Winter Approaching: Strictly Model Discussion (and highlights)

[Mike Poole]

ECM clusters this morning, starting with T120-T168:

Two clusters, roughly equal number of members, cluster 1 further south with the initial push of cold, the difference seems to be cluster 1 holds an Atlantic ridge, whereas cluster 2 is looking to split off the southern part to reanimate the Azores high.

T192-T240:

And that reanimated Azores high is what differentiates cluster 1 from the others in this timeframe - this has 18 members so is gaining traction compared to yesterday.  Cluster 4 has the strongest Atlantic ridge and deepest trough.  The other two remain -NAO with cold air in place.  The op is in cluster 1.

T264+:

Cluster 1 may hold cold air in place as the Greenland heights remain important.  Cluster 2 looks to be in transition towards a Scandi high, which is interesting, cluster 3 looks a return to Atlantic dominated.  Ural high becoming a player in all to some extent.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158247

[Met4Cast]

24 minutes ago, Nick2373 said:

 

00z for the south shows nothing special to be honest a push of milder air on the 19th usual cravats apply coastal areas sleet/cold rain higher the elevations snow, then 10 days yes its F1 showing potential return to a more west based regime. 

 

That push of milder air is associated with the slider low that moves through, naturally to the north of this low it’ll stay cold with snow on the lows northern flank but until the track is resolved it’s not worth worrying about the detail. 

I think we’re all expecting a return to a westerly regime later in the month or the following week after this cold spell, the fact models are showing this isn’t really a surprise. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158246

[Frigid]

GFS delays the breakdown by about a day or two in the latest run. 

Also, ECM is producing charts like this in two weeks (Fl I know, but a mere possibility) 

Nothing is set in stone, we have no idea whether it'll be mild or not after this spell. Less colder probably, but no consensus on south-westerlies dominating.

ECM has the best case scenario is going from this northerly to a scandi high as seen above. 

Long range does look promising, with a largely -NAO regime going into December after a few +NAO days. Too good to be true but one to watch. EC46, as unreliable as it is does show signs of blocking staying north.

Let's focus on the next 10 days, for one it's looking awesome for snow and cold potential. 

Some exceptional temperatures for both day and night. 

And snow, ECM went a bit wild! 

Some incredible charts this morning, all a variation on a theme. Let's keep the optimism!

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158255

[Met4Cast]

This plot shows how many different possible solutions there are for Tuesday low with quite a few taking it south of the UK. 

Virtually no point getting hung up on what individual deterministic runs show at this stage re: track. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158293

[That ECM]

T168 ukmo and gfs. Gefs at T168.

position of low at T180 on gfs. That’s T180!!!!!!! Not even close to nailing down that feature. 
 

what a nhp 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158124

[northwestsnow]

6Z control follows the 6z op ! Waaay more south than the 00z control ...

 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158385

[bluearmy]

I think there’s a lot of over reaction to a gfs 06z 

 

 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158403

[Ben Sainsbury]

 bluearmy No denying GFS 06z is a great run, especially this time of the year but any talk of a 'snow-event' is a good 5 days away, a lot can and will change.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158406

[bluearmy]

 Ali1977 by all means but there is a bit too much definitive analysis rather than nuanced. Most of us realise that in six hours we may be looking at only the Scots having snow next week and tomorrow amother solution but when cold is on the horizon we get a lot more inexperienced folk in here who won’t realise what a roller coaster the nwp is ahead of possible cold and snow. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158419

[Blessed Weather]

 Cirrusly Snowy Folk are talking about the Tropospheric Polar Vortex (TPV) at 500 hPA and not the Stratospheric Polar Vortex (SPV) up at 10hPA which can be a winter killer if it really fires up and couples with the trop PV. At the moment the two are not coupled and whilst the TPV is shredded the SPV is gathering strength with all 31 members of the GEFS ensemble showing stronger by forecast end. Here's the latest forecast showing the SPV stronger than the long-term average as we move into December.

Source: http://weatheriscool.com/

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158425

[bluearmy]

 Met4Cast looked through the gefs members and I don’t see that. Lower envelope is channel and upper envelope is the 06z icon.  Biggest cluster is through the midlands a little north of the op. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158443

[Met4Cast]

Huddle up.. keep it quiet, hoods up. 

