Singularity

That feature headed for areas IOW and just west is rather interesting isn't it?

The low cloud bases was something that really caught by eye when looking at the models yesterday evening. Between 50 and 200 m does look likely at times. Any lightning activity may seem especially loud and with a high risk of them being cloud to ground rather than inter-cloud. Precipitation wise, 70 mm is not an unreasonable expectation on a very localised scale. We're taking like 1 in 100 towns or something.

Cells just exploded in the Chanel south of Weymouth - from trace amounts to over 16 mm per hour in the space of 15 minutes. There's also a distinct arc of convection leaving N. France, hints of the kind of organisation that could produce some nasty wind gusts along with that tornado threat.

Yes, promising at the moment. I wonder how well the tropical air has retained its identity across all those miles. When I said 'day to remember' earlier I was thinking in terms of large rainfall totals, as for me more than 40 mm in a day (let alone 12 hours as may be experienced) is a very rare occurrence. Yet in light of Nick's convective outlook (thanks Nick), there could actually be some memorable storm rotation/strong winds or even a tornado for a few lucky/unlucky (cross out depending on your p.o.v.) people. The main issue I can see is a lack of surface heating, with large scale uplift forced by the low having to compensate. The Channel may be warm enough to give things an extra kick in the far south though. Shame this thing's making the run-in during the middle of the night, as it would be fun to compare it's track with the model predictions and attempt to anticipate any last minute changes needed for nowcasting. As it is, sleep can't be ignored for the likes of me without landing a heavy blow (v. high metabolism), so I'll have to turn in and wait to see what tomorrow morning brings.

It looks like next week will be largely defined by how strong the jet stream is as the week gets underway, as a large Atlantic trough extends toward the UK. Essentially, the stronger the jet, the more it will drive low pressure development from this trough extension. GFS' 12z operational run manages to create a well defined system which stalls out over the UK with a lot of diurnal convection thanks to cool air aloft. The majority vote, though, is for the jet to be too weak to achieve much in the vicinity of the UK - we see some more unstable conditions drift across with a day or two of slow moving showers, after which it most likely starts to settle down again. For what it's worth, I've kept an eye on 93L and it seems that although development is still expected, it has taken long enough that it's path is now looking to be toward somewhere well southwest of the Azores Isles, making it much less of a player in the North Atlantic weather patterns out to at least a week ahead.

The models seem to be struggling to capture the deep convection associated with the system - many of the higher resolution ones seem to have little precipitation associated with it until it reaches The Channel, which is a bit odd but may be related to issues with data assimilation at the beginning. Anyway, France's very high resolution AROME model develops quite a lot of downpours within the leading area of general heavy rain: ...but the positioning is further west than most models have it. If the low tracked that far west, the southeast would be at risk of seeing some of the stronger wind gusts on the eastern side (up to nearly 50 mph perhaps). There's then a fairly small region in which heavy showers and possible thunderstorms develop across S'rn England (see below), which advances quickly NE. This is just one of a remarkably wide variety of options being put forward by the models tonight, with most taking the low and areas of rain a bit further east, some keeping the leading area of rain further south, and projected rainfall totals between 6am and 6pm that on the local scale range from less than 10 mm to around 70 mm, depending on the exact distribution of those downpours. So there will probably be a fair few on here who see a 'damp squib' event, while others experience a day to remember.

Euro4 continues to have much heavier rain affecting some parts than others on a regional or even local scale. So we see a wide range of possible totals, from trace amounts to 52 mm (that 170 mm is from the rain taking place up there in NE England today...a pretty extreme projection!). Generally speaking the impacts of the storm are mitigated by the fact that it only just clips the southeast, with the rougher weather taking place over the near continent. Just need to watch out for any last minute adjustments, but really it's looking like a bit of a lucky escape, though gusts to around 40 mph could still bring a few branches down. 93L in the tropics is being sneaky this morning - not much convection but signs of consolidation nonetheless. So still a possible player in the N. Atlantic pattern down the line. 95L is not far from 93L and putting on a much better show despite being close to the equator where it can be hard to gain much spin. The expected track is more to the west though, so any affects on the N. Atlantic won't be for some 10 days or more.

