Model discussion highlights

[nick sussex]

The GFS remains isolated with its 48  hr cold snap .

The rest aren’t interested and even if the GFS is correct in its more northerly push of the high more than likely it would edge the pattern further east with time . Unless that is there’s a background signal to hold the high further north and west . 

So in effect its a bit of a squabble over tickets to see an aged band of has beens!

Given the paucity of excitement on offer I can understand a crappy band night out is better than watching One Man And His Dog.

Younger members may need to google this . The oldies will be aware of this hub of excitement! 

Anyhow for want of anything interest let’s  indulge the GFS .

Unusually it’s the more amplified over the USA and Canada . It normally used to be the flattest solution historically.

The key timeframe which is day 7 into 8 means we should get a resolution soon .

This isn’t like a shortwave drama near the UK and if the GFS is correct I’d expect a shift to that by this evening . Of course the reverse applies also .

Below is part of the latest NCEP extended forecast discussion . This explains in good detail why we are seeing the model differences .

Guidance /Predictability Assessment... Namely, the GFS/GEFS suite lately has been interacting/combining the preexisting southern stream upper low and this shortwave, which forms an east-central U.S. trough that supports a strong and faster moving surface low in the Gulf and progressing northeastward. This is opposed to the remaining majority that keeps the northern stream shortwave progressive and north of the southern stream upper low. Without the phasing of these features, the southern stream upper low will be slower and the only dynamical support for surface low pressure, so the surface low forms/strengthens later and stays more suppressed. WPC has been generally favoring the more suppressed and slower track given it is the larger cluster with multiple model suites (the ECMWF and CMC). Ensemble members tend to favor with their operational runs but do show a lot of spread.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974168

[knocker]

24 minutes ago, Ruzzi said:

I think you'll probably find that calling wet and mild until the end of the year when it's far outside even the depths of FI on the models is more of a 'mugs game' 

Essentially what you've said is: ignore background signals, have a look at what you see in the models, take that as gospel and run with the thought ls that it'll continue beyond the forecast period exactly the same. 

Nothing can be taken as gospel just now, not when the extent of the MJO amplification can't be pinned down yet with conflicting forecasts. There's no doubt that whatever verifies with the MJO in terms of amplification will have a knock on effect. 

MJO aside, it's again subjective to location, in terms of how wet and how mild the near term gets but I'd not be hanging my hat on it continuing until the end of the year, just as much as I'd not be hanging on a colder outlook. There's plenty water to pass under the bridge in the next week or so. 

I don't think that is what he is saying. But in any case  it is not that easy to evaluate the probable influence of the the forecast background signals on the Atlantic and European weather as some on here make out. But having said that they should always be taken into account along with  a keen scrutiny  of the model outputs.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974204

[Catacol]

50 minutes ago, northwestsnow said:

Maybe I'm wrong but i think the over riding majority of people here deeply appreciate the input from  catacol, Tamara, Matt  H ,Metcast and others ,I think we are blessed to have these guys sharing their knowledge with us all.

If everything goes wrong they will absolutely be able to pin point where in good time.

In the  meantime let's wait  to see if the more positive background drivers come to the fore in the coming weeks..

Not got a huge amount of time right now - but as the accountability brigade are out in force I'll put a quick analysis together. Note that the kind of stuff put onto this thread in the last couple of days is exactly why there are knowledgeable folk who choose not to post forecasts. As I have said more than once, we are playing a game of probabilities and not certainties. If people on here want an attempt at reasoned analysis then responses to changes/adjustments need to be balanced. Given - as I showed over the weekend - the MetO adjust their text forecast regularly to fit a changing situation, it might be worth considering the relative toolset of the amateur enthusiast on here and the super computer power down in Exeter that still struggles!

The answer as to why the forecast is changing at range is timing. It isn't about the signals themselves necessarily because the patterns discussed are still in place, but as I have also said more than once the bugger of an issue we can have on all this is timing.

A couple of examples. MJO forecast first:

The pacific wave which is so crucial to our understanding and forecasting of the hemispheric wave pattern has slowed down. It is still going to get through phases 7 and 8 but BOM now has the phase 8 transition as post Xmas. This is quite a significant alteration in speed, and I will be interested to read the update later today from the CPC to get their take on it.

