Model output discussion - The run up to Christmas

[northwestsnow]

Dont want Icon to verify!!

Too far west with the troughing..

[knocker]

31 minutes ago, sheikhy said:

Look what the cat dragged in!!!long time no see knockers!!!!great to have you back mate!!hopefully you join the cold camp for now!!!

It's a Cornish Rex and only entertains tasty morsels

[Scott Ingham]

8 minutes ago, goosey007 said:

 

Agree Scott ....totally understand we all have our opinions on the model output ...but to say this upcoming cold spell wont be comparible to those dates is some what premature perhaps...excluding Dec 10. If you are convinced its not going to be back it up with some data/charts and explain why it wont be at least. Its confusing for newbies otherwise. Thanks

Yeah agreed. Ive backed up my opinion.

Edited December 22, 2020 by Blessed Weather
Minor change made.

[Kasim Awan]

1 minute ago, northwestsnow said:

Dont want Icon to verify!!

Too far west with the troughing..

We are seeing a push west in some output. Could be an emerging trend hope not.

[Skullzrulerz]

2 minutes ago, knocker said:

It's a Cornish Rex and only entertains tasty morsels

When is the Beast From The West coming? XD

[Scott Ingham]

14 minutes ago, V for Very Cold said:

If you’re wrong.... it’s Rotherham high st pt 2.... cartwheels! 

Hahahaha okay agreed!

[Battleground Snow]

This is what could put us into a west based NAO, Russian high definitely foe in this set up, backs the pattern west.  Need it to do one east or north

[MAF]

Thanks for pinning the Snowking post Paul  

@Paul

Edited December 22, 2020 by MAF

[mountain shadow]

22 minutes ago, Scott Ingham said:

Agreed. Im taking a back seat and watching it unfold. If im wrong ill admit im wrong

You post about fifty times a day, hardly back seat (I'm joking)

[ALL ABOARD]

Yes further west on the icon. 

Not so good for the initial plunge. 

What it does do though as is evident on the icon, is allow the Atlantic ridge to remain in situ and leave small heights further north that would likely allow further sliding scenarios. 

All in all a great chart for December. 

 

 

Edited December 22, 2020 by ALL ABOARD

[Weathizard]

That’s a bloody impressive lack of cold pooling anywhere near us Too many winters I remember saying how annoying it was having deep cold uppers so close by and never being able to tap into them.

What is causing the lack of real cold in Europe? Russian ridging?

[Kasim Awan]

2 minutes ago, Battleground Snow said:

This is what could put us into a west based NAO, Russian high definitely foe in this set up, backs the pattern west.  Need it to do one east or north

We'll be under a pizza slice by the 00z lol

[Scott Ingham]

7 minutes ago, mountain shadow said:

You post about fifty times a day, hardly back seat (I'm joking)

Im happy to post less hahaha

[Updated_Weather]

16 minutes ago, Scott Ingham said:

I have no idea mate. Its childish and patronizing. Im on exactly the same page as you. I wouldn't be so keen if there wasnt so much evidence to back it up

I wouldn’t take it too personally Scott!

we are all grateful for your input

[NewEra21]

Icon in terms of cold is terrible. We really need to start seeing this trough take a shift east. 

[Howie]

I really hope the west based -nao signal diminishes

[ScottSnow]

17 minutes ago, Scott Ingham said:

Im talking avout the length of time of below average temperatures. Dont just look at next week look at what could potentiaply follow. 

No way mild air gets in before the middle of january at least. Backed up by the met who have forecast below average temperatures up until at least the middle of the month. The next round if amplification looks like being colder than next week and snow is looking increasingly to lower levels next week. I may be wrong and ive put my head on the line and if i am ill hold my hands up!

I hope you are right but I just can’t see it but fingers crossed!

[Scott Ingham]

2 minutes ago, Updated_Weather said:

I wouldn’t take it too personally Scott!

we are all grateful for your input

Thanks mate appreciated. Ive called what i think and well see what happens. The patronizing laughing emojis and lmaos tho are out of order. By all means if im wrong im wrong but this is my opinion and shouldnt ve rediculed really on an adult forum

[PolarWarsaw]

Obviously only worth a watch at the moment, but there's been a trend to send things west so far today.

