Model output discussion - The run up to Christmas

[Rexx]

29 minutes 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!

Long-time lurker here. Set up an account just to thank you for this cracking post! It's posts like this that really help people like me get in to the hobby.

[TSNWK]

Brilliant post @snowking we are truly blessed on this forum with the knowledge, the humor  and shall we just say the personality traits of some  this place has been a welcome distraction amongst the gloom, helped greatly that we have something of interest to look at...,

I hear reference to control runs every so often.. still not sure what they are about, their credibility? Can someone share some thoughts on this please for I’m sure I’m not alone

Edited December 22, 2020 by TSNWK

[Penrith Snow]

18 minutes ago, sheikhy said:

Seriously to hell with the gfs op if we thought that was good take a look at the control!!this is all happening at the best time of the year!!longest night shortest days!!!

Why oh why is the GFS Control Always colder than the op?

Its a bit like the ECM 240 always promising and never delivering

Andy

[onepingonly]

36 minutes 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!

This was a great informative post, thank you.

[Frosty.]

Just a quick one from me, the GEFS 6z mean becomes generally cold and very unsettled after Christmas and well into the new year, I imagine there would be snow, especially with elevation and further north but I think we would all be in the game...as for uppers (T850’s)...you can have blizzards with -2 uppers..which has been proven in the past...so, uppers, although the colder the better, don’t have to be super cold to enable a winter wonderland narnia!

 

[Old MET Man]

42 minutes 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!

Brilliant post! 

[Methuselah]

41 minutes 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!

Many thanks for that post, @snowking... It feels as if all the things I've learned piece-meal, over the years, have finally all fallen into place!

A post worth every one of its 69 (so far) 'likes'!

[Dazzer6666]

@snowking - long time lurker but complete novice here - can I just add my thanks for a fantastically well crafted and informative post. Outstanding. 

[Matt H]

46 minutes 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!

This is a fantastic post, thank you so much for taking the time to explain everything.

[NewEra21]

Everything is just setting itself up too far west, firstly next week and then even into fantasy land.

If everything can shunt a few hundred miles east we'd be in the freezer. Currently the coldest air is going into the Atlantic on each occasion. 

That Russian high hampered us at the beginning of December, and we're starting to see it do so again. The pattern just can't shift enough to the east, to let the coldest air sink down over us. 

Long term the Russian high may eventually reward us, but currently it's certainly not.

[Updated_Weather]

Got to mean something when you start seeing that rare snowflake symbol

[That ECM]

Considering this is T348 I would say it is a very nice mean. 

[Matt H]

1 minute ago, Updated_Weather said:

Got to mean something when you start seeing that rare snowflake symbol

It’s a shame that app is about as useful as a handbrake on a canoe though.

[Updated_Weather]

1 minute ago, Frosty Winter said:

It’s a shame that app is about as useful as a handbrake on a canoe though.

It’s the standard weather app for iPhones provided by The Weather Channel, 

On a serious note- early December snowfall I had, this app was the only one to suggest it’ll be snow rather than sleet or rain. 
but I never believe weather apps anyways especially regarding snowfall - good to see potential though.

[Battleground Snow]

The OP and control are very similar synoptically, but the difference for the UK is massive is terms of weather felt on the ground, I think there will be some emotional highs and lows over the coming days until this comes into the reliable,

Btw good post snowking, hopefully will stop the relentless over analysis of the 850mb temps at range.

 

Edited December 22, 2020 by Battleground Snow

[Kasim Awan]

1 hour 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!

Good explanation of the role of heights. An added bit from me for those newer to snow forecasting, on winds.

The thermal gradient through the atmosphere means that temperatures cool by around 2/3C for every 1000ft gained vertically. In winter however, there is a strong tendency for this to be "cancelled out" near the surface and cooler temperatures to be parked up here. 

In a situation with lower heights, winds are much lighter. This effectively allows the surface cool layer to remain in situ without this "mixing out". In the event of mixing in stronger winds, the non-inverted air aloft becomes mixed with the surface inversion cancelling it out. It also prevents adiabatic cooling - the process responsible for inversions.

This means the snow when nearing the surface will melt as the air gets warmer. Let's take an example with -5C uppers at 300ft. This would give a surface air temperature of 3.75C. Too mild.

Now. Imagine winds are lighter. Adibatic cooling of the surface has been allowed to develop. There is a cold layer up to 1000ft which has not been mixed out either. Temperatures between 0 and say 600 metres are 1C dew points 0C. A weather front with uppers of -3C pushes in. The layer between 600m and 1500m (uppers level) is saturated due to cloud. This means that the temperature only increases by 0.5C per 100m due to "saturated lapse rates". So at 600m our air temperature is 0.5C. So it's wet snow at 600m.. Now the snow hits the inversion. Stays as snow. Add in a bit of evaporative cooling and the air is easily below 0 throughout 

So you can start to see how -3C uppers can produce snow and -6C uppers rain depending on synoptics

Edited December 22, 2020 by Kasim Awan

[LRD]

Great post from snowking. I look forward to the replies from those who have been banging on about upper air temps being too warm for snow to challenge that post if they are so confident in what they say. Or to acknowledge they may have been wrong. I won't hold my breath

Having said that, of course, there are no guarantees that snow will fall next week (especially in my locale) but there is a chance. The weather will be cold and unstable. I believe some places, especially north of a line from Birmingham to the Wash, will get a quite significant snow event.