This means the forcing towards a high lat block is delayed, and hence our mid lat block in the ensembles is not getting the shunt we expected it to get around Xmas time. Given lag, if the speed of MJO passage is getting closer to reality then that phase 7/8 kick is more likely now to be around New Year.

Note also the context of GLAAM and the elastic nature of momentum. Here is the current relative chart

You'll note the cyclical nature of momentum,  and we are awaiting the next positive surge. A major +EAMT event is on the horizon caused by the pressure patterns around Tibet that I have posted previously, and given the lag of this it was already looking as though peak impacts would be post Xmas. Note what is happening at present with these important mountain-led processes:

Both East Asia and North America falling into negative territory....but a rebound is coming - the bigger the better. 

So - to conclude. Pacific wave appears to be slowing, momentum surge likely to chime with this post Xmas....so pattern development is delayed. And this is before we factor in any top down influence from the strat vortex. I'm out of time to address this right now.

It's only December 11th. Let's keep some perspective.

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974237

[Blessed Weather]

My tuppence worth on why the MJO has 'failed' leading to models dropping suggestions of Atlantic ridging/blocking.

Looking at the latest MJO forecasts it’s appears to me as if the MJO may have hit the well recognised Maritime Continent (MC) ‘barrier’ as it tried to progress eastward along the equator into the Western Pacific (Phases 6 and 7). Research shows that up to 50% of MJOs fail to make it past the MC. And without the MJO progressing to 6, 7 and 8 at amplitude – certainly by ECMWF and NCEP/GFS models - we have lost meaningful tropical forcing on the North Atlantic synoptic pattern in the output from those models (i.e. Atlantic ridging/high latitude blocking).

Here’s the recent progress in RMM charts that illustrates what appears to be a typical eastward propagation collapse:

EMON ECM Ens 04Dec  07Dec

BOMM 02Dec  09Dec

And this could well be the explanation:

Observations show that the MJO tends to be significantly weakened when propagating eastward into the MC region; the MJO also often detours around the MC via an oceanic pathway south of Sumatra Island and over the Java Sea in austral summer. Often, the MJO even completely dissipates over the MC and fails to propagate into the western Pacific, known as the MC barrier effect for MJO. About 50% of the total MJO events during the boreal winter are disrupted over the MC. Due to the MJO's significant impacts on downstream high-impact weather and climate events in both the tropics and extratropics, determining whether the MJO will propagate through the MC is crucial for climate prediction. The MC barrier effect, however, is poorly simulated in current GCMs, and most forecast systems exhibit large deficiencies in predicting the MJO propagation through the MC. These model shortcomings in simulating and predicting the MJO propagation through the MC are partially due to our poor understanding of the underlying physics responsible for the MC barrier effect.

There is plenty of good reading and further detailed explanation of the MC barrier in this excellent paper:

Fifty Years of Research on the Madden-Julian Oscillation: Recent Progress, Challenges, and Perspectives

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974258

[Met4Cast]

On 08/12/2023 at 22:15, Met4Cast said:

I'm not sure how long supported a Greenland high will be or whether the 2nd wave breaking event will be favourable to prop up a Greenland high, we may actually be wanting to look towards Scandinavia for height rises instead, despite current GEFS suggestions. Either way, assuming MJO progression, we're looking at a period of high latitude blocking somewhere on our side of the hemisphere 22nd Dec - 10th January. Not for the entire period, but developing sometime within that period. 

I just want to highlight the above part of a post I made on Friday. This remains true, as @Catacoldiscussed above the MJO progression has slowed and thus any high latitude blocking has been set back but other signals remain positive. I wont go into detail on the AAM/+EAMT event etc because Catacol has already done a great job of explaining the situation above. 

Timing is often the most difficult aspect of long-range forecasting to nail, a slower or more progressive evolution than expected can shift timing about, we often see this in NWP modelling & I suspect the few blocked runs we have seen have been too soon. The Pacific jet extension is what will likely feedback to our UK high being sucked out into the Atlantic, a slower MJO wave means timing wont be quite right to amplify this into Greenland, I stated in the above extract above that the 2nd wave break may not be sufficient to get there and that seems to have been the trend in the last day or so. 