Decreases the chances of a low land surprise as the uppers are even lower/more marginal. 

Even more worryingly there's more than a few west based NAO's starting to show, that would really sum the start of winter up if we manage to get there. 

Not good signs in the short or medium term today. Let's see what the strat charts have to offer. 

[snowbob]

1 minute ago, Scott Ingham said:

Thanks mate appreciated. Ive called what i think and well see what happens. The patronizing laughing emojis and lmaos tho are out of order. By all means if im wrong im wrong but this is my opinion and shouldnt ve rediculed really on an adult forum

Keep up with what you do mate. people will never agree on things. Human nature. Opinions are opinions and should be posted. No one knows what’s going to happen, so no right or wrong prediction technically. Hoping for a great 12z though I must admit.

[mountain shadow]

8 minutes ago, Scott Ingham said:

Im happy to post less hahaha

Post as much as you like. i laughed when you said you were taking a back seat, when you're actually driving, that is all.

Carry on folks, there's slush to be hunted down.

[WinterWilf]

4 hours ago, snowking said:

There's been a lot of focus on 850mb temperatures over the last few days and will they/won't they support snowfall, so just wanted to create a post for any of the less experienced members who are probably left scratching their heads about the conflicting views amongst different members and want to know what to believe.

Let's start with why everyone looks for the -6c 850mb (or hPa) line and it's a bit of a history lesson I'm afraid. I believe in terms of these (and other) forums, the magical figure of -6c was really coined as the "snow line" back in the bad old days of the late 90's and early 00's over on the old BBC Snowwatch forums, when things were so desperate that we were crying out for any sort of 2 day northerly toppler just to look forward to (which is why I do find it amusing that some members can't see the potential in the upcoming period, I think unfortunately December 2010 distorted the expectations of some). Anyway, because back in those days we were primarily looking for Northerly topplers for any sort of cold, given an almost complete lack of Easterlies between 1996-2005, the 850mb temperature needed to be significantly below 0c for the airmass to be conducive for snow falling (this is a bit of an oversimplification, but let's keep it light!). Why? Well in the lower part of the troposphere we generally expect that as we travel from the clouds to the surface of the earth, the temperature increases. However we also have to remember that cold air falls, and so what we expect is the colder air from the 850mb level to fall down closer to the surface of the earth over time. The rate of this temperature increase (or "thermal gradient") varies hugely due to an overwhelming number of factors, however if we consider this in a simplified form again, the answer lies, for us at least, in the oceans.

Because from a North or North-Westerly airflow the air is travelling a long distance over a generally warm Atlantic ocean, there is more likelihood of warmer pockets of air close to the surface of any airflow from the N/NW cancelling out some of the colder air dropping from aloft than there would be from say the East, where the air is travelling a far shorter distance over the North Sea and so is less likely to include these warmer pockets of air. These warmer pockets of air can affect many of the parameters that we won't go into in this post, but the two that are worth calling out as they are mentioned a lot are the 2m temperature and the 2m dew point. 

So if we try and summarise the above two paragraphs, in a N/NWly airflow the air is travelling over warmer oceans and so warmer pockets of air can be expected to be found closer to the surface. To override these warmer pockets of air we need the cold air aloft (at the 850mb level) which falls down towards the surface of the earth to be cold enough to override these warmer pockets of air and make the air from cloud to surface cold enough throughout to support all of our lovely snowflakes falling from the clouds remaining as snowflakes rather than falling through a warmer pocket (or layer) of air and melting into horrible rain, which from experience is much more difficult to make snowballs from.

I think the primary reason we chose the -6c line was that back in those bad old days where Wetterzentrale was the choice of most members to consume their daily dose of GFS from, the dashed isotherm line which indicated the 850mb temperatures were spaced out at approximately every 5c (although for some reason I seem to remember Wettzentrale often showing -6c instead of -5c as one of the dashed isotherm lines, correct me if I'm wrong if anyone can remember), and so we would often look for the -5/-6c isotherm on the 850mb temperature charts as our guaranteed snow line, as it allowed for enough headroom for some warmer pockets of air at the surface to be overridden by the falling colder air from above. At this time I think it's fair to say that none of our collective weather knowledge was anything like it was today, and so looking for something simple like a single "snow line" helped us in our search for snow.

