OldMetMan
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Posts posted by OldMetMan
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Not a lot can be said about today's runs in terms of any potential change of pattern. Basically the main models are in general agreement, with a SW-NE mobile pattern, with pressure fairly high over the south of the UK.
Only a couple of things I picked up on today, the first was from the GEM model which I rarely consult, which shows more of a meridional flow within 7-10 days, and HP pushing N over the UK towards the Norwegian Sea. The other is the transference of the lowest thickness values from N Canada towards to pole on the GFS, to be expected really at this point in the winter, I suppose but interesting. It does suggest the possibility of pressure buildup over Canada, and the resultant change in the downstream upper wave pattern - upper trough over the W atlantic and ridge mid- to east Atlantic. Plus the GFS seems to be building pressure more over Greenland towards the end of the run as the cold there and to the north intensifies.
The big, fat HP over central Asia is still shown to ebb away by the models but I still maintain that there is a possibility of it reforming to the W of its current position. Also, it needs bearing in mind that there was a HP for weeks over E Europe throughout late autumn/early winter so this again suggests a recurrence of the block.
In the meantime, we'll see if any of the other models start to hint at more of a meridional flow late Dec/early Jan.
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A little interest right at the end of the ECM run with that more amplifed troughing in the eastern USA.
I'm sure OMM will like this! This could be the beginnings of a change but we shouldn't get too ahead of ourselves, although the ECM has shown very good consistency in its recent runs the interest comes at day 10 but nice to see regardless.
Yes I do! Let's see if it continues. The UKMet might show a hint of it at T144 later today.
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Compare the following chart to the 1947 chart you posted remarkable pressure placements and pressure heights.
Yes it's similar but it still needs amplification of the upper wave pattern, something the 06Z GFS was suggesting but backed off from in the 12Z.
We'll see if that trend comes back in future runs.
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OMM - Point taken, but in the case of 62 decent heights were already evident over Scandi in the chart you posted and they quickly built in the 48hrs after that, introducing an Easterly by 22/23rd. This time around I think heights will build firstly across the UK, then slowly transfer to Scandi, so something of a longer process but ultimately worth the wait I hope.
Yes, that's right. Interestingly, the GFS 06Z is suggesting at least the possibility of such a development, although it's again reverting to the usual SW flow at the end of the run. But it illustrates the kind of thing I was talking about - LP off the E coast of Canada.
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Hi OMM - yes, they will need to be because given a 2, maybe even 3 week lag we are looking at early Jan as best case senario and in reality probably the 2nd week. You are right in so much as cold is possible with a deep PV over W Canada, IF it is far enough west, but it's recent and curently forecasted position makes it very difficult for what you allued too in 47 and 62 to happen again.
The above charts are for a few days prior to the onset of cold easterlies in both years. Looking at each, it is hard to imagine exactly what happened as being possible, yet it did, in quite a dramatic way.
As I said, it is a bit of a stretch. We'll see over the next few days if the models do start hinting at it.
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Yes you can still get cold with a strong PV but further to the west especially near Baffin, indeed we've seen some easterlies with a strong PV on the western side of Greenland but in terms of being able to tap into at least some Arctic air we'd need that troughing to amplify over the eastern USA to allow for your ridge to get pulled further north.
We'll have to see what the models do with that weakish rise in pressure near the Arctic, often commented on by NOAA is the lack of observational data for that region, at least the stratosphere is moving in the right direction although its taking its time.
Yes that's right, it's a bit of a long shot based on the current synoptic situation, but it can happen. I think what bothers me still is the presence of that intense Asian HP so early in the winter and how it might build/redevelop W or NW. Yes, lack of arctic data must be a major problem when it comes to feeding the models so such development of HP must be very difficult for them to foresee.
If we are to get a major blocking pattern in the next few weeks, I think the clues will be in the model output fairly soon.
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The problem remains that PV limpeted in eastern Canada, until that moves away from there then the jet is going to be too strong to allow anything other than brief PM incursions.
Nick, I don't think that is necessarily so. The state of the stratosphere notwithstanding, if you refer to the 2 coldest winters in the last 100 years, 1963 and 1947, you will see that the Canadian cold vortex was indeed strong on both run-ups to the intense cold.Back then, on both occasions a strong upper ridge was pushed N to the W and NW of the UK, in response to a deep LP over and to the E of the Canadian vortex, allowing downstream troughing, and corresponding influx of cold air over W Europe.