Where do we stand? Well, we may still see a transition to high latitude blocking but probably post Christmas now, I'd say we're likely looking into the early New Year for timing but again, this could shift. 

There remains a lot of uncertainty in the MJO forecasts with recent runs from the ECM/GFS losing a coherent signal as we transition into phase 6, this complicates the picture & increases the uncertainty on any higher amplitude blocking. We're not looking at a full zonal assault but as ever with UK weather, the picture is complicated and constantly evolving.

Thanks @snowking and others for the mention, it's appreciated!

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974248

[Catacol]

1 hour ago, Blessed Weather said:

Yes, the CPC expert analysis due later on today is awaited with bated breath Catacol. Apart from BOMM and CFS, all other models are forecasting that whilst the MJO will continue on into the Western Pacific and Western Hemisphere, it will be at such low amplitude that it is effectively 'no signal' and thus no extratropical impact.

Given the content of your earlier article, and some of the stuff I dug out a week or so ago, there is a fairly key potential turning point here, especially given the admitted reality of a lack of understanding as to the physics of what happens when the MJO traverses the MC. We could see a decent sized shift in the medium range forecast one way or another. I had known a while - and had noticed through observation - that the phase 5-8 transition was a problematic one, a wave development generally poorly modelled, but reading into it recently has furthered my grasp of the complexity and potential variation.

The BOM amplitude is a head scratcher given the way other models are reading it. Given ECM and BOM according to my research are the top 2 available (UKM seems to be unattainable now?) there is a reckoning coming.

I am learning the words to "Waltzing Matilda" as I type....

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974370

[Eagle Eye]

It appears that the +EPO wave shrinks away quite quickly with the development of a - TNH via the vertical driving of the +VE height. The jet extension happens around this time but should be displaced south enough to drive the - TNH pattern and also form the +EPO again (talked about a bit later) but more favourably and a slightly less zonal than typical +EPO. The - TNH pattern looks to force the Greenlaand - VE heights to shift poleward and disconnect the north Atlantic jet and allow for tropical forcing to have more of an impact with a slight increase around the same time. Potentially leading to a cold pattern but I'll leave that to model to model runs. 

Then a +VE EPO redevelops but a veriation of it forms. With the shallow heights centred over Alaska, then if the north American signal can stay +VE, then a jet retraction in response the the jet extension (w/about a 14 day lag, assuming no other +VE EAMT event) would collapse the high over the just south of Aleurian region and the Alaska height could well be driven southwards by being tilted on the opposite side of a Rossby wave ridge if one could form over Siberia. Then that's a great stratosphere setup for a SSW, I'll get onto that later quite a lot is on the +VE heights over the NE part of north America at least staying fairly strong (assuming the height do form which they should, just not sure how strong it'll stay) and hold out the - VE heights. Hypothetically, if it did, it'd drive the Quebec heights further north but more importantly would disperse the wave vertically, diffusing the heat up into the stratosphere with a 3 way Rossby-wave setup for a 2-Wave attack (Siberia and Quebec-Greenland) on the stratosphere with the vortex displaced over to the north Pacific. 

The Mon-development of the mid stratosphere jet looks to be partly due to the downwelling from the development of the EQBO. The 1hPa being +VE isn't necessarily something to be worried about as it acts rather erratically compared to the mid Stratosphere. It's quite easily affected by eddies compared to below it and therefore switches up more than other heights. As we aline with the 2021 EQBO, it'll be interesting to see the difference with the El Nino episode and if we can get a prolonged - TNH period, whether that can work with the EQBO to form a potentially good partnership. 

Still that eddy driven jet helping connect up with the Stratosphere but nothing particularly out of the ordinary yet. It's sitting around waiting time with the stratosphere but as I've said before, a good setup. 

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974508

[Blessed Weather]

@Catacol The CPC 'expert discussion' of the MJO just published. It certainly confirms what can be seen from the dynamical models, a weakening of the amplitude. But there's some slightly more positive news regarding the extended outlook with the possibility of "reorganization of the MJO signal over the Western Hemisphere" (i.e. emerging from COD into Phase 8 by end December).

