Model output discussion - The run up to Christmas

[Vikos]

I might be a bit of a troubled guy, but one of the most significant Temp for snow is above 850hpa (~1500m) height.

You need some good low 500hpa temps around -30 ~ -35°C for (convective) snowfalls.

Having those, the will mostly "punch" through any "warm" 850is pockets as it drags the cold air downwards. So yes, no need of very low 850is when you got relative cold 500s.

That's is for pure snowfall, sticking of fallen snow is another chapter with some other parameters like dew points, ground radiation, surface inflows etc.

[Ashme]

@inghams85 (sorry, don't know what this is. Can't delete it! )

Just for fun............like many on here, I lurk here for hours on end and never post. (Can't read models well enough to add anything noteworthy)

Does anyone else get so wrapped up in this thread that when the Mrs asks what's the weather doing tomorrow, you haven't got a clue!!!

But you can tell them the possibilities of 7 days time!! Haha

Great group, very informative, lots of bants and lots of pretty pictures (jokes)

Merry xmas to you all, hope we get some of what the models are showing (not all of it though!)

[Scott Ingham]

3 hours ago, mushymanrob said:

Yes, i think thats where these charts could go, theres a slight pressure rise to our north, troughing sinking south..  This winter really has great potential for some proper winters weather and its no exageration to think its going along similar lines to 78/9, 84/5 ..... and im no coldie... We are heading for a lengthy spell of cold unsettled...

 

Agreed. This will be historic imo for the length of below average temperatures. Im not saying a 63 (although potential is there with a ssw in our favour but thats def8nitely no guarentee) but a top 10 since the 40s is achievable 

[minus86BriJ]

2 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!

I’ve reported this post for being way too excellent, as a lurking layman these kinds of posts are pure gold

[Zak M]

I'm a bit late but @snowking, that was one insane post man. Thanks so much for taking the time out of your day to write something so informative.

Just found out that your post is currently the second most liked post on this platform. 

Edited December 22, 2020 by Zak M

[Scott Ingham]

2 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!

Excellent. This for those learning shows why its a load of rubbish to just look at 850s. Its so much more complicated.

Brilliab post mate well done

[Drifter]

Yes also thanks to Kris from me too.
 

Of course, I was just in the middle of writing something similar but deleted it when I saw yours*.
 

*if you believe that, you’ll believe anything 

[kold weather]

So I'm not sure anything needs to be added to what snowking has already said 

My best advice is 850 of -4 may well be enough, and perhaps even a little higher if we can get active fronts in the mix and/or a continental flow developing round any LP.

We have a decent cold pool coming down with this at height and reasonably low thicknesses. It's actually quite rare to get such a lobe like this straight down and still be intense. Usually they weaken which allows the upper pattern to warm up too much. Not the case this time.

I'd pay VERY close attention to any fronts that develop a secondary low, as per the ECM. IF that happens the eastern flank will feed in less modified air from Europe which will too us even more towards snow. On the eastern flank of that front could see some big totals if such a feature forms. Could quite easily see 6-9 inches in such a setup in favoured spots.

Beyond that abd I wouldn't want to call it yet other than to say unless we get a very west based -ve NAO which pulls in a southerly around Atlantic (close to the 06z op GFS actually), all other routes are cold, or very cold

 

[MATTWOLVES 3]

1 hour ago, sheikhy said:

Im actually getting pretty worried now cos all this is gona affect my morning jogs!!cant be running on the snow and ice

Firstly folks the Snowking post is absolute stuff of greatness.. Explanation and detail in that post is surreal..many thanks mate.secondly this post by Shaky also deserves praise...i would say Legendary status..the hardest cold lover on here by far,yet he's getting concerned about ice and snow causing him problems when he goes   Brilliant mate,and Decathlon are doing a cheap line in  running machines...get there before they enter Lockdown.. 

Pretty good output guys,and let's see where the 12s take us...And thanks for all your great contributions folks...its made an awful year so much more bearable.

Edited December 22, 2020 by MATTWOLVES

[Frosty.]

Here’s the updated snow charts etc from the GEFS 6z...so, it will become very unsettled after Christmas but also colder, cold enough for many of us to see some much loved snow! ️...snow charts not to be taken literally, at any range but they give a general idea that it’s game on for some wintry weather rather than the usual mild sw’ly mush!

[Slidergate '17]

Hi guy's, long time lurker on this forum but never felt the need to actually post as my knowledge is nowhere near the level to do so! However I feel compelled to thank @snowking for such an incredibly informative & helpful post !! A lot cleared up for myself there, much appreciated. 

Merry Xmas all , here's hoping for a much more seasonal period than previous years have produced.

