Winter Approaching: Strictly Model Discussion (and highlights)

[Mike Poole]

Looking into the medium term, first here’s the clusters T192-T240:

Clusters 1 and 2 both suggest an end to the cold spell with westerly winds, and a burgeoning Euro high - this option was there this morning, but the likelihood has increased to 37 members in both.  Clusters 3 and 4 keep us in the cold trough.  All 4 clusters retain some heights over Greenland.

T264+:

Cluster 1 here (20 members) maintains the cold and Greenland block - I’m not sure how this follows from the previous timeframe, the numbers don’t add up!  Cluster 2 Atlantic dominated (16 members).  Clusters 3 and 4 make a decent attempt at a Scandi high by slightly different routes.  The minority (for now).

ECM 46 dayer heights:

It is worth noting there are above average heights at high latitudes through much of December on the 46 dayer.  Meanwhile, the strat vortex above runs unfettered it seems, but there looks little prospect (to me) of a strat/trop coupling.  

The regime chart pretty much trends to steady probabilities by 15 days, with Scandi blocking favoured, +NAO a constant possibility (<25%) and Atlantic ridge -NAO also in the mix.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160102

[Mike Poole]

Here’s the track of the centre of the low for the EPS at T84 on the 12z suite:

Contours are the control run, and the blob with the yellow ring on it is the control.  Interestingly, there appear to be a good smattering south of the op/control run.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160107

[bluearmy]

Ref yesterday evenings post, Eps postage stamps have reduced the chances of Monday nights low bringing snowfall to the n Home Counties. of course have to wait for the 06z update to make sure it isn’t a dodgy suite. that should theoretically close the argument that it may be a v weak feature that trundles across in the base of the trough.

looking forwards to the last week of November, (and avoiding the hooha on the sou’westers possible as the Atlantic pushes across), the ec AI 18z run looks a very good representation of the three latest ens suites to day 15.  We are very much not done with n Atlantic blocking but we could do with losing euro heights although they won’t necessarily be a problem re frost and lower than av temps. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160398

[Blessed Weather]

A few thoughts on what a couple of the key teleconnections are suggesting as we move into December.

sPV
The sPV is forecast to gather in strength and according to the latest update (11th Nov) and chart from the Atmospheric and Environmental Research group (AER) it is likely to couple with the tPV as we head into December, quote:

“Negative Polar Cap Heights are predicted to expand into the mid-and lower stratosphere and then eventually all the way to the surface. This suggests coupling between a strong stratospheric PV and a positive AO that favours an overall mild pattern across the NH.”

Daily polar cap height (i.e., area-averaged geopotential heights poleward of 60°N) anomalies:

Source: https://www.aer.com/science-research/climate-weather/arctic-oscillation/

The latest Zonal Wind forecast from stratobserve shows the sPV westerly winds strengthening and gradually descending through the atmosphere:

Source: https://www.stratobserve.com/anom_ts_diags

MJO
Arguably the precursor to the current synoptics were at least in part triggered by the MJO tracking through Phases 6, 7 and 8 during the last week of October and first week of November (then adding the typical lag of 12-15 days). The MJO is currently close to (or in) the COD in Phases 1 and 2 where it is too weak to exert any influence on our weather patterns, but from around the 20th of November forecasts predict the MJO will move through Phases 3 (Indian Ocean) and 4 (Maritime Continent) at reasonable amplitude and composites show this likely to bring about a positive NAO with mild and unsettled weather (after the usual time lag) by the start of December. Latest forecasts:

GEFS Extended:  ECMWF Ensemble:

Typical influence of MJO Phase courtesy of Met Office, Marco Petagna:

 

Possible developments
The good news for those who like colder weather around Christmas is the above RMM forecasts suggest the MJO will make good progress back to Phase 6 by as early as the 8th December. IF progress continues into Phase 7 (Western Pacific) then a return to northerly blocking (GH high) would be nicely timed for around Christmas/New Year period.