• Since early December, an active MJO has continued to propagate eastward across the Indian Ocean and Maritime Continent and has recently begun to constructively interfere with the low frequency El Nino base state over the equatorial Pacific.
• Continued eastward propagation of the MJO signal into Western Hemisphere is favored in the RMM forecasts towards the end of December, however dynamical models generally point to some disorganization and loss of amplitude likely tied to competing tropical variability as it crosses the Pacific during the next two weeks.
• Dynamical models generally favor a MJO signal that slows and reduces in amplitude as it shifts across and Western Pacific during the next 2 weeks. This behavior may be tied to a convectively couple Kelvin wave propagating across the Indian Ocean in the forecasts.
• Extended range solutions favor some reorganization of the MJO signal over the Western Hemisphere with continued eastward propagation towards the end of December and into early January.

Full update: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjoupdate.pdf

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974601

[nick sussex]

I’d be wary of outputs that take the PV eastwards at longer range .

We have seen this before and then as the time counts down a chunk is left over ne Canada .

You really need to see this modelled at closer range .

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974743

[Nick F]

Don't think I've seen a 1050 hPa Azores high forecast before, as shown by EC at T+216, perhaps enhanced by the very strong jet arcing across the N Atlantic?

The danger of such a strong jet aimed at the UK is potential for one or two nasty deep lows in the run up to Christmas, which will be in stark contrast from the settled anticyclonic conditions prior taking over later this week. 

GFS shows the jet stream pushing further south by and beyond day 10, hence the threat of colder arctic incursions by Christmas, but still bit too far away for my liking to be confident of some cold and wintry weather a few days before and over Christmas. Especially, given the strong jet stream forecast over the N Atlantic, it stays flat and zonal and doesn't sink far enough south. More runs needed.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974812

[Cambrian]

Some notable differences across the models with regard to the rate of the westward removal of heights from around the UK and Ireland towards day 10. 0z EPS at 240h vs. 6z GEFS at 234h. 0z GEPS was somewhere in the middle. 

Partly as a result, there’s also differences in the southward extension and shape of the trough around Iceland. GEFS is much keener on dropping a trough with a round base well to the south of Iceland. The EPS trough is much more elongated and remains much further north. 

 

There are consequently marked differences for the projected T850s, the GEFS looking quite seasonal. 

 

But the contrasts are only so profound at that stage, the differences are much more resolved by day 12, with the trough slowly working its way towards Scandinavia. We see a lot of this, where there is seemingly better agreement on the ultimate destination than the road taken to get there.

 

If this happens, this is a very interesting place to be for Christmas Eve, the tightness of the contours suggests an active jet stream, with the UK and Ireland on the northern, colder side of it. 

Pacific jet, Atlantic jet, both up for it. Both North American and Siberian heights, pinching a cross-polar trough at the pole. The point is that’s a cold trough, hanging north-south. The mixing air masses underpinning the cyclonic developments that will be heading our way are from a long way further north than for our usual upstream pattern, with plenty of Arctic air mixed in. 

The EC46 heights anomalies have over recent days slowed down the eastward progression of the trough to our north, but have been settled on the ultimate outcome for the turn of the year for several weeks now, with the polar trough slowly moving east to bring us under the influence of flows more directly from the north. 

These are yesterday’s charts for 25th-1st and 1st-8th and it’s good to see the NWP outputs falling more in line with these as time goes by. 

 

And whatever it’s like on the day, almost two weeks away, it’s the whole feel of the season that matters to me, and that is a long way from a normal Northern Hemisphere profile - the precise timings are not going to be clarified until only a handful of days beforehand - surely why the developments around this core theme are going to keep us especially riveted over the days and weeks to come, for Christmas itself and through the New Year. 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4974896

[Mike Poole]

ECM clusters T192-T240:

Clusters 3,4,5 more rounded with the high pressure to our west, and therefore faster moving the vortex to the east.  Clusters 1 and 2 slower, 2 includes the op and control.

T264+:

These two rather underwhelming, and for me, quite different to this morning’s clusters at this range, suggesting the model hasn’t got a handle on this timeframe yet at all.  A suggestion maybe that the runs that didn’t see the Atlantic ridge in the previous timeframe are seeing it in this one, amidst a predominately westerly flow?

