Winter Approaching: Forecast and Model Discussion Commentary

[WYorksWeather]

The longer range output is of course very interesting, but just trying to avoid for now going beyond the 8th December range, which is roughly the timeframe we've been looking at for a while now as the start of another potential period of interest. That's now at day 9, so worth a quick comparison of the 12z runs.

GFS looks underwhelming (block too far south), whereas GEM looks much more promising.

 

So, a good way of looking at this is to wind the clock back and see where things diverge. Starting at day 3. Not much of a visible difference here.

Roll it on to day 5, and the differences are more obvious. The GFS has two deep areas of low pressure in the Atlantic to our west, whereas the GEM only has one deep feature - the one closest to the UK is much shallower than on the GFS.

Going on to day 7, and you see the result. GFS keeps two fairly separate areas of high pressure, one over Greenland and one over the Azores, with a deep Atlantic low poised to erode the top of the high. By comparison, the GEM shows the western lobe of the high starting to link up towards Greenland, and the Atlantic is held back well to the west and cannot interfere.

It is these sorts of details that will now start to affect the outcome. Probably another day or so to start to figure out the strength of the day 5 low I imagine. We know that GFS has a tendency to blow up lows in the Atlantic, but every once in a while it gets one right (like Storm Bert recently), so I don't think either scenario can be discounted.

Will be interesting to compare with ECM.

[TSNWK]

Sobering ecm day 8 not sure there is any room for the heights to go at day 9 apart from being flattened by energy south of greenland

 

[Ball Lightning]

 Jeremy Shockey Hopefully that verifies just a week later instead...

[Mcconnor8]

 TSNWK Yes the ECM vs GEM at Day 9 show the range of possibilities as to how the high builds in the Atlantic, the AI run has been showing similar to this ECM run pretty consistently.

[damianslaw]

 northwestsnow Key feature on these runs is the shift of the PV to Siberia opening up opportunities for mid atlantic heights to build north, but there also appears to be a segment/lobe of PV to our NW spilling runner/shortwave features towards the UK. I can see the evolution to high pressure to our SW/W with colder shots of air from the NW as we move through to mid month. All rather average. 

 

 

 

[Cold Winter Night]

Those Low heights sneaking into Europe through Scandinavia, SE of the Atlantic Ridge, can make all the difference going forward.
Many a cold spell (Scandi High type) started like this.

GEM has that feature more pronounced, with the al important warm T850 'finger' towards Iceland, and Scandi High potential after 240h:

EC does this too, but the smaller weaker Low isn't enough on this attempt.

However, in the extended, EC does try again with an Atlantic ridge/Scandi Low:

[Dennis]

good +PNA 12z -6z  critical point on a Low or not N from Greenland 

Edited November 29, 2024 by Dennis

[Legend.Ice]

 

Aleutian Low, Atlantic Ridge and weak Ural Blocking , who will change the game ?

[Dennis]

 Legend.Ice

signs in EC charts shows the Ural (yesterday)

[HarvSlugger]

Latest ECM and many other models showing a storm around Friday-saturday

 

[Legend.Ice]

 Dennis

thanks , maybe UB will make a nice trigger for SSW mid Dec.

[Dennis]

 Legend.Ice

could be better so lets kick it 

 

Edited November 29, 2024 by Dennis

[Dennis]

OK into 5 MJO soon by EC.... still worries , sometimes im very angry on that 

but a nice sign to blocking is getting in 

Edited November 29, 2024 by Dennis

[WYorksWeather]

EC clusters below.

Days 3-4

No real pressure signal for the UK - a predominantly westerly pattern at this stage.

Days 5-7

Quite a few clusters here, so just a quick rundown of each.

Cluster 1 (33%) is westerly and mild at days 5 and 6, with a transient north-westerly influence at day 7. High pressure to the south, low pressure to the north, high pressure over eastern Scandinavia into Russia. Cluster 2 (24%) is broadly similar to Cluster 1.

Cluster 3 (18%) remains westerly from days 5-7, with no north-westerly influence. Mild.

Cluster 4 (14%) is mild at days 5-6, becoming milder still at day 7 with more of a south-westerly.

Cluster 5 (12%) has more of a north-westerly at day 5, then sets up a high slightly further west, closer to a true Scandinavian high. A bit of a battleground by day 7, with possibilities of north-westerlies or westerlies further west and south, and a possible easterly for areas further north and east.

The OP was in Cluster 2.

Days 8-10

Cluster 1 (51%) builds a mid-Atlantic ridge just to the west of the UK. This brings a north-westerly around the top of the high. However, the ridge then topples into the UK, with a very quiet spell of weather following with a large ridge from the Azores through the UK and up to Scandinavia. Probably quite a mild ridge, with a link back to the Azores, but could still be cold at times depending on cloud amounts.

Cluster 2 (25%) has a more elongated high to the west of the UK at day 8. This would allow a brief northerly, before the high then topples into the UK. Probably a colder high than Cluster 1, as there is no link back to the Azores. Perhaps a good opportunity for surface cold, perhaps frost and fog.

