OldMetMan
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Posts posted by OldMetMan
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And so it goes on - how can there BE so much rain?
The models are offering little hope of any change, in fact, the GFS seems to be emphasising more strongly in recent runs a renewal of a strong, low-latitude jet heading in our direction in the mid-term, basically repeating the pattern we are in now - slow moving upper low on top of the UK.
There are a few variations on this theme amongst the other models, the UKMet latest run is suggesting a weaker flow and the ECM is building pressure across us at the end of the run.
As I see it, the only real chance for this pattern to break down is for the new jet to propagate further south still than the models show with the main LP activity heading much further S into Spain and the Mediterranean, with HP building further N across the UK. I keep on about the strong upper high over the US and it is really remarkable in its persistence. At present, it does seem to be retrogressing somewhat into the W and NW States and W Canada, but, as I said before, we are still left with all the jet energy heading across Canada and SE into the Atlantic, then on to us.
I notice a fairly strong upper low developing in the Pacific NW which could perhaps start to erode the US ridge, maybe enough to disrupt the static wave pattern and give the downstream flow a chance to re-arrange itself in our favour - with the possibility of a more significant mid-Atlantic upper ridge finally blocking off the endless train of LPs coming our way.
But we still have the next few days to get through, with the current LP doing a cyclonic loop and further troughs and minor lows adding to the soggy mix. I notice we have a rare red alert for the SW of England from the Met Office - not good. The short-term model forecast seem to be shifting the rain emphasis further south in the next day or two - yet more flooding likely - until the whole sorry cyclonic mess finally moves off to the east.
A couple or three more days from now we should begin to see in the model output if this cyclonic pattern really is going to continue until at least mid-July.
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There's not much change of any note in the latest model output, only to say the ECM now seems to have fallen in line with other models in keeping LP over or near the UK for the foreseeable future.
As I mentioned the other day, I remain convinced that while the extreme heights (and associated extreme temps) remain entrenched over much of the US, I can see no clear way in which the downstream flow will shift significantly. There do seem to be signs that the US upper high axis is beginning to shift W and the medium term model output suggests more of an upper trough forming over and to the E of the US and Canada, but from our point of view that only serves to amplify the downstream flow with another upper high forming mid-Atlantic, and with a sharper upper trough over us.
In the long term, all the models are in reasonable agreement with changes occurring only slowly, However, the trend seems to be to move the main area of LP over the near continent so I still think we will see HP starting to build from the NW or N, but it may take another 2 weeks or so before it happens. Historically, this is a situation I have seen happen before.
I think it is still important to watch the trends in the Atlantic hurricane season this year as I believe this will have a significant effect on the longer term trends here. At the moment, it has gone quiet but the tropical waves coming out of Africa are showing signs of growing more active and, with the Azores High and the subtropical ridge staying south this would guide any storms on a more westerly course, with the threat of more US land-falling storms than usual. This is bound to affect the flow over the US and thus the downstream flow over the UK – one effect would be to strengthen the flow along the US east coast as any ex-storms carry their hot airmass NE.
In terms of the models, I am keeping an eye on the tropical Atlantic forecasts.
Meanwhile, swirling LP over us – plenty more rain to come – and when the pressure gradient slackens, watch out for big, slow-moving storms.
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July is here and summer seems as elusive as ever - at least for the moment.
With the strong upper high over most of the US and another over much of central and eastern Europe, there seems little likelihood of any imminent change in the pattern with an upper trough sandwiched in between near the UK.
The prospect of slow moving LP over the UK from mid-week on seems pretty consistent across the models now, but in the mid- to long-term, only the ECM has been quite consistent in building the Azores HP across us.
Looking at the rather tortuous path of the jet it is still obviously way south of where is should be across the Atlantic, but, as I mentioned the other day, there is a definite weakening trend being shown by the GFS, which would fit with the presence of a slowly filling LP over us. It would also, in my view, tend to support the ECM's insistence on a pressure build by the 2nd week of July, with a gradual change over the US towards upper troughing near the east coast, plus the jet taking a more northerly path towards Greenland, like it should in July, favouring rising heights across the mid- to east Atlantic.