This is the first Euro4 run extending far enough ahead in time to show us what it makes of the convective potential associated with the deep low arriving from the southwest (there's predicted to be a lot of relatively warm, moisture-rich air within the heart of the storm). While most models have been developing what look like banded features, this model seems to be organising the convection into multi-cellular systems. By that I mean clusters of convective cells which may or may not be thunderstorms. Some suggestion, then, that the initial push of wet weather could be more hit and miss than the likes of GFS have been suggesting... but on the other hand, Euro4 does have a habit of getting carried away with the organisation of convection in this manner. I'll be watching this model's output closely tomorrow, as from what I've seen it usually gets somewhere close to the right organisation mode (chains of storms, multi-cellular systems, or banded features?), despite tending to overdo the precipitation rates.

Still we see ECM a little deeper than GFS, with just under 980 mb as of 1pm Wednesday compared to around 982 mb. GFS has adjusted west with the track a bit though, which raises the risk of some strong wind gusts affecting the southeast. Just how strong seems to depend on the extent to which the low develops a secondary centre on its NE side, as this shifts the strongest winds into the North Sea on the GFS 12z operational run. In fact GFS has been fairly persistent with that outcome, while ECM has been equally persistent in making little of the secondary feature, keeping the strongest winds far west enough to affect Southeast England. I can still see the potential for a few 60 mph gusts, though for the most part the peak is now looking more likely to be nearer 50 mph. The low then slowly but steadily clears off to our NE to leave us with a very well timed ridge of high pressure for the weekend. The airmass looks warm enough to allow for day temps to reach the high teens widely, perhaps low 20's in the south. Chilly by night though - but having said that, frost isn't being indicated at this time. The Sunday chart from GFS is a fine one indeed: Then it looks like head scratching time, as there's expected to be a broad trough over NW Europe, and any venture of the Atlantic jet stream toward that region is likely to result in a low developing and influencing the UK to some extent or other. Currently we have GFS liking the idea, but ECM's not keen, instead allowing high pressure to ridge across from the Azores - albeit weakly, with the risk of shallow disturbances bringing some cloud with a chance of rain in places. There is a caveat to add regarding developments beyond the weekend, though - it all assumes that 93L doesn't kick off, when actually it still could despite a poor show today, or so the NHC thinks: "A low pressure system located several hundred miles west-southwest of the Cape Verde Islands has produced little shower activity today due to dry air nearby. However, upper-level winds are expected to be conducive for tropical cyclone formation and this system is still expected to become a tropical depression during the next couple of days while it moves generally northwestward across the central tropical Atlantic. * Formation chance through 48 hours...high...70 percent * Formation chance through 5 days...high...80 percent" So it could be that the models have to make some abrupt changes in the near future if the NHC are on the money.

http://www.bbc.co.uk/news/science-environment-34226178 This seems a bold statement given that a sharp transition to La Nina was reflected in a similar sharp drop in global temps after the 1998 event. Could there be indications that La Nina will fail to manifest next year? That would be one for the books!

Ah... now I see it. It all makes sense having looked on that site - thanks for the link. I was confused by the way the low seemed to develop from nothing in 5 days time on the N. Atlantic charts. Turns out GFS keeps it too shallow to show up on the charts until that time... lesson learnt! In which case, there's every chance that the behaviour of the Azores/N. Atlantic high pressure beyond 5 days range could end up substantially different to what's currently modeled. It'll all depend on how strong the tropical cyclone becomes... I see the GFS 06z has backed off a bit and it seems that a weaker low tends to end up further west with less chance of influencing our weather much.

This isn't the system that GFS develops in 5 days time - it's an area of disturbed weather some way SW of the Azores Isles which I have seen rated at near 0% for the next 48 hours and the same again for the next 5 days. I got that info via this blog post: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3111. I think it may be the one that comes into the field of view in the past two GFS op runs at about +252 hours. GFS then had it doing a dance with the one near the Azores on the 12z operational which was fun to watch. The 18z clears the Azores one quickly NE via the UK (as a weak remnant) and then traps the one that originates near the CV islands underneath a strong mid-Atlantic ridge. That 'trapping ridge' keeps on coming up in the GFS operational runs at the moment.

We do need to watch out for an area of strong winds that could affect the southeast on Wednesday. It looks like gusts to between 50 and 60 mph are not out of the question should one of the more aggressive model solutions land close to the mark.