Word on the street is that internally the Met Office are considering next week to be a significant cold event for the time of year, Cold Health Warning updates are likely on the next couple of days. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158480

[Met4Cast]

For what it’s worth the 03z UKV is just coming into range and has Tuesdays low barely scraping the south coast. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158490

[fromey]

 Met4Cast I work in the NHS and they have already mentioned it, they are waiting for further infomation. all they have mentioned so far is low temps nothing else

Edited November 14, 2024 by fromey

[bluearmy]

Ens update 

gefs and geps mean do not get the zero c 850’s into the U.K. - even on Monday 

the eps 00z brought it to the M4 

the eps 06z takes it to the M62

that means the eps are firming up on Mondays feature being further north.  But as has been memtioned in the past, the eps can cluster around an incorrect solution.   

onto the 12z’s - going to be really interested to see where Ukmo goes on this. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158534

[bluearmy]

 LRD been thinking about that zonal flow chart and I think it’s more complex than just looking at the mean.  The distribution is v relevant.  each blue line is an ens member. Only one of those members can be close to being right.  The mean reverting to average simply reflects the uncertainty increasing unless we see binary clustering.  We were looking at the odds increasing whereby it was just as likely that we saw a stronger than average zonal flow at 10hpa as we saw a weaker than average.  Yesterday that drifted back to it looking more likely to be sig stronger than average.  But that is in tandem with the mean spv being on the Asian side of the pole. 
I think it’s probably more important to see whether the trop is leading the strat or vice versa. 
 

incidentally, ec 06z op is a bit further north on Monday night 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158499

[Cambrian]

If the emphasis is on getting proper cold in and having the right kind of core setup in place, it’s very difficult to be disappointed by the 6z GEFS at day 6, a genuine 500mb to surface cross-polar flow for the UK and Ireland and uppers 7-8 degrees below normal for the end of the third week of November…

   

…on an ensemble mean!

Longevity of the cold spell - we won’t know for a few days, probably until the models are able to take account of how the cold digs in, down through to the base of the polar trough. Tracks of low pressures and other features interacting with and around that base of the trough - we probably won’t know for sure until the very day. As ever, we countenance ever narrowing ranges until then.

However, the persistent signal for cold that has been modelled for days on end, continues to grow in its depth.

I’m greatly looking forward to the next few days.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158611

[Robbie Garrett]

 

Shared by Marco Petanga on X

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158637

[sheikhy]

Control and mean have the low even further south!!!!

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158770

[Derecho]

Usually when assessing the longevity of a cold spell the first thing I look at is whether pressure is rising more in Iberia compared to what it is over Iceland.

The little low that forms at T168 helps weaken the Greenland block on the Icelandic side and the push of milder air from the west is initiated. However it's at T168 over a region with little data so expect it to be gone on the next run.

My concern about longevity however is related to the cut off Azores High, it is not building north behind the slider lows that cross us... as a result the Atlantic train leaves the station and we end with this.

Might be a lot different next run but I'm less confident that this will be a prolonged cold spell. It might be good but lets keep our feet on the ground. Comparisons to November 2010 are silly. That was in a different league.

I wouldn't even rule out a scenario where the south never really turns that cold at all, though at this stage it's unlikely as crossing lows tend to correct south and the GFS develops them too much.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158781

[ICE COLD]

1st post this season . I’ve been watching and reading getting all excited . Just read this and it don’t really get much better

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158617

[Mcconnor8]

Not one single GEFS member brings a breakdown in the cold by T204 for Northern England, that's quite a signal!

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158842

[Kasim Awan]

Ecm builds the ridge back into Iceland by day 10.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158938

[Ali1977]

Incredible snow depth on the borders and southern Scotland  - but that’s about it !! Strange how it’s so different !! 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158896

[WYorksWeather]

Snow depth charts - issues

Just worth looking at a few caveats when it comes to snow depth charts. We all like looking at them, but just worth a few notes of caution, beyond just the uncertainty about whether the snow itself will fall, but also the way the models handle snow when it does fall, and how this is converted to depth.

I'm using the ECMWF in this case, because they are very transparent about how their model works on their website. They provide a full list of issues here, but for the purposes of this I will focus on the most important issues relating to snow.