The exact evolution and timing is, I think going to be hard for the models to agree on day to day, but it seems clear that a lot of the energy is ebbing away from the Atlantic upper flow, a good time for a pressure rise. As ever, a great deal depends on where any such HP centres itself, but I have a feeling we may see it centred to our NW or N, not necessarily the best place but, if nothing else, we might at least get a week or two of relatively dry weather.
Before that happens, of course, plenty more scope for deluges as the LP slowly fills over us, where and when is anyone's guess but I fear there may be yet more local flash floods before we see the back of it all.
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But, oddly, the models seem agreed on a large very shallow LP dominating NW Europe for a good while. At times and on some models it's so shallow as almost to be a col. I am very uncertain as to what kind of weather this is likely to give rise to - any more experienced posters assist in that regard, including you OMM? However, I wouldn't have thought that flooding is likely to be a major concern, although I am conscious that sometimes flabby lows can give surprising and of course slow moving downpours.
I'd be interested as to what people think this kind of synoptic situation would give rise to, assuming it happens, which with good agreement it seems likely to.
Hi Weatherboy,
Yes, slack LP over the UK and near continent are certainly capable of either slow-moving heavy/thundery showers, plus they also tend to breed small troughs or even lows, equally slow-moving with persistent, sometimes heavy rain. One of the gloomiest setups, especially in the S and E of the country, is a light to moderate NE coming over the North Sea which can produce very dull cool weather.
But, on the up-side, they often tend to be replaced by HP building from the N or NE.
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This is turning out to be quite a summer!
Looking at recent model trends, there has been a certain amount of wavering between maintaining the run of LPs across the country and building the pressure at least across the south of the country. UKMet seem to have now settled for mobilty, at least for now. It is interesting that the GFS is showing a gradual weakening of the jet so maybe the longer term trend is for some Azores HP offshoots next month.
I think the persistent upper high over much of the USA is strongly influencing the downstream flow over us, with the persistent upper trough over much of the eastern Atlantic. In the mid-term, there is some consensus for there being a slow down in the eastward moving LPs with a slow-moving LP settling over the UK and near continent - bad news in terms of the threat of even more flooding events. But certainly no sign of any real summer heat.
I think the later runs will again show some uncertainty between LP and HP development over the UK. If the slow moving LP shown over us in the mid-term starts to consolidate more, then I suspect that we will see HP building beyond that.
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Model output today continues to intrigue.
There is a reasonable consensus in the short- to mid-term - continuation of the marked meridional flow all the way from the US to Europe with the UK still stuck under the eastern flank of the upper trough, hence the continuing run of LPs coming at us. The one developing to our SW at the moment is showing growing dense high cloud on satellite - I think the MetO warnings are more than warranted.
There are a few similarities in the mid- to long-term model output that look plausible. If anything, the meridional flow may increase in the near-term. However, the models are suggesting that eventually we may get a cutoff upper low to our SW which may allow for pressure to build over the UK. The ECM though is keeping us firmly stuck under the influence of the upper trough - personally I think it's a bit extreme - but we'll see.
I am convinced that the Atlantic tropical storm development will have some bearing on our weather in the next 2-3 weeks. TS Chris is about to go extra-tropical but it is taking an easterly course which is only serving to strengthen the southern arm of the jet so will not do much to help from our point of view. However, the GFS shows a significant LP in the long-term up the east of the US and Canada which suggests to me the LP depicted would likely be of tropical origin - perhaps based on the possible development that is taking place in the Caribbean at present. It is worth noting that having 3 named tropical storms at this point in June has only happened twice since the 19th century. All 3 have formed in similar locations - just E of the US - so I think if this trend continues, it could affect things here by pumping very warm air NE into the NW Atlantic - enough maybe to disrupt the blocking pattern, or re-establish it in a more favourable pattern from our point of view. Pure speculation I know but I feel it needs something significant like that to shift the stuck pattern we are in.