I agree the jet is very strong but as I suggested last night, it is quite possible for an intense, and more slow-moving LP to form to the E of Canada. Of course, there is the upstream pattern from that to consider, but I still think it might happen - not just yet but maybe after Christmas.
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Not much change in today's model output but 2 things today get my atttention. First, as I mentioned, yesterday, intense cold building over Central asia with the corresponding very high pressure and second, correct me anyone if I am wrong, but the Canadian cold pool seems more intense and further south than normal.
Many here are looking for a more amplified pattern, as the models persist in zonality but the 2 features I mentioned above suggest that such amplification will occur in the not too distant future. Very strong baroclinicity exists off the Canadian seaboard, so it is entirely possible that an emerging LP wil deepen drastically and distort the upper flow significantly. Also, there is still what I consider remarkably high thickness values pushing up from the sub-tropics, adding I think to this possibility.
I am surprised at the way the models warm out the intense Asian cold so quickly so I am inclined to think that it may be more persistent - much depends I think on the intensity of LP running to the north of it and pulling in warmer air.
If an intense LP forms off Canada, then we can expect a strong mid-atlantic ridge, with an amplified upper cold trough in our vicinity.
I think it worth mentioning again that historically changes from mildish zonality to cold meridionality have often occurred suddenly, check out what happened in late December 1962, for example. Others are more qualified than I to explain this phenomenon, personally I believe it has to do with setting up a stable, and thus slow-changing, upper flow pattern but why such patterns exist sometimes is beyond my technical knowledge.
I think as far as the models go, even less faith than usual can be put in the GFS beyond 4-5 days, but the ECM and UKMet would likely pick up on such a switch.
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Yes, am watching and I believe something to watch over next 10-14 days. Chart watching could become very interesting come Christmas week.
As an aside, ECM and BOM suggest Boxing Day stormy, very windy / wet / snowy for NI and Scotland and High levels to NW. A period to keep and eye on with hints that a pronounced NW/SE axis wants to develop.
BFTP
Indeed. Another unusual thing I've noticed is the presence of high thickness values in mid-Atlantic, >564dm (or "red air" as I have always called it!). The low that zipped up the Channel yesterday was essentially sub-tropical in origin if you track it back - strange thing to happen in December. The implications of this high thickness could be an increased chance of very intense LP over the Atlantic, with a very strong baroclinic zone and jet. Or if the upper pattern amplifies, a strong mid-Atlantic upper ridge and corresponding deep downstream trough over us.
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A very interesting read there Old met man, I know most on here would love to see some weather from the northeast this winter. This would also fit in with those metoffice charts that were posted the other day. When would you expect to see any FI eyecandy following the influence of the Asian high?
Thanks for that, hard to say, as I mentioned, most likely the ECM would pick it up soonest, perhaps within the next few days.
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Others might not agree, but I think today's GFS and ECM show some interesting potentialities. First off, apologies if anyone has already discussed it, but has anyone noticed the intense HP forecast for central Asia in the next few days? So what you might ask. For one thing, bit unusual to see HP > 1060mb this early, for another, I have seen a tendency in the past for the HP to build westwards from that location, or to re-develop to the west.
We still have cold air feeding SE into Europe. Beyond 5 days, I think the GFS loses the plot somewhat, but the ECM looks more like it, a mobile pattern but the jet alignment NW-SE - likely to continue to introduce cold air here and into Europe at intervals. So I think we need to watch out for rising pressure over NW Russia/Scandinavia within the next 10 days or so as the cold deepens.
The GFS clearly has been defaulting to southwesterlies beyond a certain point in time for a while now so I think the UKMet and ECM will most likely pick up on any HP build up to the E quite early, if it is to happen.
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Some early thoughts on what's happening today.
Models now seem pretty well in agreement with handling today's low, although I am inclined to think they may have deepened it too much. The satellite images show a fairly uniform mass of high cloud, there is a hint of our low, with a bit of a baroclinic leaf forming at about 40W, but water vapour images do not show it to be that dense, so that suggests that there is not much vertical motion going on - i.e. not much deepening - yet, plus I don't see evidence of a short-wave trough forming, usually quite easy to see on water vapour images
This is still on a knifedge really, I would think that satellite is pretty much all the forecasters have to go on, with surface data sparse. So obviously model output will be affected by lack of data. I think we will have to wait at least another 6 to 9 hours before things become clearer.
Historically, this type of LP development tends to be exaggerated by the models in my experience, so as I suggested last night, we may still just get a fairly shallow feature in the Channel with any deepening taking place over the Low Countries. Certainly most interesting!
