Will The Mild Air Win Out And When?

[johnholmes]

below is the pdf which I hope will help explain the problem

The thickness change equation-re 20 dec onwards.pdf

[Stuart]

Thank John for that :smiliz34:

[NL]

Understandably a job I'd not want to undertake..!! Thanks John for a small peek at the huge task the senior forecaster and his/her team have to do, hope he/she has all available and up-to-date data at hand and comes up with the likely outcome whether it be mild/cold.

[Guest]

Thank you John for your explanation, though the maths are far beyond me.

In the winter of 62/63 I was a humble scientific assistant at Heathrow when we had the situation on a number of times when the warmer Atlantic airs made an approach but beyond getting as far as the far SW peninsula of the UK they were really unable to penetrate to any great extent. I think we had one occasion where a warm front approached from the SW to be preceeded by snow and the precipitation to remain as snow behind it. This appears to have been a rare occurance and brought some gasps of amazement.

If my memory serves correct the final break through came from the north west. For most of the time North West Scotland (Stornaway logged up quite a few +4C's) was pretty mild in comparison to SE England and it seemed to me at the time that the attack was coming from the weakest point and a bit like using low gear to drive a car up a hill, whereas in high gear the engine would stall.

I also get the impression that it must take a terrific amount of energy to dislodge a well established "cold pool" and it really does become a battle of the air masses, the energy of the warm fighting against the inertia of the cold.

It is interesting to note that over the past few days the timing of the warmer incursion has steadily been moved from just before Christmas until after.

So despite what Joe B is saying I am watching this space with interest, though I am not putting any money on it either way.

Edited December 20, 2010 by mike Meehan

[La Bise]

Interesting read John, a much simpler form of it is a standard part of mountain walking manuals actually, to help us judge what conditions we can expect on the tops compared to the valley floor in respect to the various types of air masses floating around.

[Rollo]

Many thanks for that detailed explanation John,let me say that I only understood parts of it but it is obvious to me now what sort of headaches these situations can cause to the professional forecasters.

[NorthNorfolkWeather]

Thanks,

Excellent reading

Must improve my maths

[johnholmes]

Thanks,

Excellent reading

Must improve my maths

glad you have found it helpful,

and to mH, I too was a lowly observer that year stationed at RCAF Langar, station 449!

a week of sub zero temps, today here is the 4th, so its only the second time in all those years I've experienced such prolonged low temps.

Edited December 20, 2010 by johnholmes

[johnwirral]

Thanks for putting that together, can't say I understand the maths, but as a computer programer the old saying is garbage in garbage out. Is it difficult to measure the density of these cold air masses, and is this one of the drivers of the current uncertainty of the models?

Edited December 20, 2010 by johnwirral

[johnholmes]

Can't say I understand the maths but as a computer programer the old saying is garbage in garbage out. Is it difficult to measure the density of these cold air masses, and is this one of the drivers of the current uncertainty of the models?

I cannot think that it is difficult but we seem not much further on dealing with a cold block with a warmer less dense airmass trying to move it.

The models are given the latest data, from below the oceans to their surface and the land along with upper air way into the Stratosphere so the question of density is not really one we need to try to measure other than density and temperature are inextricably linked.

So its not garbage in at all-just in these situations for whatever reason, I simply quoted one of the many from my experience, humans and models find it difficult to master.

Edited December 20, 2010 by johnholmes

[johnwirral]

I cannot think that it is difficult but we seem not much further on dealing with a cold block with a warmer less dense airmass trying to move it.

The models are given the latest data, from below the oceans to their surface and the land along with upper air way into the Stratosphere so the question of density is not really one we need to try to measure other than density and temperature are inextricably linked.

So its not garbage in at all-just in these situations for whatever reason, I simply quoted one of the many from my experience, humans and models find it difficult to master.

Thanks for the reply, I suppose its easier to model the more static air masses in the centre of continents, than the ebb and flow around the edges for rare conditions that only happen a few times over decades.

Edited December 20, 2010 by johnwirral

[Guest]

John,

Yesterday evening a forecaster was on the box explaining the co-relation of the jet streams and the positions of the highs and lows relative to them.

In a normal westerly flow the lows follow the jet streams but when we have a high the jet streams go round these.

Now what I was wondering, do the positions of the highs and lows determine the positions of the jet streams or do the jet streams determine the positions of the highs and lows?

[Interitus]

Interesting stuff, but the models obviously use a bulk equation for advection and diffusion and for it to underplay moving a cold block would seem somewhat of an oversight. In this particular instance a few of the model outputs didn't really grasp development of low pressure on the continent which is associated with pressure build up over the UK.

[johnholmes]

John,

Yesterday evening a forecaster was on the box explaining the co-relation of the jet streams and the positions of the highs and lows relative to them.

In a normal westerly flow the lows follow the jet streams but when we have a high the jet streams go round these.