Meanwhile, more wind and rain - the drought seems a distant memory now!
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This is turning into a most unusual summer.
There is a fair amount of consistency amongst the models in maintaining the broad cold upper trough over much of the central and eastern Atlantic but what I am most surprised by is the northern extent of the upstream upper warm ridge with thickness values up to 564dm getting very close to S Greenland - don't recall seeing that before.
Thickness over the near continent is fairly steady although there is still a chance of the "Spanish Plume" event I mentioned the other day with the jet being at such a low latitude - in fact we have something of a split flow so some of the hot air over S France and Spain could well get caught up in the developing low to the SW in the next day or two with thundery activity developing over France.
In the longer term, the upper pattern is shown by the models to remain broadly similar with the static wave pattern persisting and the jet still a long way south. However, the UKMet in particular is showing the possibility of slowly rising pressure over the SE and S of the UK - but again much depends on the strength of the jet.
One final factor which may have a bearing on the longer term upper flow is that there are signs of tropical activity beginning both off the eastern US and the Caribbean so, if that takes off, we may find it having the effect of strengthening the jet over the NW Atlantic - could give us a breather from LPs coming in from the SW at us again and again!
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I remember that, too...Late June or early July, and 33C in London? Was that the year when the Macclesfield game was called-off after several players were struck by lightning?
Yes, you are both right, that was quite some weather in those few days but there was also a LP that travelled from N Spain around 10th and produced massive amounts of rain and thunderstorms across the SE, plus another event around the 15th/16th September that produced 3 days of torrential downpours as an occlusion got stuck over the SE - I remember parts of Surrey were very badly flooded. It was similar to the situation of a few days ago that caused all the flooding in Sussex.
I shall be following the UKMet particularly in the next couple of days - as TWS suggests, the jet may weaken enough to let HP develop. Interesting situation this!
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I am finding this summer especially interesting in terms of contrasts and extremes and, at the moment, the models seem to be in a sort of no-mans-land as to what is likely to happen following this current LP, that is, after the next 3-4 days. The consensus at the moment puts the UK in a slack vaguely low pressure field after the current low fills with a weak HP possible settling things down temporarily but I have seen some hints of another significant weather event within the next 7-10 days.
Given that the jet is still way south, and there is still likely to be a broad upper trough over much of the E Atlantic for some days yet, what I am looking at is a steady increase in thickness values, and thus surface temperatures over W Europe and I have seen this pattern produce "Spanish Plume" events in the past, with hot, humid air heading N very close to the SE of the UK and engaging the upper trough and jet. This is being shown at intervals in the GFS and especially the ECM today. The previous unusually deep LPs this month have not had this added element and they produced more than enough rain!
The significance of this is clearly the threat of a major thundery LP crossing the UK, most likely the S and E, but accompanied by very high rainfall amounts with the cool Atlantic air undercutting the hot, humid continental air. If this is going to happen, we should see the models picking up on it within the next 2-3 days.
2 previous instances of this happening that I remember most vividly at July and September 1968.
In the meantime, the weakish HP looks likely as per the GFS early next week, but I can't see anything yet to suggest a longer settled spell. The upper pattern is very meridional but, like I said in an earlier post, there has been alternating upper high/upper low blocking for much of this year so we need to see the models suggesting a more zonal flow, with the jet moving back to where it usually is and the Azores HP being given a chance to push in. At the moment, that is conspicuous by its absence.
One last point, the slack pressure field early next week, with fairly low thickness values gives a significant risk of some local heavy - and slow moving - downpours.
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Is there any sign, from the models, that the weekend low will have any less (persistent) cloud in its circulation than the last one did?