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If the low tracts about another 50 miles south I guess that could happen.
I don't seem to remember many lows that start to track south bounce back again, might be wrong..
Will be interesting to see latest BBC updates tonight
Yes indeed. Trouble with this situation is that we are dealing with what you could almost call meso-scale systems, this is why I think the models have been having such trouble. Small disturbances on this frontal boundary can form/dissipate very quickly, which is why only satellite will be able to pick up the detail, and then perhaps no more than 12 hours ahead, maybe less. Rapidly deepening lows have very distinctive satellite presentations so this is what the forecasters will be looking for.
I don't know what the equivalent model is today, but after the October 87 storm the UKMet developed a fine-mesh model capable of picking up the sort of detail we're looking at in the next 24 hours, but it used to only really work for short-term forecasts. Perhaps the forecasters here can clarify this.
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It seems we are going right down to the wire with tomorrow's LP, as although there is finally more agreement between the models, it's clear that the detail is proving to be a nightmare.
I would think that satellite analysis must be critical now, as whatever finally happens tomorrow looks as if it will happen fast. I am particularly intrigued by the changes in the surface analyses, with a newly-added frontal wave running ahead already - this would seem to be in answer to the high-cloud streak which has been moving rapidly across the Atlantic since this afternoon.
The 00Z UKMet analysis for 16th is most interesting. It has now stabilised with a small, tight LP racing up the Channel. I find this forecast disturbingly similar to the infamous "Lothar" storm of Boxing Day 1999 (chart below). I don't see much evidence of this happening at present though. I think a lot depends on whether the jet gets engaged as to the intensity. We could just end up with a shallow wave giving rain to the coastal strip and little wind!
One thing's for sure, the forecasters will be glued to the satellite pics!
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I tend to support most of what you have posted, especially about the warm front wave not being initially forecast nor that its necessarily correct yet in how its being dealt with.
The comment from Nick F in anwser to a post that it may be tomorrow before any firm 'handle' is made on the actual evolution is perhaps about right.
It is certainly a wake up call to all forecasters as to what/why and how events are being handled by the models for Thur-Fri. I would be amazed if UK Met do not set up their own operational review of this. Perhaps an article, probably pretty technical, in a few months time, in R Met Soc journals?
Thank you John, I must say I do find it baffling that all the models seem to have gone awry here, although we are still 2-3 days away of course so that could still change again.
I have always trusted the UKMet output above all others and it came as a revelation yesterday when I watched the video by their chief forecaster on their web site explaining just how they interpret the model output. I believe you posted about this matter recently? Anyway, I guess I rather naively thought that it was much more straightforward. I should have realised, as I often see this process that the chief forecaster uses in judging the likely outcome when looking through the National Hurricane Centre's discussions on tropical disturbances - often the judgement of the forecaster is crucial in getting it right when different model output gives different outcomes.
Yes, I think you're right, this particular forecast has changed so radically, it certainly calls into question, either the models themselves, or their interpretation so perhaps they will learn vauable lessons as a result.
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I've been trying to figure out the change of direction the models are taking today.
I talked yesterday about the possibility of a warm frontal wave developing ahead of the potential storm, and today the UKMet has picked up on this idea more noticeably. I still think the wave is being overdeveloped by them, such waves usually are quite shallow and fast moving - as the GFS has depicted.
It seems the cold upper trough associated with the deep low to our NW has dug down further than originally thought and the jet is displaced further S also. Looking at the GFS it seems to be simply running the warm frontal wave ahead of the developing system and then leaving the main LP behind, whereas before it was moving the whole thing bodily NE. I would suggest that the reason for this is that the LP is not expected to engage the jet fully, with only the wave being caught, although someone with more technical knowledge than me, do put me right if you think that's wrong.
Meanwhile, cold air will continue to sink south across the country so the risk of snow is now increasing steadily, well, at least for places away from the coastal strips.
It's been a remarkable series of runs from all the models soit will be extremely interesting to see how it actually pans out.
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Looking at the charts for the weekend, I don't mean to be alarmist here, but I was wondering if anyone has been considering the risk of a storm surge in the North Sea? My own view is that things are moving too fast, so sustained N/NW winds along the east coast are unlikely to last long enough to generate a surge.
I wonder if anyone has any thoughts on this?
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18Z GFS is still telling the same story for Friday, although I can't say I think it looks right.
