swilliam

Agreed but I think we have to expect some of these as well as the wonder synoptics (and Nick S I am not moaning just commenting so please don't bar me!) I bet that does not stop you looking at the models though (and using it as an a reason when we don't get what we want)!

The HP in FI never properly connects with the arctic high - hence a likely toppler in far FI. Still lots of cold and wintry weather though. I still think the battleground situation next week looks marginal even though this run probably gave one of the better alignments. This may change (the marginality) nearer the time though.

Hi Kold, Yes I agree with you in general - certainly about the reloads on the GFS evolution which I mentioned earlier. As for the ECM it does look like the natural evolution for a massive reload of the Northerly due to the PV appearing like that. But I also think it possible to get the SE flow that I mentioned - particulalrly if the PV starts heading SE rather than S. I think this is rather an usual situation so there are not many analogues to look at - may be something for Christmas evening if, as is likley, I get bored with the telly. I expect everything will be differnet by tomorrow anyway but very interesting model watching at the moment. Some excellent previous posts BTW.

Convection is when air is warmed at the surface, usually in the winter by passing over warm water such as the North Sea, in the summer by heating of land by the sun, causing the air to rise like the smoke and hot air from a fire. This causes clouds to form and eventually rain or snowfall. In winter in lowland places we generally get three types of snowfall, convective, frontal and those caused by troughs in the general flow (usually due to some warm air aloft or low level convergence of wind flow). Convective snowfall is obviously more likely the closer to the source of the warming - in this case the North Sea. It will depend on the wind direction and strength where the convective snow actually falls. For the East of England (including Leeds) the best direction is the Eastern quadrant. The nearer the coast you are the more likely you will get some precipitation, However because you are close to the source of warmth (the sea) the more likley it will be rain rather than snow (which is why the upper air temperatures matter so much in this situation). For inland areas such as West Yorkshire it is also necessary for the the wind to be strong as well as in the correct direction. The convective activity dies down as the showers move away from the source of the warmth. Therefore a strong wind is needed to get significant precipitation a long distance away from the coast. This can change when troughs are present which enhance the shower activity. What matters is the orientation and movement of the troughs. If the movement is at right angles to the length of the trough then usual most places get a spell of snow with the length depending on the speed of movement. These troughs are usually due to some upper air feature. Sometimes though the trough is aligned along the direction of flow and this can be mean the trough is virtually stationary. In this setup some places get many hours of snowfall whilst either side not many miles away nothing falls at all. This is what occurred last Thursday/Friday so that some places got a lot of snow whilst other places not much. Frontal snowfall has two main variants. The most common is warm air replacing cold and in this situation we get a spell of snow followed by rain and a thaw. This occurred yesterday although a lot of us down South and in the Midlands missed out on the snow first due to the upper air temperature profile. Sometimes the warm air pushes forward and then is rebuffed by the cold air and we get a long spell of snow and then the cold air wins and the rain does not arrive. This gives us (in lowland England) our biggest snowfalls. This is what is being projected for early next week with warm air pushing up from the South but strengthening NE winds pushing it back. How far the warm air gets determines where the snowfall will occur. The setup next week is complicated by the fact we are not very cold to start with so it may start as rain and then change to snow. All the discussion on this thread is to do with where the warm air will get to which is why there was much mention of snow in the South or North. At this stage it is far to early to know either how cold it will be when the main precipitation starts where it will get to. This will only be approximately known 24-48hrs beforehand so you should not worry about this till then. The other main frontal situation is when the cold air is initially introduced which can lead to rain followed by snow. This rarely gives us a good snowfall as evidenced by last Saturday night when a front of this nature passed through us all. So on this thread we are discussing all the different model runs and what that might mean for cold weather and what type and where we might get precipitation. However this is not usually done from an in my back yard (IMBY) perpsdective. This is mainly because the detail such as this will always change beyond the 72hr mark and the models are running out to 384hrs. So I think if you are interested in the will it won't it snow in Leeds next week type of information you should use some of the other threads on this excellent site. I believe there is a will it snow thread and also a North of England cold spell discussion. You should use to these to get your local iformation. Use this thread to see what the general evolution of the synoptic pattern is likley and possible. Hope this helps Cheers Stewart