Cluster 3 (24%) starts with a more unsettled north-westerly at day 8, then builds high pressure a little further north in the mid-Atlantic, allowing a north-westerly or northerly. By day 10, signs that this high will again topple into the UK.

The OP was in Cluster 3.

Days 11-15

Cluster 1 (51%) has high pressure to the SW and NW of the UK. Quite a cold pattern, with a persistent north-westerly wind. A potentially stormy spell at day 15.

Cluster 2 (33%) has high pressure more to the north of the UK. This leads to a potential for more easterly influences, especially for the southern half of the UK. Likely cold, but the depth of cold will depend on temperatures over central and eastern Europe.

Cluster 3 (29%) is fairly similar to Cluster 1, with north-westerly winds throughout.

Summary

Overall, the next week or so looks generally mild, with no real support for anything of interest beyond the briefest of north-westerlies. For the day 8-10 timeframe, which was the initial period of interest for December around the 8th December, the clusters are going for more of a middle ground. All three options show a cold synoptic at that range, but it has been downgraded to more of a north-westerly or brief northerly toppler, followed by a quiet spell with high pressure over the UK, rather than some of the more persistent cold charts that were showing yesterday.

In the extended range, there continues to be interest in colder options, with a potential easterly on Cluster 2, and Cluster 1 and Cluster 3 both feature persistent north-westerlies. None of the options look all that notable though in terms of the persistence and depth of cold.

I would say that in the last 24 hours we have seen a trend away from a more notable cold spell, but I would still say it could look a lot worse. The second week of December looks likely to be more seasonal, with the potential for temperatures to drop below average, and who knows, there could still be some snow risks for prone areas, even with only a north-westerly or brief northerly.

[Dennis]

if this is staying in the forecast then we can forget the whole party (combined to a bad MJO ) 

so i do hope for better charts next week 

[Mcconnor8]

Unfortunately not a good EC AI 12z at all, high builds so close to the UK that it topples almost immediately, worse than the 6z in this aspect as well, with the vortex setting up shop around Greenland.

[Dennis]

 Mcconnor8 the charts need a good sleep the next days to feel better next week

[WYorksWeather]

 Mcconnor8 Probably the only consolation from that run is that at least it looks dry, with a lot of high pressure close to the UK, in fact at one stage we have a 1040mb high over us.

Later on, way out in FI, we end up with a 1050mb high over us, which is even more ridiculous.

If we are going to have a mild pattern, at least this would be a dry one. After recent flooding concerns, if we are going to have mild, then mild and dry is preferable in my view.

Of course, far from guaranteed that it will play out this way.

[TSNWK]

 Mcconnor8 I remain unconvinced with ecm ai..

During last week's cold snap it on several occasions extended the cold with high ridging towards scandi and sliders including on more than occasion suggesting storm Bert would slide and go under.

As is the law of sod ecm ai has probably nailed it for next week

 

[Don]

41 minutes ago, WYorksWeather said:

I would say that in the last 24 hours we have seen a trend away from a more notable cold spell, but I would still say it could look a lot worse. The second week of December looks likely to be more seasonal, with the potential for temperatures to drop below average, and who knows, there could still be some snow risks for prone areas, even with only a north-westerly or brief northerly.

I think the consensus from some long range forecasters was for any cold to occur during the 2nd half of the month, so perhaps the models have been a tad premature with the pattern?

[Don]

24 minutes ago, Dennis said:

if this is staying in the forecast then we can forget the whole party (combined to a bad MJO ) 

so i do hope for better charts next week 

It's a stressful game this! 

[Dennis]

 Don

for sure it is , on the MJO ideas the best phases for west europe is late 6 and a 7 (scnadiblock) in a winterseason to get good East winds

start dec we see a 5 - it can help a block - but not always a long time 

then we have the PV and its layers  (1-1000)hpa , there we see a strong PV 60N near 50m/s into dec start - its not the best but in the lower latitudes we see a strong sudden decrease after 5/6 dec (can help to disturb the jetstream (zonal)

and we do see a shift of the PV position that is at the background

So all is amazing to watch as an weatherfreak

Good weekend 

 

[Mike Poole]

 Dennis It’s a question of where the trop vortex, or the pieces of it, are that is important.  And on the GFS 12z run that the chart you quote is from, it is kept away from Greenland, so not too bad:

What we don’t want is runs like ECM AI, as @Mcconnor8 posted, that one is ominous!

[WYorksWeather]

 Don It's possible. I think ultimately there's also the difference between a cold-conducive pattern at the broader scale and one that actually delivers for the UK. All of those EC charts could be very cold with a few tweaks (high a little bit further NW, lows from the west a little shallower with more tendency to slide rather than blast through). But you do need those details to be right for a notable UK cold spell. This is of course why most of the time we get brief, sharp cold spells, or a more prolonged north-westerly influence, rather than a long-lasting northerly or north-easterly.

[LRD]

Massive step back today. No getting away from that. Not to say it won't turn back the other way but we know how this usually goes

EC46 pretty disappointing too. Are we heading for a 14th not-especially-wintry December in a row?