NOGAPS is rather similar to previous runs today but is tracking the low a bit further north, with strong gradient over the south of the UK.
This serves to strengthen my belief that this low will be complex and that even 24 hours ahead it will not necessarily be clear what will happen.
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Here is another with excellent overlays http://www.eumetrain...date=2011120718
Thanks, yes lots of options there.
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http://www.yr.no/sat..._animasjon.html
this is a good satellite link.:-)
Thanks for that. Very smooth animation, makes a change!
Need to watch the GOES-East satellite over the W Atlantic in the next couple of days to see how this LP develops.
Also worth looking at the WV images too. The presence of a dark streak (i.e. dry, cold air} behind the developing storm 'head' could be fuel for the 'bomb'. This was clear as Bawbag developed last week.
Yes indeed, the GOES-East have a variety of different images, needs close watching I think.
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I keep on seeing people say "due to the complexity" of Fridays system. Could anyone elaborate a bit more on the complex factors involved? Would be nice to try to understand and follow some of these factors building into it.
Hi Jon, perhaps you were referring to what I said above. The models have been changing over the last day or so to depict the developing low for Friday as a more complex feature, rather than a single centred depression. The UKMet chart I attached above has what is essentially a warm frontal wave, and this doesn;t quite look right.
The following definition of a warm frontal wave might serve to explain it better:
"A secondary disturbance, often accompanied by a shallow closed-low circulation, that forms at some point on a marked warm frontal boundary a good way (at least 1000 km) from the parent (occluding) depression. Once formed, it moves quickly away from the parent depression (in the Northern Hemisphere east or southeastwards). Although not common, they are often responsible for considerable forecast errors, and are of particular importance in winter (snow-situation) forecasting as mild, maritime air attempts to displace a cold, continental blocking anticyclone."
I think the models seem to be suggesting this as a posssible outcome, but as the definition shows, this is very tricky from a forecasting point of view. One notorious instance of this was the October 1987 storm I referred to, when the models completely missed the development of the LP that caused all the damage. Granted, forecasting wasn't so sophisticated then, but I think we might be facing this situation Thursday/Friday.
We are looking at a potentially explosive situation here in my opinion. The baroclinic zone where this low is developing is very strong and development will be accompanied by an intense jet, so frontal disturbances will be fast moving and very difficult to accurately model. That is why i think the models so far today are giving mixed signals.
I think on balance the NOGAPS looks more realistic - at the moment - as if this low does turn out to be complex, then the final low in the chain is more likely to be the major one, as it engages the upper trough and deepens, accompanied by a rapid pressure rise behind it.
This is why I keep on about the satellite images as these will be key in identifying exactly how this frontal boundary is developing.
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It seems the models are having some trouble with the Friday low, in terms of its complexity. They all seem to have picked up on the fact that there will likely be warm frontal waves running ahead of the system but the attached 72 UKMet chart shows the front-runner as the deeper, with a shallower wave following behind.
The NOGAPS on the other hand, shows a warm front wave followed by a much more vigorous low, with a strong pressure surge behind it - such as would give a spell of intense NW winds.
In particular, the GFS seems to be sticking with the idea of a fairly shallow wave running across the very south of the UK.
Given the sparse data in the area of predicted development of this low, I think satellite data will be crucial nearer the time. Rapidly developing secondary lows can be dangerous beasts - remember October 1987!
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I think the GFS is backing off way too much for Friday. The NOGAPS is sticking to its guns for the 06Z.
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Lovely downgrade from the models for Friday in the past 12 hours hopefully this will continue in the days to come.
As was mentioned yesterday, the central pressure for any LP is not the crucial factor in determining how strong the associated gradient will be, so even if the models "downgrade" as you put it, there might still be very squeezed isobars to the S of this predicted system. Also, the speed the system is actually moving has an effect on the surface wind speed, so the faster the movement, the greater this effect will be.
Model Output Discussion - 19th - 25th December
in Forecast Model Discussion
Posted
Interesting shift in the 06Z GFS. After 26th, cold air now flooding S over Scandinavia and HP over Greenland, warming over Canada, suggests as I mentioned earlier, upper trough digging SE to the E of Canada, warm ridge pushing N towards Greenland, HP starting to build to the N of the UK. Bit far ahead to be trusted but certainly a different trend.
Whether it's always true or not I don't know, but the GFS seems to be continually defaulting to a SW flow beyond a certain point, perhaps 10 days or so. I think it's definitely picking up a trend here after Christmas.