 

So, on to the next part, what exactly is the 850mb (or hPa - they both essentially mean the same thing) temperature chart. Again I will keep this as simple as I can, so apologies to anyone offended by the oversimplified statement I may be about to make. Well to answer the first part, it's exactly what it says - it is the temperature of the air at the point in the atmosphere where the air pressure is equal to 850 millibars (mb) or hectopascals (hPa). But the exact height above the surface of the earth of where pressure is equal to 850mb can change, and that is what all of the pretty colours on the "height" charts we see are. So we've been talking about the 850mb level of the atmosphere, let's take a jump to the other common charts we see - the 500mb charts, such as the one below:

The colours here represent how high above the earth the 500mb pressure level is. The more towards blue/purple the colour is, the lower the height (or closer to the surface of the earth) the 500mb pressure level is, and vice versa for the greens/yellows/oranges. This also has a knock on effect on how high/low the 850mb pressure level below it is.

So what we are saying in the context of our upcoming cold spell is that under that huge low pressure system, the height above the surface of the earth of both the 500mb and the 850mb pressure level is lower. What this means is that the colder air aloft, at the 850mb level, does not have as far to fall down towards the surface, and therefore more colder air is able to come down and help override any warmer pockets/layers of air towards the surface. This means that we don't necessarily need the 850mb temperature to be below -6c to support snow falling to the surface. We also then need to bear in mind that as that low pressure falls further South, and we change our feed of air from being from the North/North-West across the Atlantic ocean to instead coming from the East from the near continent, fewer of these warmer pockets of air are likely to exist.

Just to give you an example of this from the most recent 6z run, here are two 850mb temperature charts, the first one at +138 hours, and the other at +168 hours:

It would stand to reason that if it was as simple as colder 850mb temperature = colder surface temperature, then we would expect to see a lower 2m dew point temperature in the corresponding +168 chart right? Well see for yourself:

You can clearly see that despite the 850mb temperature being 1-2c warmer at +138 than at +168, the dew point temperature is 1-2c lower at +138c. 

You can again see the reason for this by looking at the 500mb height charts:

We can see here that the lower heights at +138 aid the cooler 2m dew point temperatures.

So in summary for what is a very long post, you do not require -6c or colder 850mb temperatures, even in a North-Westerly airflow sometimes, in order to guarantee all of the relevant parameters for snow being the right side of marginal, it is significantly more complex than that, and that is why the upcoming period could well deliver a lot of surprises - the word could being crucial in there!

Thank you for such a great post explaining and dumbing down things so people like me can learn. I come here every winter and read the model forum every day and try and learn about what to look for in charts and gain knowledge rather than being dumfounded and confused. I never post just in case I say something stupid. Just lurk reading everyone's thoughts. Posts like this really help brilliant.

 

I read this post on page 38 or 39 and commented then went back and read from there always do after work...

Having caught up now it looks like I was jumping on the bandwagon haha. 

I did not even see it pinned up at the top.

 

Edited December 22, 2020 by WinterWilf
Other thoughts

[Battleground Snow]

GFS looks a smidge further east at the early stage to calm the fear 

[Eskimo]

The GFS 12z drops that trough slightly earlier which may indicate a westerly shift. 

EDIT: My post now contradicts the one above 

Edited December 22, 2020 by Eskimo

[Kasim Awan]

 

20 minutes ago, Weathizard said:

That’s a bloody impressive lack of cold pooling anywhere near us Too many winters I remember saying how annoying it was having deep cold uppers so close by and never being able to tap into them.

What is causing the lack of real cold in Europe? Russian ridging?

You need to be creative with meteorology. If you look closely the UK is actually exceptionally cold.

Edited December 22, 2020 by Kasim Awan