+240 as it shows similar deep cold to the GFS operational run. This is the average of all 20 ensemble members, plus the control and op runs. Very few members show less extensive cold on the Asian continent but there are some that show more of the cold extending into Europe. GEM is similar and NAVGEM (only out to T+180) is well on the way.

So, I'm delighted to close with some better news. If only someone can give that AAM a big nudge to get it spiking upwards that we might just manage some synoptic patterns to help deliver that cold to our shores. If anyone wants to discuss any aspect of this post, particularly the first part, then you'll be very welcome on the new teleconnections thread.

 

                                             

Thanks for that amazing informative post. Very interesting 

[northwestsnow]

18Z shows heavy precip here for like 36 hours, its either going to be a snow lovers dream, or an otters dream...

[captaincroc]

1 hour ago, Nick F said:

The 500mb heights are pretty low across the UK on the models by middle of next week, lowest to the north (see the purples which are below 500 dam), so probably why the air is forecast cold at all levels

With regards to whether snow may fall, there are various parameters required. 

Dew point is crucial, needs to be 0C or below

Partial thickness between 1000 and 850mb ideally needs to be below 129 gpdm as absolute upper limit

0C level useful too, don't want too high but looks fairly low next week at this range

Finally, wet bulb temperature not any higher than 2C for snow to fall.

Seem to be ticking all the above for next Wednesday morning, though GFS does have a tendency to have these parameters too low at this range in a WNW flow.

So we will have to wait nearer the time to see if the criteria are likely to be met, but ultimately looking out the window / reports will be the decider!

Thank you so much for your time and effort making this. 

[feb1991blizzard]

Just now, northwestsnow said:

18Z shows heavy precip here for like 36 hours, its either going to be a snow lovers dream, or an otters dream...

My guess is somewhere in between actually, some snow, some sleet and some rain.

[chicken soup]

In the name of sanity can people please not quote the whole of bb6263s excellent posts 

[bluearmy]

if only for purpose of consistency of reporting, the Ec46 looks quite blocky over the Atlantic weeks 3 to 6. Nothing overly strong but much more than previous output 

perhaps indicative of the model sniffing a new route forward?? 

[pureasthedriven]

27 minutes ago, terrier said:

To be honest looking at various output unless your at 1000ft. Then I think next week will be more wet than white. The latest week ahead wasn’t really showing a cold outlook with temps for most around 3-5c. Can’t really see much for most of us. 

Current (stress current) modelling for my location (Gatwick) has cold, wet and occasional wintry mix - especially with convective showers - written all over it. Here’s hoping for those very minor favourable adjustments. 

[PsychedelicTony]

3 minutes ago, I remember Atlantic 252 said:

cool post Tony, but don't tell the Express! will be on front page, polar vortex to hit Britain, 6 weeks of snaw

I can give em their headlines  for the next 6 months"!

"Dementia Cure on the way"

"Arctic blast to cripple Britain next week"

"Statins increase risk of Alkzeimers"

"Britain colder than Moscow this weekend"

"Storm Henry to Bash the UK"

"Coffee reduces Heart Attack Risk"

"Heatwave Brtiain..UK Hotter than Greece"

"Brexit fears unfounded, Wages rise as immigrant leave"

"UK to face flood risk"

"Biblical floods return in washout summer"

(Pics from Wimbledon/Glastonbury)

I'm giving up around June..!!

 

[Matthew.]

2 minutes ago, northwestsnow said:

18Z shows heavy precip here for like 36 hours, its either going to be a snow lovers dream, or an otters dream...

I would say rain for the immediate coast but snow inland and at elevation. With temps around 4 max 0 min very interesting for central areas away from the mild sea

[MattStoke]

8 minutes ago, markw2680 said:

True and the one in December gave us just short of a foot of snow, I can’t see this been anywhere near that tbh. Let’s see if the upgrades continue in the 18z

Had powder snow here that lasted on the ground for days and day time temperatures struggled to get above freezing. All the lakes and canals froze over. It was the most potent NW’ly I can remember in a very long time. Next week looks far less potent to me, and far more marginal (not just from an IMBY perspective). I would be amazed if delivers anything like that, but hopeful of some snow.

Anywayy, probably going to get a slap on the wrist for going off topic!