Important differences between GFS (top) and ECM (bottom) at just 72 hours range. GFS has a slightly weaker system than recent operational runs, which crosses the far southeast, keeping the strongest winds over the near continent, where 40-50mph gusts are projected. ECM sticks with a deeper low than I have seen on any of the GFS op runs, with a 980mb feature in the above chart that is tracking west of the GFS path, meaning some of the strongest winds could impact the southeast. Wind gusts could be as high as 60mph, which would cause the trees some real trouble. Some very heavy rain is expected, with embedded convective elements that I doubt GFS has fully resolved. By Friday the system is clearing away northeast... but it could linger based on the GFS 12z operational. There's no such hanging-back from the ECM version of events, though. So we could have a lot of showers or just a few - in both cases tending to fade away during the evening. We then seem to be in with a good shot of seeing a ridge of high pressure for next weekend. However, it may not be enough to save the far west and/or northwest from some rain before Sunday is over and out. Curiously, GFS has become very keen on a low sliding SE from the North Atlantic to sit across the UK by Monday next week, with great run-to-run consistency over the past 24 hours. Meanwhile ECM has been more uncertain, despite being the first model to detect the jet taking a more southerly track again after just a brief break during the weekend. Beyond that, a key player could be a tropical storm or hurricane which GFS has been very keen on for a couple of days now: That could bring all manner of results for the UK depending on its track - you can see in the chart shown how it favours a ridge to build ahead of it should it track that far west, but if it tracked further east it could head toward the low pressure over the UK, which would be... interesting. Remarkably, GFS now develops the cyclone 5 days from now, yet ECM refuses to develop it at all, and has been sticking to that across all of the past few operational runs. The official NHC discussions for America are currently discounting development of this system, suggesting that GFS is off on one. The Azores Isles will be relieved if that proves to be the case!

I believe bluearmy is referring to the potential for these recurving systems to interact with the mid-latitude westerlies and bring us some interesting or at least unpredictable weather at times? GFS does seem keen on having a number of tropical cyclones wandering about in the subtropics, though it may be overdoing the intensity in some cases. - - - - - - - Short term, Euro4 shows nicely the wedge of relatively warm, moist and unstable air moving up across the south from near midnight, here's the 3am position: As the day progresses, that wedge moves north and some drier air cuts in from the west to lie across the south by mid-afternoon. Consequentially, shower activity looks to kick off in the middle of the night across the south, then become more vigorous during the morning but with the strongest activity trending north. By the afternoon convection is expected to be suppressed across southern parts, though showers may manage to keep going in some guise. Meanwhile some diurnal heating of the surface looks likely across the middle third of England and Wales during the morning, with the convective activity arriving during the afternoon and potentially utilising this surface heating to kick things up a gear, with a higher risk of thunderstorms featuring cores of very heavy or torrential rain and of course some electrical activity. In terms of severity, the surface low associated with the warm, moist wedge of air (it's the warm sector) does offer some storm-relative helicity (a measure of rotation potential) to convective cells, however directional wind shear looks low, meaning storms aren't likely to stick around long enough to achieve dangerous levels of organisation. There is some suggestion of the convective cells merging into some form of organised line by late afternoon, along the occluded front marking the leading edge of the warm sector, but again the risk of severe conditions is looking on the low side to me. Things then complicate a bit going into Tuesday, as a secondary low develops on the SW flank of the main low and draws the warm sector back southwest, this then moving across the south while the remainder clears steadily away from areas further north. The result in terms of precipitation looks to be a more pronounced back-end to the warm sector crossing the far south into the early morning, while areas further north see another day of showers, though these could tend to fade out by the afternoon as the warm sector clears to the east. Then comes Wednesday's trouble, which the models still seem to be having issues pinning down the details for, so I'll leave that alone for the time being. It does look like a real soaker of a day though.

That ECM 12z operational run sure had it in for S'rn England... a nasty low at just +96 hours range, but are we to trust that model over the others? There would probably be some 50-60mph gusts out of that, even inland. Low risk 70mph perhaps? Looks like HP will get a chance to ridge in behind the resulting LP complex as it lifts out NNE, but another area of low pressure west of the UK may simply flatten that to sustain changeable westerlies... OR it could drop south and become cut-off from the Atlantic jet according to the ECM 12z operational. The change of heart from the morning run - which was very flat with the jet pattern - seems to depend on the exact timing of the Wednesday low plus a tropical storm engaging with the jet in about 9 days time. So we're talking about an outcome that could easily disappear as suddenly as it materialised. Which is a shame because I find it a lot more interesting than the idea of a standard westerly pattern with pressure highest across the south.