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975189

[bluearmy]

1 hour ago, Cheshire Freeze said:

The point is we were staring down the barrel of an entrenched Euro HP a couple of days ago (if we took the modelling at face value)…which never ever looked plausible at all.

Said many times that I was unconvinced by the modelling post the ridge. But there was a period where the ridge was a bit uncertain and then the nwp fairly quickly firmed  up on it.  Then post ridge then became uncertain and now we see the modelling firming up on the movement of the tpv from Canada towards n scandi with associated deep nw euro trough.  Now we become uncertain on what follows this trough.  usually we would have a handle on later week 2 and feel that it made sense. for pretty much all of December this hasn’t been true.  
the past three gfs ops have downwelled the weaker strat flow into to the lower strat and trop and the NH profile looks more wintry moving forwards for the mid lats as a consequence- then it becomes about how the dice fall 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975346

[MattH]

Morning...Just wanted to jump back to late Nov and a screenshot that I posted, one of the key points back then, which is still relevant is highlighted...

It is and has been the amplitude of the MJO event that has been the troublesome aspect of the forecast and this has had some influence on the expected rise in the AAM and the GWO as mid-December approaches. However, there is still expected to be some recovery looking ahead. I have touted the final third of Dec (20th-31st) as the key period of interest for some time and as we are now seeing that period remains of interest looking ahead. The -ve VP200 anom signal is still progressing eastwards and I believe this is something NWP is still yet to properly get a grasp of - Granted the high impact amplitude of a phase 6-7-8 MJO event isn't there, but there are still potential influences here from the upstream developments.

There are some very unusual anomalies, in terms of temperatures setting up over in the USA looking ahead, but this, overall, isn't of much relevance to the North Atlantic. The key evolution here is the eastward movement of both the sPV and the tPV off the pole to be more towards the Russian side of the H Hem. Clearly, all details aside, we are seeing a highly cyclonic but increasingly cold polar maritime NW'ly flow develop as we move through the end of next week onwards.

We can see this pattern and evolution well on the loop below - What will be key for the British Isles is how far east the main lobe of the tPV gets and just how much ridging takes place over the W and NW of the Atlantic. This is never going to be a significant cold setup, seasonal and wintry yes, but a widespread UK sub -10c 850mb event - no.

One of the primary variables to be aware of with this pattern though is the potential for "last minute" rapid cyclogenesis events that could stop the cold air from getting to our shores. Using the 240-hour EC as an example, we can see a weak low or an open wave exiting the NE USA. On face value, it's a cold trend of course. However, the N Atlantic would be a breeding ground for rapid cyclogenesis given the high thermal gradient (baroclinic zone) that would be present. Again, the warmth over the USA has little relevance to our outlook, IMO, as it is the N Atlantic in its own right that'll determine the Christmas outcome. If these small-scale features exit into the N Atlantic and rapidly blow up then you cut off the polar maritime or, perhaps, arctic maritime flow, but these are all details for further down the line.

Over the next week, the keyword is 'trend' - If you're looking for a seasonal Christmas week, which I believe is now more likely than not, especially with still some influences from the GSDM, then keep tabs on how far eastwards the main lobe of the tPV gets, while keeping a close eye on the amplification over the W and NW Atlantic and whether any small scale features potential immerge.

As things stand, a seasonal Christmas looks a good shout - A significant and widespread UK cold spell is, unlikely, but in this synoptic regime higher ground of the north can do very well, with low tropospheric heights, cold 500mb temperatures and 850mb temperatures of -5C or below can bring wintry precipitation to rather low levels.

An upgrade would be a more 'clean' N or N-NW flow, which remains possible, but that final third of December still looks like delivering something far more seasonal/interesting in time for the Christmas week and I feel it's just another starter before the 'main course' through Jan and Feb.

With kind regards, Matt.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975385

[bluearmy]

Ec op and control diverge on the trough 

ec op is in the smallest cluster 

ec control is closer to gfs (a day slower perhaps)

ec control remains troughy out to day 15 and even throws in a channel runner late on, ending with a mid Atlantic ridge 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975386

[Mike Poole]

This morning’s clusters from T192:

Not entirely sure what to make of them, so I’ll just make one observation.