Alas, for me, I may have to wait a little longer but I'm still not ruling anything out (or in) here either

[Dave Kightley]

I think we can expect a mix of rain, sleet and snow throughout next week.. very hit and miss in this set up but better then Westerly nonsense 

[Snowonder]

1 hour 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!

For someone who is trying to learn and understand charts etc can I say that this post is amazing so thank you. I know it must have taken a while to put together but more posts like this showing and describing what the chart means would be so helpful. Please all keep up the hard work, may not post much (had to rejoin) but I read everyday and am slowly understanding different things. Always think the weather can surprise at almost any time and looking at the next 7 days I suspect some surprises may come about. Have a great xmas to all  

[fromey]

Thank you @snowking for a well written post that everybody can easily understand, your thoughts are backed up by some on Twitter including Matt Hugo, I hope this puts the 850/uppers debacle behind us once and for all so we can get back to enjoying the charts which are currently being produced! And hopefully in the not so distant future enjoy some of the white stuff falling from them.

[markw2680]

1 hour ago, Severe Siberian icy blast said:

What a fantastic post, showing your worth there my old boy

I'v came out the shadows after 2 years, I allowed my personal life to leak through and became nagative rather than constructive, so for the good of the forum I took a break! 

So after, a marriage break down, a mental break down, a bankruptcy, a house loss, I'm back lol, happier it's all over and as a single lad and know nagging wife behind my back I can come back and pick up my obsession of winter cold!!!! 

Promise to add meaning and illustration to my posts moving forward to a put a smile on you all and hopefully bring a 3 wk period of severe winter weather with disruption to travel

Good to see you back sister, may I say things are looking pretty impressive around the new year, let the upgrades start

[Vikos]

Who remembers?

 

Edited December 22, 2020 by Vikos

[BlueHedgehog074]

1 hour 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!

Dude, your post absolutely rocked

Concur with the other responders of this thread. Really helpful and well explained, so much insight. The way you showed how the importance of low heights at the 500mb level can mean not having to have particularly cold 850 hpa temperatures at times for low level wintry weather. I learnt some new aspects from that post. It just shows the 850 hPa temperatures aren’t everything. Indeed, there are several factors that determine if it will snow in your area such as evaporative cooling, wet bulb temperatures, dew points, slackness of flow etc. 

Some of the more exceptional spells of weather like December 2010, and the late February/March 2018 Easterlies, I certainly feel as though had some affect on expectations - the fact that mega cold 850 hPa temperatures aren’t required for falling, and even settling snow. Not that I believe everyone on here thinks that very cold 850 hPa temperatures are always needed for snow, as reading through numerous discussions on here, I know that’s not the case  

With the testing times we’ve been going through, I can in a way understand why there’s been quite a few getting hung up on certain aspects of the charts (such as the upper air temperatures) as I guess a lot of us are eager to see that first flake falling. Which fortunately there’s still plenty of time for not being that far into Winter yet. And looking at the models, next week does have real possibilities to produce some surprises. No doubt, the higher up you are the more likely to see some white stuff, but falling and/or settling snow could quite easily be possible lower down, too.

Nevertheless, your post is still a great reminder for some of us not to (often) become too concerned/focused about the 850 hPa temperatures. To be honest, when we start up the next Model thread I’m going to quote your handy post making reference to it for that very reason, and just generally in regards to some of the various parameters for snowfall. Of great use I’m sure it will be

It’s what’s so cool about sites like this. Lots of varied and great contributors with varying levels of knowledge that make this a great place to be

Edited December 22, 2020 by DiagonalRedLine

[northwestsnow]

Just to add to the others, thanks very much Kris 

Not just for sharing your knowledge, but for the time and effort involved in sharing your knowledge 

 

[beefy201]

1 hour ago, Severe Siberian icy blast said:

What a fantastic post, showing your worth there my old boy

I'v came out the shadows after 2 years, I allowed my personal life to leak through and became nagative rather than constructive, so for the good of the forum I took a break! 

So after, a marriage break down, a mental break down, a bankruptcy, a house loss, I'm back lol, happier it's all over and as a single lad and know nagging wife behind my back I can come back and pick up my obsession of winter cold!!!! 

Promise to add meaning and illustration to my posts moving forward to a put a smile on you all and hopefully bring a 3 wk period of severe winter weather with disruption to travel

Good to see you back old mucca sorry to here you've gone through such a horrid time ive been there so know how it feels....looking forward to your informative and excitable inputs