That is one vicious little low spinning into the southwest on the ECM 12z operational in five days time. This is a little concerning, but at this time only GEM even comes close among the other models, so it's a case of waiting and wondering whether ECM's high resolution is picking up on some small-scale structure that the others are missing? In the case of GEM we're talking about a model that has a habit of making the absolute most of any intense LP potential, so the default position has to be one that assumes it's overcooking the storm. So overall it still seems to be more likely to be a 'rainmaker' than a wind event. At least we're now seeing good signs that high pressure will have a go at following on behind within a day or two. What will be interesting regarding future prospects is whether the jet is adjusted south nearer the time of not, as that would represent a backing down from a typical El-Nino driven tendency to raise pressure over Europe.

I've just checked out the past 22 years' worth of N. Atlantic SST anomalies and 300hPa zonal wind anomalies for the period Nov-Mar (using NCEP/NCAR Reanalysis charts). Overlaying where the peak zonal winds occur onto respective SST charts, 12 of those years display some hints of a correlation between the two, with the peak zonal winds (hence the jet stream, for the most part) tending to favour a track that takes it over or just south of negative SST anomalies, or over the northernmost extent of positive SST anomalies. Here's a snapshot of the lot - I'll admit that 2008 is stretching it a bit as the jet was a bit further south than my proposed relationship with SST anomalies suggests. ...but then there's the remaining 10 years which tend to defy the theory in its entirety. I'm still looking at those, as closer inspection may reveal some reasons why (though I'm not expecting much. Also, it appears I may have 1999 in the wrong group (should have been included above). This is all work in progress - if you have any ideas or thoughts then I'm all ears! ...or should that be eyes?

ECM continues to make a lot of the potential, GFS not much at all, and UKMO is somewhere in between. Typical! Interesting longer-term potential for some exceptional warmth to envelop the UK and NW Europe as the trough retrogresses in the aftermath of the tropical remnants getting involved. After all, the tropical import helps to bring about 850hPa temperatures a whole 12*C above the usual levels across NW Europe on the 7th day of the ECM 12z operational run: ...so there's plenty of heat potential (relative to the time of year) should we see high pressure develop across Europe and circulate the air around to the UK in some fashion. Way too much uncertainty with regard to the complex low pressure behaviour next week to place much faith in this yet, though.

Another set of runs, and no closer to knowing how the ex-tropical low will behave early next week. Key differences between the operational runs are already evident as early as +96 hours. See how GFS (first image below) has a small low SW of Greenland that's having nothing to do with the remnants of TS Hilda moving north of the Azores (the vague 'sausage' of lower pressure west of the UK trough). Yet ECM (second image below) has the two lows interacting, with their combination resulting in a powerful low southwest of the UK 48 hours later. Even the GFS 18z operational, which also had a powerful low to the southwest, took 24 hours longer to produce it, as it was the result of disturbances combining just west of Iberia. The results were quite similar though, despite the differences of timing (see below images, GFS first, ECM second). In both cases, the import of warm (even very warm) air into the UK combined with rapidly falling pressure suggests a recipe for some severe weather in the form of heavy rain (with convective features) and strong wind gusts. Relative to this, the GFS 00z operational is very tame, the only real trouble being caused by some trailing frontal boundaries in the manner warned about last night. I was going to look at the ensemble clusters again now, but my PC is forcing a restart with no option to postpone it... how rude!

Briefly on the rain potential next week, that currently looks to be highest in association with a shallow low running up against high pressure across Europe on Wednesday, following Tuesday's potential windstorm. The alignment of the flow will make a big difference to the totals though, with a more SW-NE alignment as per GFS reducing the risk of a frontal boundary 'trailing' across part of the UK to deliver an extended spell of often heavy, sometimes torrential precipitation, when compared with something closer to S-N such as ECM has come up with tonight. I still think Tuesday's event could produce some very heavy rainfall, but the speed at which the low looks likely to move is probably going to keep the totals from being excessive. Perhaps 20-30mm generally? There's also every chance that Wednesday's threat does a vanishing act, as it wasn't there at all on the GFS 06z operational run, for example. Shorter term, Sunday's low seems to be causing the models a lot of headaches. The large range of solutions and lack of severe outcomes within that range is the reason for my looking past it for the time being. They say ignorance is bliss...