I think what stands out is the T264/day 11 charts - there is consensus across the 4 clusters that the PV has moved east and drained from Canada and Greenland.  There has been this trend on the models to properly move the PV east for some days now, we’ve talked about it in here, but it has been uncertain - I think we are now in a position to say this is very likely to happen.  And it may bring with it cold and possibly snow to some in the UK.  But what is of interest to me is what happens next to the PV?  Does it regroup and head back home, in which case it is not clear much has been gained?  Does it remain displaced?  How do the MJO and AAM uncertainties play into that?  Does it pay us a proper visit (can’t discount this)?  What is the role of the strat, and the interesting developments up there (near SSW at the end of last 2 GFS runs), and how does that factor in to the positioning of the trop PV for the start of January?  More questions than answers at the moment.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975413

[Nick F]

All looks very nice and wintry on 12z GFS from Thursday next week right up to Christmas Day (when mild air invades from the west).

But there is the danger without the amplified flow over N Atlantic, with upper N Atlantic ridge further north into Greenland, that the strong and flat jet stream / sharp baroclinic zone from New Foundland to SW UK could spawn a few lows that could scupper cold getting to the south for long or altogether before Christmas.

We are very reliant in the south for the jet stream to get south of the UK. North looks best placed in this kind of set-up with low heights and more likely to get north of the jet for longer.

When there is not much amplification to work with, we are relying on the trop PV pushing S and E towards UK and Scandinavia to push the jet stream south of the UK. Tall order, but we may get lucky even in the south. Albeit it could be brief without amplification upstream.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975724

[WYorksWeather]

In relation to what some were saying earlier about the impact of the low pressure on snow chances, here's a GFS T+240 chart from Meteociel which shows the T850s and the height in dam (for anyone unaware, 1 dam = 10 metres).

Hypothetically, if this run did come off (I think it's doubtful it will), the snow chances would be very good, as we have around -7C at 1200m.

By comparison, when we're under a strong ridge of high pressure, for example at T+72 with the mild forecast for the weekend, we have the 850mb heights up to around 1600m.

Another factor that's worth considering if you're looking at snow chances.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975788

[Kasim Awan]

Usually in a showery flow there is a corridor of temporary accumulations where dew points are marginal, i.e. where accumulation forms in heavy showers and melts inbetween as the showers only temporarily drop the snow line. There will then be a limit to this where the dew point is constantly below freezing and does not rise above between showers, in this area snow will accumulate nicely. Stronger wind means more intense showers and more of an inland reach, but also much faster moving so less potential for evaporative cooling and more of a warming effect near the coast. So generally these flowers deposit more snow inland / at altitude unless the flow is very cold, cold enough to eradicate the warming effect such as 2013/2018. Contrary, slower flows tend to produce more sustained precipitation and colder temperatures allowing better low level prospects. Slower flows can also improve formation of troughs allowing for heavier inland snowfall, particularly in low heights. Though not too slow otherwise trough formation may not occur.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975751

[Kasim Awan]

1 minute ago, weathercold said:

?

Uppers are kinda irrelevant more about the type of flow heights / wind speeds.

By definition lower heights means the 850hpa level will be lower down in the atmosphere so 850hpa temperatures don't have to be as low. In very low heights the 850hpa level can be as low as 900metres, vs 1600metres in higher pressure. This means in low heights -4C is the absolute equivalent of -8C in higher heights. So using 850s alone is a very uninformed guide. It's one factor. The exception to that would be a showery flow in strong winds above averages of 20mph, where the temperature profile aloft then becomes the main factor for snowfall as other surface and thermodynamic operators become less relevant.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976006

[Allseasons-Si]

It's all about the scenario's on whether you get snowfall below elevation's:-

in a PM airflow it has to be below -5 uppers to get snow down to modest levels and the thickness dam below 528

a battleground scenario is slightly higher in terms of the uppers from the legassy of a cold pool in the vicinity where stagnent air has pooled to the surface so 0c uppers or lower would be required dependant on the dewpoints and wet bulb at the time

A continetal flow has dryer wet bulb and colder dew points due to less moisture track from the continent,this direction could be from the NE/E/SE.

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4975987

[Cold Winter Night]

The EPS 0z today has only one cluster in the extended, the familiar “we-just-don’t-know-cluster,” but inspection of the whole panel of individual members reveals some interesting hints.