Out to T144, the GFS is keeping the jet slap bang over the UK, so, assuming there IS a significant deepening, then even after the main frontal zone has moved through ( and which will quite likely get stuck somewhere over N England/S Scotland) , there seem likely to be cold-frontal waves running up from the SW and S, so it's showers or longer spells of rain - and a lot of cloud. If the low tracks further to the W then there would be a risk of hotter air from France getting caught up in the circulation - a very bad combination as we saw in 2007 - but, to answer you question, notwithstanding the possibilities of frontal waves, the best places for less persistent cloud would be the eastern and perhaps southern coastal fringes.
I think the 12Z GFS will give us a more accurate picture.
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I do wish that the GFS would stop playing the hokey cokey with that depression...Are we still really any the wiser as to what it's going to do?
Looking at the upper air pattern today, I'd say it's a pretty safe bet it WILL deepen significantly. Combination of a broad upper trough and the jet a long way south, it would seem likely to engage both and develop. Not much warm air over the near continent at present so I guess the thunder risk will not be significant but a lot of rain nevertheless.
The longer term GFS is showing no signs of the pattern shifting any time soon, although it seems to be hinting that we may see a deeper upper trough developing in the W Atlantic which may finally allow heights to rise east of it, with associated HP near or over the UK. Seems to have been the pattern for so long now - upper low, upper high interchanging.
Meanwhile the jet so far south suggests it may be quite a while before summer comes back.
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Hi Bill, looking at the longer range GFS, the last few days does seem to be showing a stready build up of heat over mid- to eastern Europe. The pattern this year has shown repeated blocking with alternating upper ridges and upper lows over the UK, so if this pattern continues we could see the current very broad upper trougn replaced by an upper ridge again. Then I think there's a distinct possibility of some very hot air heading our way out of Europe. Obviously, the timing is crucial and it could be that any upper ridging starts say late June and into July and then by later that month, the Atlantic "monsoon" will get pulled in, just in time for the Olympics!
But, like I say, it's been a year of extremes, so anything is possible. I hope for the athletes' sake, it ISN'T hot late July!
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Hi everyone, haven't been here for a while but what a remarkable year of weather extremes we are getting!
Today, well bucket loads of rain. I cannot recall such a cold day for nearly mid-June - 11.2C here at present. I thought at the end of May that the Atlantic jet was looking ominously far south so we're stuck with this for a while yet I think.
There certainly looks like the potential for local flooding in our area today - there is an axis of very heavy rain associated with an occlusion that is moving VERY slowly W so a good inch or more seems llkely in Sussex, Hampshire, perhaps into Dorset and Wiltshire, although I expect the front will weaken slowly as it travels W.
The rest of the week, well cool, cloudy, and with a slack area of LP over much of England until mid-week, some hefty downpours seems likely, perhaps merging into areas of heavy rain with troughs forming. Then it looks distinctly thundery late in the week with a deep LP to our SW.
Personally, I don't mind this, it's the heat I cannot stand. In May, we had the highest temperature I can ever remember for that month, 29.2C, although I suspect my thermometer may be reading a degree or so high, but that' still mighty hot!
Interesting to see what July brings - what's the betting it turns hot and steamy for the Olympics!
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Hi folks, winter still has a sting in its tail! Snowing here in Southampton for about an hour, temp 2.8 now, it started snowing with a temp of 3.5, amazing. Since 9am the temp has gone down from 8.8 to 2.8! Some drop.
Around 1110, a front went through, very marked wind shift, from SW to N, the frontal clouds were showing a distinct swirly upward motion, sort of thing you see when the front is capable of spawning tornadoes. The MetO were analysing it as an occlusion but it looked much more like a cold front to me.
Seems to be dying out now, I expect we'll see the edge of the frontal cloud soon.
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Morning all.
Most of the snow gone here, temp 3.1. Seems a small LP did form, but it was much further west than I thought last night, but it has had the effect of bringing winds in off the Channel for a longer time, and with a longer sea fetch, hence the rise in temps. Had it been further east then winds would have most likely have stayed SE to E.