Similarly, although the sPV is forecast to continue growing in strength and couple with the tPV, over the course of a normal winter season there is often a number of coupling/decoupling occurrences, with decoupling in particular as the sPV becomes disturbed by atmospheric planetary waves from the troposphere. ( @joggs sorry for delay responding on this point). There are certain tropospheric weather patterns, such as low pressure over the North Pacific and the high pressure over the North Atlantic/Europe, that generate wave activity into the stratosphere.  And the stronger westerly winds with an established sPV are more accommodating for atmospheric planetary waves (which can be 800 miles across) to move up from the troposphere into the stratosphere and 'break' causing disruption.

So it’s interesting to see a couple of members of the GEFS pick up on a sudden weakening of the sPV zonal wind strength in the latest forecast. Has recent Wave 1 activity had any influence on this?

Current Wave activity:

Source: https://www.stratobserve.com/anom_ts_diags

Latest GEFS sPV forecast:

Source: http://www.weatheriscool.com/prod/interactiveTserie.html

In summary, there are good indications of a period of unsettled and milder weather as we move into December, but also some early indication that this situation may turn-around to blocked/colder by end December. As always, the behaviour of the sPV a wild card in our prospects.

[Mike Poole]

 Blessed Weather Interesting.  I do feel with December, we are likely to get caught up in a race against time, between the possibly inexorable coupling of the strat and trop vortexes, and the progression of the MJO towards the favourable phases.  

If the strat vortex wins the race, the MJO may end up having little effect or getting flattened into the COD at just the wrong time, if the trop vortex can remain disconnected, then the MJO cycle might just provide the amplification needed to develop a blocked pattern in the second half of December.  Could be a close run thing.

 

[captaincroc]

I would recommend anyone who is confused about the models struggle with this low, to watch the Friday Met Office Live. They talk about it in great detail, basically there is the main jet stream and mini scraps of offshoots too. This low is forming amongst them all which means it could stay on the warmer side of the jet and not develop or move over to the cold side and develop. TINY differences early on make BIG differences later on. That's why it is so volatile as the low is just forming and it's how it interacts with the many parts of the jet will massively effect it's development. We should know very soon but until that we shall be in this limbo.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160676

[minus10]

 IanTlooking at the eps you can see this in the spread over the shorter term period of the low and then the reduced spread in what follows with the straighter line over the next week where it is colder...

..so yes i think there are situations where short term is more uncertain than longer...this is also referred to sometimes in the met's deep dive...

 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160624

[Blessed Weather]

 Mike Poole Hmm. Got me thinking there Mike. I'll see if I can dig out some research to give a definitive answer to the question you raise.

[Daniel_c150311]

Judging by the current area from where the low pressure is at the moment it looks as if it’s forming more south of where it should be at the moment so I reckon the centre of the low pressure will be going over south wales down to Kent bringing more widespread snow above 200m> 

 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160672

[IanT]

 captaincroc I don’t think it makes sense to say that the models are “struggling”. Rather there are many potential outcomes from the current set of stating conditions, and model output necessarily reflects this. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160689

[Derecho]

On a more formal note, I've refrained from commenting on the model output in the last few days as I was expecting a period of high volatility.

Still that volatility remains, after Tuesday's messy event we could get some wintry little features in the cold air behind its wake. I'm more confident in this then I am with the events on Monday night / Tues

 EC

GFS

Hopefully something forms! Otherwise I'll be just looking glum at the cumulus clouds passing by to my east in the North Sea lol.

Aside from that though the rest of the forecast is very tricky. Personally I think the low on Monday will pass towards the south of the ensemble average, it tends to be the case. Ideally for snow chances here we'd want the low to take a WNW to ESE path to allow cold air to the north to dig in ahead of it.

If I am in the path of the low here in the NE I expect the situation to be very marginal.

Then next weekend looks highly uncertain, I feel like I'm a spoilsport by saying the milder air will push through at ease but I'm less confident of that today.