Edited January 11, 2018 by MattStoke

[tight isobar]

The divert- or otherwise point..away from anything else..

The complexities of track and switch- of cold upper transition hold many cards!..

The warmer gradient has hold/rotate analysis as its the evoling of height align or not to run with an' immediate overide into an northern/northern easterly flow..

Edited January 11, 2018 by tight isobar

[nick sussex]

I wish that trough would get a move on and sink.

It’s taking longer than waiting for the Eurovision results to come in. 

 

[stewfox]

8 minutes ago, Draig Goch said:

Thanks for that amazing informative post. Very interesting 

Remember the scroll bar when you re quote a lengthy post.

 

[captaincroc]

1 minute ago, MattStoke said:

Had powder snow here that lasted on the ground for days and day time temperatures struggled to get above freezing. All the lakes and canals froze over. It was the most potent NW’ly I can remember in a very long time. Next week looks far less potent to me, and far more marginal. I would be amazed if delivers anything like that, but hopeful of some snow.

Anywayy, probably going to get a slap on the wrist for going off topic!

I totally see  your point about it not being as cold and I had some great - and memorable - falls of snow too not far from you from Decembers event but this is exciting for different reasons. For me there seems to be a LOT more precipitation about and stronger winds and a lot more energy in the atmosphere so could be some interesting events within. Anyways, it's better than mild mush! haha.

[andymusic]

1 minute ago, nick sussex said:

I wish that trough would get a move on and sink.

It’s taking longer than waiting for the Eurovision results to come in. 

 

loving the way it's dropping it's load nicely and taking it's time over it to place each flake of snow

[ACCOUNT DELETED]

47 minutes ago, MattStoke said:

December had uppers at -8 to -10 widely. Colder than anything showing next week.

Not trying to put a downer on things. Just cannot see how it is looking like the coldest NW’ly in years when only a few weeks ago we had a colder NW’l. It’s as if December just gone has been forgotten already.

Even so, here on the south coast of Wales there was no snow (however we did have some torrential hail showers which brought some accumulations) but now with the sea temperatures throughout the Bristol Channel around another 2C colder since 10th Dec (now between 7-9C which is not far off their seasonal coldest), maybe the coast might get more chance of a dusting of snow this time perhaps.  

[feb1991blizzard]

6 minutes ago, bluearmy said:

if only for purpose of consistency of reporting, the Ec46 looks quite blocky over the Atlantic weeks 3 to 6. Nothing overly strong but much more than previous output 

perhaps indicative of the model sniffing a new route forward?? 

Perhaps its the Shoveling Snow Way forward.

Anyway for the record people - here they are.

 

[nick sussex]

1 minute ago, andymusic said:

loving the way it's dropping it's load nicely and taking it's time over it to place each flake of snow

No you want it clearing off se as quickly as possible.

You must get a decent ridge to the nw to stop the next upstream low from moving in and phasing .

If you want to extend the cold you have to clear the limpet trough sufficiently se.

[andymusic]

a full week plus of wintriness starting Monday night

[feb1991blizzard]

1 minute ago, nick sussex said:

No you want it clearing off se as quickly as possible.

You must get a decent ridge to the nw to stop the next upstream low from moving in and phasing .

If you want to extend the cold you have to clear the limpet trough sufficiently se.

Yes, that's whats worrying me. I think Phillip Avery thinks the same as well, Westerly veering Northerly for a short time then Westerly again.

[andymusic]

2 minutes ago, nick sussex said:

No you want it clearing off se as quickly as possible.

You must get a decent ridge to the nw to stop the next upstream low from moving in and phasing .

If you want to extend the cold you have to clear the limpet trough sufficiently se.

tonights beeb agrees with the gfs going from a stonking north westerly to a northerly and then even back to another stonking north westerly apparently

[Ice Day]

3 minutes ago, nick sussex said:

No you want it clearing off se as quickly as possible.

You must get a decent ridge to the nw to stop the next upstream low from moving in and phasing .

If you want to extend the cold you have to clear the limpet trough sufficiently se.

It's on it's way Nick at 204, bringing down some cold air from the north east with it!

 

[feb1991blizzard]

2 minutes ago, andymusic said:

tonights beeb agrees with the gfs going from a stonking north westerly to a northerly and then even back to another stonking north westerly apparently

Surely we want it veering NE'ly after the Northerly though.