As the pattern shifts over the next day or two, seems the warmer air will try and push right through the area this time, but we could get snow ahead of it. Best we can hope for. There is still some uncertainty next week. Last night the 120hrs UKMet had a strong northerly blast coming right over us but the models seem to have backed off that idea today. We'll see!
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Still light to mod snow, temp steady at 0.6, perhaps 1/2 cm, but roads/pavements are clear fortunately.
Time to go bed, nite nite everyone!
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Many thanks for the detailed response OMM. Snowing hard now, albeit very small flakes.
You're welcome Luke. Curious the Met are apparently talking about snow turning to rain, maybe there IS a LP coming south, which would bring about a change of wind direction to onshore, but only temporarily before the cold air comes back from the E.
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Hi OMM - what affects do you expect the wind shifts to have?
Hi Luke, when the temps are marginal, that is, a bit above freezing, any shift of the wind to onshore can be enough to lift the temp so that you get rain rather than snow, whereas the reverse is true, which is what seems to be happening now with an E to NE drift. But with such a slack pressure field as we have, you can get local winds changes due to local geography say, or small scale features such as the LP that might be nearby, as I mentioned earlier. The observing network is not dense enough to pick up these small features very well, plus we don't have the benefit of visible satellite which can pick up cloud patterns/motions, and therefore windflow, that the IR can't.
However, the cold air is spreading back, undercutting the frontal zones which on its own would favour snow rather than rain.
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Heavier in Southampton. but the temp is still rising - odd.
Absolutely no wind at all, totally calm.
Probably because the cloud is a bit thicker. I noticed this afternoon that the temp DROPPED half a degree when the sun came out - weird. Now, as I said, I think we will see some wind shifts which may affect what happens on the coast. Shame it's dark because it would be possible to tell a lot more from the low cloud motion.
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Excellent analysis OMM.
Thanks GTLTW, as ever I don't envy the official forecasters!
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Having a good look at the satellite and the surface charts, quite a complex - and developing - situation. I've a feeling there may be a small LP somewhere near, perhaps over N Hampshire/Berkshire, not enough data to be sure. In any case, it's clear from the cloud motion that the cold continental air is pushing back again. The 2 frontal zones seem to be very close together or even merging, which may account for the intensification we've seen in recent hours, that together with that possible LP.
This situation is different from the weekend in that we don't have a slightly warmer airmass coming in from the NW behind the precipitation as happened then, but the very cold, dry air coming in from the E instead.
Our region is well on the cold side of the fronts so rain or snow will largely depend on local wind shifts (onshore or offshore)
Most interesting and no doubt giving the forecasters a hard time!
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What is it that determines the size of flakes? Is it the rate of "rain" or something else?
Hi Stormforce. Big flakes tend to occur when the temp is just above freezing point, so the flakes melt slightly then re-freeze and stick together. Yes if the temp were well above freezing and it was heavy rain, then near freezing, heavy snow and big flakes!
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Thanks OMM as ever, sounds pretty good for a lot of us, will parts of Dorset be getting any do you think?
Thanks GTLTW, well, judging by the radar one of the brighter echoes seems to be headed in that direction, but the temps/wind direction are a bit marginal, so it'll be a close call, especially for the coastal strip. Mind you, I didn't expect it to snow here! Dew points are low so if it comes down heavy enough could see a lightish fall right along the coast - for a change!
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I wish I could like your post, OMM, but they are so MEAN on this board...
There you go Kate, I'll like yours instead!!
Model Output Discussion 30/06/2012
in Forecast Model Discussion
Posted
Good point, TWS, but I have seen past similar occasions in summer of a fairly deep LP in Biscay or over and to the W of Portugal spiral around for a day or two and finally move off to the E, as pressure builds over us - yes cool NE winds to start but then the jet re-establishes itself nearer it's normal position well to the NW, with HP finally settling over us.
Bit of a long shot I know, clutching at straws really! Much depends on whether the jet really does set out from N America as per the GFS - the Atlantic flow is quite weak at present.