Some of the long-range model guidance is suggesting that blocking highs over Scandinavia could be a common feature during the second half of this month and a large part of October. ECM's output for 10 days from now ties in with that nicely, but GFS has the jet stream proving too energetic and running over the block, resulting in the high being pushed south until it's E of the UK rather than NE. That brings a southeasterly flow with some very warm air for a time that lifts temperatures into the mid-20's for a couple of days. A Scandi High offers less in the way of warmth, but could do a better job fending off the Atlantic storms. Unless of course the jet undercuts the high or the lows grind to a halt just as they begin to influence our weather, in which case you have slow moving areas of rain to contend with. As usual, the reality is most likely to be a combination of these things, with the exact distribution depending on the strength of the Atlantic jet. Here's hoping that it isn't getting a boost this season - I've been really enjoying the usability of the recent dry weather and it'll be a shame to lose it Friday night.

This cluster is composed of 20% of the 12z GEFS and appears to represent the scenario in which a vigorous low races across England from the southwest without merging with the broader scale Atlantic trough. It's the most threatening in terms of high wind gusts for the country, and among the wettest scenarios for England and Wales. This cluster has 15% of the GEFS and is similar to the 12z ECM op, as the tropical remnants merge with the trough to create a large, deep low that focuses the rain and high wind gusts across the western half of the UK, the coasts getting hit hardest. This is the largest cluster, with 40% of the GEFS, and shows little appetite to develop a nasty low in the vicinity of the UK. I didn't check this morning but I suspect that this cluster was smaller back then, with an overall move toward the tropical remnants heading too far south to engage with the jet stream and bomb out.

GFS is still keen on using that ex-tropical material to deliver a nasty blow to the UK from the southwest next Tue/Wed (and some very heavy rain for western parts, not shown): With the trees still in leaf, such strong gusts (nearing 60mph even inland) would be damaging to trees. Something tells me this is the right sort of setup - with a rapidly developing low featuring a compact core - for sting jets to pose a threat - definitely something to keep an eye on. ECM also gives reason to be concerned, albeit with a slightly different approach. A very compact 994mb low races across England on Tuesday night (see first of two charts below), bringing some heavy rain and the risk of very strong wind gusts depending on the mesoscale structure of the storm. I know some of you will be reminded of the storm of '87 (also a compact system, though it was a sting jet that did the most damage), but these events are rarely that extreme despite their potential. I can't rule it out though - having tropical remnants in the mix inevitably brings such events into the achievable range. Strangely enough, ECM brings a second compact system up from the south during Thursday, which if anything looks more threatening than the first. I can't quite work out which system is associated with the remnants of the tropical cyclone currently starting to get its act together near Bermuda. Perhaps the associated moisture breaks down and is involved in both? Such an outcome, with two moisture-laden systems so close together, would produce some very high 2 or 3 day rainfall totals. Those who have been wishing for more interesting weather next week might get just that... but with a nasty catch?

Seems GFS has finally moved toward the multi-event solutions favoured by ECM lately. We're still not seeing much sign of anything extreme within the next 6 days, with fairly typical wind and rain events for the time of year. Perhaps ECM would produce some large multi-day totals as it continues to make more of the secondary lows while also taking them through the UK more slowly. Then comes a potential troublemaker, as the models are keen on bringing an ex-tropical system our way, after it passes close to or over Nova Scotia this coming Saturday. As it arrives in the vicinity of the UK from the southwest, it comes up against some impressively warm (or even hot) air across NW Europe, which drives a strong jet stream capable of giving the ex-tropical system what it needs to become a meteorological 'bomb'. The GFS 12z operational showed us what that could look like this evening, as the rapidly deepening low was modeled to bring wind gusts to near 60mph across Southern England, and stronger still going northwest from there, exceeding hurricane force across the North Sea as the low departs, with similar gust speeds hitting the far NE of England and SW of Scotland. I need not post the charts as Polar Maritime has already given a good overview with their choice of charts ECM's not so violent but I suspect that some extreme rainfall would still be an issue for some part of the UK, as a very large amount of moisture is associated with the system: P.S. Anyweather, I'm concerned that you'll damage some brain cells if you keep on ramming that wall like that every day