Model Output Discussion Autumn 2024

[I remember Atlantic 252]

 sheikhy aye was meant to be snow here too, but there wasn't any, this one low is too far east but time for tweaks

[AO-]

 Scandinavian High.

A chart with potency. The pump near the eastcoast of North America keeps the ridge posted. Curious about the ensemble. No signs anyway that the Atlantic is going to play ball in the next few weeks. 

[That ECM]

 Scandinavian High. not worth over analysing a chart at T240 other than maybe from a learning point but it would depend on a number of things. It would feel bitter in the wind. 

[bluearmy]

Eps mean now bringing rising euro heights on two consecutive runs as we reach the end week 2 

Given the time of year and sst’s it will take some quite notable sypnotics to bring proper lowland snowfall 

bit we are certainly in t(e mix 

[sheikhy]

 I remember Atlantic 252 yes if i remember correctly just few miles further west it was sleety rain!!

[TSNWK]

 bluearmy the coldies nemesis.

eueo heights 

[bluearmy]

 TSNWK it’s two weeks away / it’s the mean / we dont live in Siberia

let’s enjoy the chase because a week ago, few could see this possibility 

[I remember Atlantic 252]

 sheikhy

[TSNWK]

 bluearmy agree.. I was playing to the gallery a smidge..

[LRD]

11 minutes ago, bluearmy said:

Eps mean now bringing rising euro heights

'Twas ever thus. The Met Office do see an unsettled end of the month and a return to normal or mild after whatever we get next weekend

[WYorksWeather]

 Met4Cast Some of the snow-related known issues in the ECMWF are summarised below - I picked out the three most important from the ECM summary of issues page, which can be found here. Since it is usually considered the most sophisticated of the global models, I imagine many of these issues will apply to other models, though difficult to know to what extent. Presumably, high-res models like UKV will do better, as usual.

Credit obviously to ECMWF for this explanation. Nice that they are pretty transparent about their known issues.

Quote

 

Snow

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.

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.

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.

 

 

[Daniel*]

Good to see the mod thread busy again recent weeks have been truly dead! ECM 12z left cold tap running one would imagine increasingly lowering temperatures and wintry possibilities opening up away from hills.

[nick sussex]

Not a bad end to the day for coldies.

Variations between the outputs between the cleaner GFS with stronger blocking . The ECM has a shortwave running se and then reinforces heights to the west nw.

The UKMO has a shortwave day 7 likely to run se but it looks a bit too flat  over Canada earlier  . I'd not have much confidence in its upstream set up so I think it's the outlier solution at that time.

We should temper expectations somewhat as we're early season and would need the coldest possible source at this time of year.

Certainly though there could be some more wintry weather on the way especially further north over higher ground .

Still early days but at least it gives us something to get our teeth into!

 

 

[Pauski]

 Daniel* It doesn't look good if you think of Scandinavia and Finland, the heat continues in Finland.  Too much westerly cold snap, not a good thing.

Edited November 9, 2024 by Pauski

[Lukesluckybunch]

ECM mean day 9

[TSNWK]

 Lukesluckybunch I struggle with means.

Is this day 9 good for coldies? Trend since last mean, Lukesluckybunch

[LRD]

EC46 - a mild, wet December on the cards if it's correct. Would only be the 14th mild December in a row...

Edited November 9, 2024 by LRD

[AO-]

Still no done deal for the Netherlands with the current charts. We need a follow-up with Northeasterlies for winter. SST's are too high to deliver proper cold. Preferably a Scandinavian high to deliver the real cold. But first this will do.  There is a lack of Northeasterlies in FI. Same holds for northerlies and easterlies. I prefer the control obviously.

[AO-]

 TSNWK

great mean though. Look at the 552 dam(?) line which goes well Southeast into Europe.  Signal for hp over the Atlantic as well as an Arctic high. Add this to the Vortex over Siberia and voila. Nice meridional pattern. A little contrast to my previous post

[Lukesluckybunch]

20 minutes ago, TSNWK said:

 

 Lukesluckybunch I struggle with means.