It is difficult to know how many of these issues would apply to other models, but it's likely at least some of them do, since the ECMWF is the 'best in class' model in terms of verification.

I've adapted and reformatted this text, but otherwise quotes are verbatim from the ECMWF website. I've provided a simplified explanation of each issue.

Issue 1

Quote

 

S1. Snow drift in convective situations

When snow falls through cloud or beneath cloud it drifts with the wind. For large-scale (dynamic) precipitation IFS physics accounts for this. For convective precipitation however it does not; there is no drift, the precipitation arrives at the surface instantaneously once the convection is diagnosed, in the place that it is diagnosed. As a result snow arising from convective processes may be misplaced in the model (too far upwind), and the errors will be larger if winds along the snowflake path are stronger. Errors can be of order 100km. Precipitation issue P1 relates. The same issues exist for rain, but given the faster fallspeed of raindrops relative to the IFS model resolutions these errors are negligible. Clearly one also has to take account of the melting level.

2017 IFS changes (43r3) included detrainment of some hydrometeors from convective into large scale precipitation, bringing a small positive impact as those hydrometeors then drift with the wind.

 

Essentially, this issue means that where snow falls as part of a storm, rather than a more organised feature, it doesn't drift correctly as it falls. This means that the resulting snow can be misplaced in windy conditions.

Issue 2
 

Quote

 

S2. Snow on the ground takes too long to melt

In both ENS and HRES small amounts of snow on the ground tend to take too long to melt, even if the temperature of the overlying air is well above zero. This is because, for melting purposes, the snow that there is is assumed to be piled up high in one segment of a gridbox. For smaller nominal depths, the pile becomes higher, though at the same time covers a much smaller fraction of the box. The reason this is used is to improve the handling of screen temperature; by confining the snow to gridbox segments the impact on the temperature of that snow is reduced, and on average we find smaller errors and biases in 2m temperature as a result. The main downside is that snow cover pictures can look misleading, particularly at longer leads (when they can not of course be rectified by observational data). The cut-off above which snow is assumed to cover the full grid box is a 10cm depth - this is why a green hue used on standard snow depth charts on the web, which suggests to the eye the presence of some vegetation, disappears at 10cm.

Cycle 48r1 which went live in June 2023 had a minor positive impact on this problem, via introduction of a multi-layer snow scheme; previously there had been a single layer. Increments applied through the snow analysis scheme tend to be slightly smaller since then.

 

In simple terms, this issue is caused because the model uses what you might call a bit of a bodged solution to another problem. An even, thin layer of snow has too large an effect on the temperature due to physics issues, so the model 'piles up' small amounts of snow so they don't cover the whole grid. That fixes the temperature bias that would otherwise occur. The issue with this is that it means that thin snow layers stick around in the model for longer than they should once the temperature rises above freezing.

Issue 3

Quote

S3. Mixed rain/snow leads to snow accumulation

In marginal snow situations, when precipitation at the surface comprises both rain and snow, the snow component accumulates as lying snow. In the vast majority of cases this is wrong - it should melt instantaneously. This behaviour occurs because small snow depths within the model are assumed to be piled up into a small segment of a gridbox, and as such it is very difficult for them to melt quickly (as in Snow issue S2 above).

One related coding bug was identified and removed during the winter 2017/18, which helped a bit, but the problem of correctly representing the physics remains.

This is another issue that tends to create an upward bias in the amount of snow. It means that in situations where the snow is marginal, the accumulations will often be far too high, because the snow part of a rain/snow mix is deemed to accumulate as snow, when it shouldn't. Again, this is because the snow portion is 'piled up' in each grid as in the issue above.

Summary

Overall I think the picture from this is clear. Certainly in the case of the ECMWF it'd be wise not to expect anything like the amount of snow shown on the depth charts to actually accumulate, due to these issues. There have been some recent improvements, though, as the ECMWF note that they have been using a multi-layer snow model since last year. Many of these issues will likely apply to other models as well, though it's not possible to know for sure, as I don't know where to find similar information.

This most likely explains why the Met Office are usually quite conservative with snow accumulation estimates in their forecasts, particularly for lower levels where it is more likely to be marginal. When snowfall is marginal, the depth charts can be highly inaccurate.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5158998