Hi Kold, I am not as convinced as you about the PV sliding South and prolonging the cold on the ECM. Looking at NH chart it seems the PV cannot move much further South due to the HP block which cannot go anywhere due to the LPs propping it up. I think the most likely evolution would be for the HP block to extend SE and for Mediterranean air to come NW towards us. I think I prefer the GFS set up at 240 for likley continuation of the cold spell. Of course we know what happens with GFS evolution which does prolong the cold but I am not as convinced by the ECM. I think I would prefer the PV to stay to the West of our block giving a stable upper air pattern. It might not be as cold as if the PV sunk South but it would be cold enough I think!

Control run is even better than the OP. It keeps the low further South and it then run along the channel ala 1978. Then it runs through various Northerlies with regeneration of the HP Block to the NW. Then as a final present it gives us a beasterly in deep FI with <-12 uppers and extensive precipitation - dream on. My only concern is as was pointed out by Geordiesnow if the low stalls out to our West or SW and the north easterly winds are in the Atlantic. What makes that low want to suddenly move East like that?Should we be concerned about this?

My only concern about this battleground situation next week is that most of it might actually be more cold rain if the orientation is not favourable. The cold air is not well established initially with highish uppers (-3/4) and not much surface cold in England and Wales). This is highligted by the forcasted temperatures and dewpoints at the peak of the event as forecast by the last run. If the trough does not align correctly then we could have a lot of rain in England and Wales with only a brief transition to snow as the final low scoots through. We have all been fondly remembering 78/79 New Year when the cold air became established and then the battleground developed as the wave depsression passed along the English channel. The alternative though is what happened in Jan 1982. We had a similar situation to then as is projectd now The siutaion remained static like this for a while until the low finally shot through The result of this was copious amounts of rain in England and Wales with realtively mild temperatures. There was only a brief changeover as the low passed rapidly through. In this case though this is forgotten as there was follwoing low and new battleground became established with massice snowfalls across the whole of England and Wales. In our case we do not have a low following so we would not get this situation. What we do have is much better establishment of the block and the potential for a longer lasting cold spell with snow. So I think we need to watch the developments early next week and not get too excited yet by the potential synoptic situation. Although we are better placed than Jan 82 it still looks bit marginal for a major event There are several things needed to go in our favour to get a large snowfall from this situation. Edit: Meto update shows same concerns

I think what was interesting and slightly different about the last GFS run was that the HP never makes it to Greenland (in terms of an upper blocking high) and meanders around in the vicinity of Iceland e.g. Despite this there is virtually uninterrupted cold and wintriness once the block is established. This is because of the absence of the PV to our North so we do not have stuff coming round the top of HP introducing Atlantic air again and putting pressure on the block. As someone mentioned previously the PV is nicely positioned to the West of Greenland with another important bit in Scandi. Our blocking HP is actually linked to the arctic/Siberian HP - almost the reverse of the usual situation. So instead of sinking highs we have rising highs (should that be floating?). Quite an unusual situation I would suggest.

Comparing this run of the models and the 12Z run yesterday not much has changed really. For early next week we are still looking at the exact positioning of the LP to the SW and the angle of the trough extending East from it. Looking at the ensemble for the Midlands there are still are fair few (including the OP) showing a more Northward position and the trough extending NE sending up mild air over the South and Midlands. There is then even firmer agreement for the cold NEasterly early in the New Year. We then have the large scatter depending on the setup of the blocking anticyclone - cold if it it initially sets up towards Greenland or later retrogresses NW, mild if it sinks towards us (like the control run - although even that still keeps us cold but with less snow potential). I think there are more members going for the 1st option this time so encouraging signs.

Looking at the ECM ensembles there is good support for the OP and the control run goes off on one in deep in FI - maxes in Holland of -10 and min of -15 BY the way there is now a link of the meteociel ECM page to the De Bilt ensembles - the link showing when they have been updated.

HI Pete, A polite request - could you add the link to the new thread when you close the previous one - saves trawling through looking for it. Thanks Stewart

No suicides here - we're trying to assess what the most likely evolution is at T168-240 and what is needed to get the Northern blocking over Greenland that most of us would like..