[knocker]

There is guide on forecasting snow that has been lurking in the learners area for many years written by John Holmes who knows a thing or two about it. And I will just add that when dealing with 850mb temps make sure not to get the thermodynamic equations muddled

https://www.netweather.tv/forum/topic/23729-will-it-snow/

 

[Scott Ingham]

Lookiny through the models and background signals can anyone make a compelling case as to why or when the UK will warm up after the 28th?

If you actually sit back and think about it there is nothing in any model or teleconnection yo suggest it. 

Thats the mad thing about our current prospects. The actual length of time below average is forecast for.

When was the last winter we were looking at a below average last 10 days of December and January...

[mushymanrob]

1 minute ago, knocker said:

There is guide on forecasting snow that has been lurking in the learners area for many years written by John Holmes who knows a thing or two about it. And I will just add that when dealing with 850mb temps make sure not to get the thermodynamic equations muddled

https://www.netweather.tv/forum/topic/23729-will-it-snow/

 

great to see you posting m8.... ive even joined another site to read your informed and balanced comments.

[Daniel*]

Yes great post SK as you are one of old timers on here it’s nice to see you posting again, unfortunately I fear the worst for @TEITS I know he was in ill health, physical and mental, the last time he visited this forum was a year ago he would have been all over this, with his keen eyes on strong cold clustered ens.   I thoroughly enjoyed his posts, he wasn’t a fan of teleconnections so there were some tensions there sometimes, but he often went on based on experience and they were insightful. I pray that he is just not posting anymore but..  

Here’s hoping for some snowy weather so we can all be cheered up! Next week seems likely to throw up some surprises. 

[starstream]

@TEITShas been on my mind as well.

I hope he’s still with us, and not anything else .

apologies Mods but, it’s the season for thinking of others.

The old names who’ve been around for so long, are noticed when they go quiet. 
 

[That ECM]

I feel sorry for snowking, he’s got about 150 emails in his indox to get rid of Quality post

[sparky1972]

i love to see all the talk of snow and cold weather, but surely there must be a concern for the amount of rain thats due over the next cpl of days, are there any charts that show how much rain is due now the weather warnings have appeared, asking on behalf of the Wife as the horseyard and paddocks are close to flooding already and she is a little concerned.

[moorlander]

Being known as the weather nerd at work and trying to explain what i am seeing in the charts to my colleagues.  Has also always been a problem without losing their interest but now thanks to the wonderful well thought out post by @snowking i think i'll try again his way

[northwestsnow]

*Metoffice upgrade alert *...

Snow possible to lower levels in the period between Christmas and NY....

Eyes down for 12zs.  .

Upgrades please NWP !!

 

[ScottSnow]

1 hour ago, Scott Ingham said:

Agreed. This will be historic imo for the length of below average temperatures. Im not saying a 63 (although potential is there with a ssw in our favour but thats def8nitely no guarentee) but a top 10 since the 40s is achievable 

You must be looking at something different to me. I have been on here since 2004 and I could name 5 spells just since then that this WILL not compare to. Feb/Mar 05, Dec/Jan 10, Dec 10,  Jan 13, Mar 13 and Feb/Mar 18. 
There is potential for snow on higher ground but it’s not cold enough for disruptive low level snow.

I would love to be wrong but just don’t see it IMO.

[Sky High]

I'd just like to 2nd 3rd 50th 175th the comments made above regarding Snowking's post. To take the time and the trouble to set it all out in layman's terms is marvellous, so thank you.

Today, the surfboard has stayed afloat whilst it rides the waves of excitement unhampered by the tumultuous swell of disagreements. It's a joy and a pleasure to be here ladies and gents. Long may it continue. And may my knowledge grow in your warm embrace field of expertise. 

[sheikhy]

58 minutes ago, knocker said:

There is guide on forecasting snow that has been lurking in the learners area for many years written by John Holmes who knows a thing or two about it. And I will just add that when dealing with 850mb temps make sure not to get the thermodynamic equations muddled

https://www.netweather.tv/forum/topic/23729-will-it-snow/

 

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!!!

[V for Very Cold]

A work of staggering genius @snowking

how nice to see everyone support such a great and thoughtful work as well as be full of festive spirit 

 

“it’s beginning to look a lot like Christmas “ 

Edited December 22, 2020 by V for Very Cold

[That ECM]

6 minutes ago, Snow White said:

You must be looking at something different to me. I have been on here since 2004 and I could name 5 spells just since then that this WILL not compare to. Feb/Mar 05, Dec/Jan 10, Dec 10,  Jan 13, Mar 13 and Feb/Mar 18. 
There is potential for snow on higher ground but it’s not cold enough for disruptive low level snow.

I would love to be wrong but just don’t see it IMO.

We’ve got the meto on board, let’s hope Prince Charming comes along and gives you a kiss so you can awaken to the potential.