Is this day 9 good for coldies? Trend since last mean, 

 

Yes it would be a decent mean..showing a well defined ridge in the Atlantic stretching to the southern tip of Greenland,the trough to the east being the main player bringing in east to northeast winds

[RJBingham]

One for the GEM fans.

[Lukesluckybunch]

1 hour ago, TSNWK said:

the coldies nemesis.

eueo heights 

Exactly blue ,or the annoying Azores high

[WYorksWeather]

EC clusters below.

Days 3-4

Becoming cooler as high pressure starts to pull to the west, but probably brighter as well, bringing an end to the anticyclonic gloom.

Days 5-7

Cluster 1 (29%) has high pressure gradually retrogressing, meaning we go into fairly cold north-westerlies by day 7.

Cluster 2 (27%) is even faster with the retrogression, with a northerly at day 7. Cluster 3 (27%) is similar.

Cluster 4 (16%) has a messier pattern. A low to the south and some trailing heights to the SE may pump up some milder air, but colder air coming down from the north. Maybe a N/S split in terms of the temperature.

The OP was in Cluster 3.

Days 8-10

Cluster 1 (29%) is quite notably cold. A northerly flow sinks a very long way southwards, right into mainland Europe as well. Cluster 2 (27%) is similar, though perhaps slightly less extreme than Cluster 1 in terms of the southward progression of the cold air.

Cluster 3 (16%) has a failed retrogression. The high moves out to the west, allowing a north-westerly, but then ridges back across the UK. Still on the colder than average side though for a time, but milder again by day 10.

Cluster 4 (16%) has the block further west at day 8, more towards the far side of Greenland. This results in more of a north-westerly. By day 10, turning colder with a true northerly.

Cluster 5 (12%) has a cold-looking northerly at day 8. By day 9 and 10 though, signs of a bit of an interruption, with low pressure moving up from the south which may re-introduce milder air.

The OP was in Cluster 2.

Days 11-15

Cluster 1 (55%) has high pressure to the NW at day 11, keeping it fairly cold from the NW or N. Beyond that though, high pressure ridging from the Atlantic through the UK, and mild westerlies or south-westerlies from around the high. Likely mildest relative to average in northern areas where the wind blows around the high. Relatively dry overall.

Cluster 2 (45%) also starts with some blocking and fairly cold at day 11, winds from the NW or N. Similar to Cluster 1 it then goes mild, but more of an unsettled pattern. Lots of low pressure moving in from the Atlantic. Possibly very wet at times, but also mild or very mild.

Summary

Overall another interesting update. The ECM is definitely firming up on an opportunity for some cold from around day 7 to day 12. Overall, I would say the support is around 70%. There are still a few too many runs for my liking showing untidy setups, with features like lows running up from the south or blocks not quite in the right position.

To be clear, the other 30% are not especially mild, but just not as cold. At this time of year if you are looking for snow we do need the perfect setup - any imperfections are likely to result in the air being too mild for snow to low levels.

In the extended range, the ECM is pretty adamant about a swift termination of the blocking signal and a quick reversion to a much milder setup. This does tie in with what some of the teleconnections posters have been saying as well. However, I would caution that this is beyond the reliable. It is still possible the cold spell could be extended beyond what is currently shown.

 

[clark3r]

Nice to see colder weather showing on charts early in the season compared to recent years. My gut feeling is we have a higher chance of a colder winter this year.  I feel with the solar storm on the sun that is bringing chance of aurora until March 2025 May well impact our weather e.g colder spells. No scientific evidence but my general feeling. Good luck all for the coming winter. 

[AO-]

 clark3r

You know that the colder winters happened during solar minima right? The only exceptions are 1947 and 1979 which happened during a maximum. The others ('41, '42, ('44), '63, '85, '86, '87, '96 and '97) happened during the minimum. I don't count '56 as it only was an exceptional cold February.