We need a shallow block to our NW and a disrupting low as it approaches our shores. The AIFS is more akin to what I'm hoping could happen....

The cold remains nearby for longer because instead of a pressure rise over Spain during the weekend, the high quickly moves east and builds there instead which allows for low pressure to dig further south rather then ploughing through like a knife through hot butter (as we have seen so many times in recent years).

 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160771

[sheikhy]

 northwestsnow literally at the last moment i MAY have nicked the snow!!!!!if 12z gfs remains the same then its game on!!!

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5160839

[Blessed Weather]

3 hours ago, Mike Poole said:

Interesting.  I do feel with December, we are likely to get caught up in a race against time, between the possibly inexorable coupling of the strat and trop vortexes, and the progression of the MJO towards the favourable phases.  

If the strat vortex wins the race, the MJO may end up having little effect or getting flattened into the COD at just the wrong time, if the trop vortex can remain disconnected, then the MJO cycle might just provide the amplification needed to develop a blocked pattern in the second half of December.  Could be a close run thing.

Mike - I've found an interesting blog from the World Climate Service that seems to confirm your thinking that the best result (for blocking/cold) is when the MJO cycle reaches Phase 7 whilst the sPV is weak:

"....it’s the combination of a weak polar vortex with MJO phase 7 that produces the strong cold signal for Europe. For instance, past occasions with strong MJO phase 7 and a strong polar vortex have warmth in Eastern and Northern Europe...."

The full blog is worth a read.

MJO Forecast and Stratospheric Warming: Impact of Extremes.
Summary: Historical analogs point to the likely impacts of a weak stratospheric polar vortex and an amplified MJO forecast.

 

[Mike Poole]

 Blessed Weather That’s interesting, I’m not sure I can recollect any exceptional cold in 2021, but the principle seems sound.  It may be simply that with a strong strat vortex, coupled to the trop, it is difficult for any driver to promote significant amplification in the Atlantic sector, which is maybe one reason why the MJO has form for failing to deliver in the heart of winter.  The likely timescales may make it a close race this winter, I think.

[Kasim Awan]

UKMO/UKV South.

This is a classic Nottinghamshire, Derbyshire, Staffordshire, Cheshire, Manchester, Lancashire, Yorkshire event. Reasonable low level accumulations below 200m down to near sea level of 1-8cm in these areas. Allow for sea air influence near the east coast to widely stop accumulation within 10 miles of the east coast, and significantly reducing accumulation within 25 miles of the east coast. This will produce a linear increase in snow accumulation inlandward. Accumulations of 5-25cm are more likely above 150m asl in the central region as air adiabatically cools, increasing precipitation to snow ratio. A moderate foehn effect will establish around the central east Cheshire and Greater Manchester lowland region. This will likely reduce average precipitation intensity and increase surface dewpoint resulting in lower snow accumulations and potentially only slight accumulations in Manchester City. However this effect is likely to only be moderate therefore some snow accumulation is likely in these areas. This will quickly be phased out west of the M6 resulting in better snow accumulation towards west Cheshire and west Lancashire, before a slight reduction towards the coast, however not as significant as an effect as the east coast. Currently confidence is strong for snow in these areas. The above scenario at a 75% likelihood. The southern and northern extent of precipitation is still uncertain. The snow gradient will be steady on the southern extent, resulting in linear gradient full accumulation to slight accumulation within a 40 mile distance.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161026

[johnholmes]

Perhaps the main reason, despite deepening, that the low on UK met charts shows little sign of turning left of its track is that the feature is actually not quite in what is called the left exit area of the jet. There seems two parts to it on the GFS charts and the low is somehow between the two highest speeds.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161058

[Kasim Awan]

 The Northern Ramper It's more of a wind shield effect than urban heat island. Easterly winds descending result in drier air and warmer dew points. This will be present however wont be as much of an issue this time, the main issue for lower levels will indeed be precipitation intensity and surface temperatures. Also further east the effect of sea air may be significant. However that northern flank looks adequately cold and intense for significant accumulation to lower levels. At first this will be moderated by a slight foehn effect and sea air, however the precipitation looks more than prolonged and intense enough for significant snow to lower levels. 