No-one said it did on the ECM but it did and now does on lots of the ensemble members including the control run this time. We have to consider that this is also a possible outcome - maybe at this stage probable. The ECM had a rather unusual route to the greenie high by having a relativity low latitude mid Atlantic high which retorgresses and grows northward. Not to say it won't happen though

Yes you are quite right that is is likely to sink. As I pointed out earlier many of the ensemble members on previous runs have shown this scenario so we should not be surprised that the op run has now shown it. The difference between this and getting the greenie high is quite small so we should expect to see some runs showing this at this range. As I also previously showed even when the high sinks - in this case over us, which is what you expect (although the ECM showed an alternative but perhaps less likley outcome) this does not necessarily mean the end of cold prospects. I would only become concerned if the Greenland high scenario does not appear on some of the runs and is not shown by many ensemble members. Stewart

Still lots of scatter on the ensembles in the South and Midlands for the period 29 - 31 Dec so no guarantee of cold next week yet. Looking at the members this seems to be mainly because the trough extension from the LP to the SW is aligned poorly or further North allowing much milder air Northwards. eg pb 1 They do all then converge on the 1st at a T850 of -8 here - indicative of the Northerly Thereafter there is enormous scatter again and this is due to whether the blocking high becomes established over Greenland or not. A lot of members have this hightoppling e.g. pb 11 leading to + zero uppers This then goes on to be a huge Euro high But to illustrate that if this happens we should not panic - even this evolves into something interesting with -5 uppers. Indicative of the large amounts of cold air around us at present. So for medium term cold we need to keep a watch on the position and direction of the troughing from the LP in the SW and the building of the ridge to Greenland. Hopefully the higher resolution of the OP and Control (which is also stonking) mean that they will be the favoured solution and the ensembles will trend towards them.

A new ice age is definitely about to occur! I guess we would not all be obsessed about model watching if this was the reality - although we may all be desperately looking for blow torch SWs

Yes I thought it was more than T240 that the first indications were given. I remember being 'told off' for discussing it as it was obviously implausible. I think the current predictions for the cold spell are more marginal as it is very easy to see how it can all go wrong. As NS has stated several times we are relying on not having a link up between the two depressions shown below - one over us and the other to the SW. Looking at the ensembles it is clear that many members do go for this link up leading to the possibility of much milder air over us as indicated by the GEM GME models. Then we have to worry about the troughing and undercutting Eastwards. So plenty needs to go in our favour but at the least the possibilities exist. For me I think the marginal situation is highly preferable as it can lead to some serious snowfall instead of the routine falls we have had to date. Also provides much more will it won't it situations and if so where so lots of interest.

A very interesting post if what you say is true. It is certainly true that the GFS produces lots of dartboard lows that never materialise and cold scenarios that frequently downgrade (although it did particularly well with this spell - easterly predicted well in FI at times and the parallel run very consistent unlike the Euros which showed some horrible charts). Do you have any evidence or information that the model has a programmed bias as I would be interested in reading about it. What about the other models - do they have a programmed bias? Thanks Stewart

Looks like that was a heavier pulse which moved SW to NE along the band. Currently it has stopped here and I do not see good prospects on the radar for more pulses to move through. Maybe when the trough starts to move again the precipitation will pep up again.

The snow has really intensified over the last 1/2 hr here in Northampton. Huge flakes and the snow is now accumulating rapidly despite the temperature being slightly above freezing (0.5).

I think this is because it is very light and sublimates before reaching the ground. We had a couple of hours of that in Northampton but it has been snowing for about 1/2 hour now.

The ECM is a shocker leaving us all in a mild Southerly by Christmas day

That is certainly true for the 850s but is not so pronounced for the actual temps on the ground due to cold surface layer. This is shown on the progged temps where 5C is not reached until almost the New Year

Yes and look at the dip in 500HPa temps after the cold front comes through in a couple of days time. Now that is really cold true arctic air. I think he was being sarcastic!!!

The answer seems to be yes with a completely different evolution now. 00hrs 06hrs Not sure if it was for the reason I mentioned previously but certainly a vastly different outcome and very interesting/welcome it is.