After the initial move of the tPV East, nearly half of members indicate renewed ridging near the American Pacific coast/Rockies, generally pulling a trough away from the main vortex towards Canada, SW of Greenland. This American amplification in turn increases opportunities for ridging near the British isles.
To pick one as an example, p5:

Keep this in mind: In spite of the bad name they have, a solid tPV (or segment) West of Greenland and an Azores/Iberian High are actually essential starting points for serious cold spells (longer than 2 day affairs).
If you don’t believe me, look up the most severe cold spells in NW-Europe, then check the NH setup 5, 10, 15 days before, and you’ll see what I mean.
What it needs is amplification though, and that usually starts in the Pacific, so it’s good to see some hints of that deep in the fog of the extended.

It could be something that disappears again, but worth to keep an eye on IMO.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976326

[johnholmes]

Well some 3 days since I posted the NOAA and ECMWF latest charts, the 6-10 day issue takes us into the Christmas period. I wonder if they suggest a cold one?

NOAA 6-10 out to 23 December and it is showing a very strong westerly into the UK. This from a fairly marked trough off the west coast of north America, into a slight ridging east of this with the westerly over the rest of the chart into the UK. Heights around the Border region are on the lowish side at about 540 DM; (I might be 6DM out as the gradient is so strong making it hard to decipher the actual height). It also suggests, with a large temperature gradient in the east of n America, that there is a good chance of it being a windy and unsettled period in much of the UK. With fairly low heights then wintry type ppn from about the Peak District north on high ground and low ground in the more northern parts of Scotland would be probable. At this distance suggesting anything more detailed is a waste of time. The 8-14 shows 534 DM in the Border area so remaining on the cold side of normal and further disturbed weather likely. That is assuming these two charts are on the correct lines!

Turning to ECMWF, period 20-24 December, and it shows a 500 mb trough just east of the UK, extending SE with a strengthening north of west flow which has backed slightly and also decreased a bit in strength by Xmas Eve. The 850 values are around zero C for much of the country most of the time; the colder air -5C is predicted to be largely well into Europe even in the centre of the extended trough.

Both sets of chart do suggest colder than rather than above normal temperatures. So I’ll be watching how they both develop over the next  3-7 days!

Looking just above my post I see Mike is in about the same place as me!

Climate Prediction Center - 6-10 Day 500mb Outlook

WWW.CPC.NCEP.NOAA.GOV

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976321

[Mike Poole]

I am struggling to see the wood for the trees at the moment in the model output, to be honest.

Some kind of cold plunge seems very likely to coincide with the run in to Christmas, but the detail is not discernible, and one reason for that is any snow will depend on being the right side of marginal for many, and the models just can’t tell us that yet.

Beyond Christmas, I have some ideas (which I will post when I have more time) but I feel a little hamstrung because I can’t see a signal in the EPS that remains consistent in the clusters from suite to suite.

I am genuinely interested in a longer cold spell given the current unusual pattern over the hemisphere, the strat, the background signals, but we’re not close enough to it yet to work out exactly how or if it might actually come about.

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976317

[Derecho]

From the model output I can foresee three possible scenarios

1) We get a northerly and a wedge towards the Greenland area that helps sustains a colder pattern - can't see this happening. Looking at the previous round of GFS ens this is the closest I can find.

2) We don't get northern blocking but high pressure avoids Europe for a little while allowing for alternating periods of chilly and mild up to christmas or just after christmas. Potentially a stormy pattern and at least holds some interest as the cold shots will be sharper and pushing further south.

3) High pressure leeches into Europe and as a result mild air wins the battle in the south with fleeting chilly and mild spells in the north. This is the scenario we don't want but it could happen. Time and time again in recent years (and the cold spell at the start of the month) pressure has somehow found a way to increase over Spain or central Europe to end our cold spells early. SLP trends over time have shown that pressure has increased over the continent as our climate warms.

I often therefore sit on the fence when committing to a cold spell unless I can be sure heights over Europe won't be an issue.

It'll be difficult to see some decent cold for the rest of December if the outcome of this pre-festive period is a poor one so hopefully we can get 1) or 2) but prepare for 3)....

 

Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4976487