In terms of the 850s debate and freezing heights, in this scenario -1c at 850hpa and a freezing level of 250m is required for snow. This is because of the release of latent heat into the atmosphere in humid conditions which reduces the lapse rate going towards the surface. This equals less cold 850s required. Also consider evaporative cooling which transfers heat vertically, this reduces temperature near the surface. This can make the temperature at 850hpa and at ground level the same. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161277

[Met4Cast]

Regarding the longer range; 

It seems inevitable that we’ll see a period of +NAO driven weather, how/exactly when that comes about regarding the breakdown of next weeks cold spell though remains to be seen, the EPS are fairly evenly split between driving milder air in across the UK & prolonging the cold, the EC det ends up with a cold high sat across the UK for example; 

This is where teleconnections aren’t necessarily helpful & short term meterogology takes over. The continued disconnect between the strengthening stratospheric vortex & the troposphere is interesting & models don’t currently have much of an appetite to develop a strong tropospheric vortex over Greenland, even the zonal keen GFS keeps weak heights in the area to the end of the run on the mean. 

This suggests it’s plausible that we see a south shifted jet through the rest of November & into early December, that’s not to say it will be cold but it does present a question on the potential for a battleground scenario setup beyond next weeks cold spell with further bouts of cold, albeit not necessarily sustained. With this comes the potential for some stormy weather & deep lows..

The MJO is continuing to transition east through the Indian Ocean, as a result AAM has been falling. 

The inflection point where AAM started to fall is what triggered the UK high to move west, the jet stream hit that road block, amplified & next weeks cold spell was born. 

Running the clock forward you’d expect the MJO to progress through the Maritime continent triggering a rise in AAM tendency before the MJO continues into more -NAO phases putting mid-late December the next “teleconnectively” period of interest for potential blocking, depending on the behaviour of the sPV & tPV by that point. 

Lots to play for..

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161332

[Derecho]

 WYorksWeather I don't think uppers are really relevant though there is often a relationship between uppers and surface temperatures. It's a combination of air temperature and dew point. As a very basic rule, If air temperature + dew point is zero or less you are good for snow, a degree or 2 higher and it is uncertain.

As long as the uppers aren't above zero then the rule above holds, if you have air temp + dew point below zero with uppers above zero you run the risk of freezing rain. This is most likely after a spell of deep cold and is more common on the American continent.

Often in March when you get very cold northerlies you can see snow falling at higher temperatures as dew points are often lower.

With heavy rain you need light winds, the rain cools the atmosphere and if the winds are light enough that cold air builds in situ, it's a balancing act between the intensity of the precip and the wind. 

I'm glad I never came across an evaporate cooling event when roads forecasting 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161314

[Mike Poole]

This evening’s plot of the ECM EPS centre of the low at T60 now:

More tightly grouped, and I think a little further south, but little sign of anything as far north as the UKMO solution.

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161350

[sheikhy]

Im so nervous right now looking at gfs 18z that im farting soo much!!!!IBS farts!!!

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161463

[bluearmy]

Ec 18z pretty much the same as the 12z run. Possibly a smidgen south but nothing in it 

snowfall same as the 12z - a little further south with the back edge stuff 

looks better to our west at day 6 than the 12z run with cold trough further south 

to bed!

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161734

[Nick123]

 That ECM you're killing me God Imma need a chill pill soon

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161938

[Kirkcaldy Weather]

 

[That ECM]

 Ali1977 it does tend to flip a bit. great to watch though. For those wanting to learn it’s a great run to view and see why it gets to where it does. 

Original post: https://community.netweather.tv/topic/100917-winter-approaching-forecast-and-model-discussion-commentary/?do=findComment&comment=5161945