Now what I was wondering, do the positions of the highs and lows determine the positions of the jet streams or do the jet streams determine the positions of the highs and lows?

meteorology is often chicken and egg and same here although the upper air and the temperature differences are what causes the jet streams and subsequently its the jet that creates the surface features not the other way round. The major problem for this area arises when very cold surface air develops from a deep cold air outbreak, such as this. Then it does have a feedback into how the upper air reacts. Its this reverse role that seems to cause all the problems for human and model forecasts.

[johnwirral]

John,

Typically as an total amateur I look for the 528 dam line to make a guess about the precipitation type. On the forecasted breakdown/battleground event would this be less relevant due to the presence of colder low lying air?

[johnholmes]

John,

Typically as an total amateur I look for the 528 dam line to make a guess about the precipitation type. On the forecasted breakdown/battleground event would this be less relevant due to the presence of colder low lying air?

a short answer, forgive me as I'm rushing to get things sorted before I leave for a few days, and its yes.

The surface cold will be important, more so than the 528DM line.

an update for my lrf into January

So on to the end of December and the first 2 weeks of January

I am sorry but due to time I am not showing any charts this morning but I’ve storing dozens of them over the past week or so.

Let’s start at the top so to speak.

Well the sun is still fairly quiescent. Little sign at 30mb of any sudden temperature rise but there is higher up which may well have a bearing towards the end of this lrf.

The tropic signals are, I think, rather vague or at least not all that conducive to blocking, but others may disagree with my view.

Lower down the teleconnections and the MJO is in phase 7 which generally favours blocking. Just where it may go from here I admit I have no idea. On to the AO and NAO. Both show upward trends in the longer term, more so with the AO than the NAO. Remember though, just like the models, their accuracy decreases with time. Pretty good for 7-10 days, less so further out.

The 500mb anomaly charts are more synoptic type that those mentioned so far. With 3 to look at each day (I don’t use the 12z ECMWF-GFS issue simply because of storing even more data) and they are not that far off one another in their ideas. All 3 have backed the idea of the major +ve area being just west of Greenland over east/north east Canada. Generally they have suggested a fairly westerly flow across the Atlantic which seems to have decreased somewhat with time. ECMWF has been less keen than GFS on building heights to the NE of the UK; NOAA seems to sit between the two. It prefers to suggest some extension, rather weak so far, of an extension of +ve anomalies east from the upper ridge as mentioned previously. They each show a 500mb flow south of west into the UK but with heights rising to the east and north east, especially the latter few days.

Down to the models out to T+240 and there is a well documented variation day to day and model to model. I attach less weighting to these than the other signals. At T+144 UK Met does seem to be more consistent than either of the other two.

So where does that leave us after Christmas?

I think that high pressure will dominate this period. Obviously its location is crucial to whether its cold with night frosts or very cold. I would favour the weather in terms of precipitation and probably winds in parts of the far west/north west, calming down as a high develops possibly over the UK but with a tendency to shift more to the north east. This will lead to possible precipitation threats as surface features try to move east or north east off the Atlantic. With the major upper trough in the western Atlantic but with very cold air over much of western Europe and Scandinavia along with this country, away from western fringes, will probably prevent any really mild air by early January standards making much progress east. Its way too far for me to suggest when/where any snow might fall. But it does look like cold or in some places very cold for many away from the far western fringe. Even here it may turn colder again. Freezing fog could be a problem in any light winds with severe frost likely to occur at or a bit above the average frequency for early January.

have a good Christmnas everyone-whatever the weather.

[doctordoolittle]

Thank you John for your explanation, though the maths are far beyond me.

In the winter of 62/63 I was a humble scientific assistant at Heathrow when we had the situation on a number of times when the warmer Atlantic airs made an approach but beyond getting as far as the far SW peninsula of the UK they were really unable to penetrate to any great extent. I think we had one occasion where a warm front approached from the SW to be preceeded by snow and the precipitation to remain as snow behind it. This appears to have been a rare occurance and brought some gasps of amazement.

If my memory serves correct the final break through came from the north west. For most of the time North West Scotland (Stornaway logged up quite a few +4C's) was pretty mild in comparison to SE England and it seemed to me at the time that the attack was coming from the weakest point and a bit like using low gear to drive a car up a hill, whereas in high gear the engine would stall.

I also get the impression that it must take a terrific amount of energy to dislodge a well established "cold pool" and it really does become a battle of the air masses, the energy of the warm fighting against the inertia of the cold.

It is interesting to note that over the past few days the timing of the warmer incursion has steadily been moved from just before Christmas until after.

So despite what Joe B is saying I am watching this space with interest, though I am not putting any money on it either way.

Interesting. Makes sense to me. I notice that the models often underestimate the stubbornness of cold. This last week has been a great example. 3 day forecasts constantly predicting breakdowns of cold - as they are now - that never materializes. watch this space!

[Guest]

Michael Fish is not totally sure either according to his latest forecast - I expect we will all be watching developments with bated breath.

[The PIT]

Bloomin mild here right now at 5.1c. Not bad for Dec at night.

[paul tall]

1.8º here with a max of only 2.3º