stodge

Evening all The forthcoming evolution is one fans of cold and wintry weather would love to see in January. The trough to the north and east of Iceland plunges south east into the North Sea as Fiona's wash of ex-tropical air pushes north over Greenland and encourages a mid-Atlantic block and heights over Greenland. The 850s on the GFS 12Z OP at T+102 explain it far better than my verbosity. We get a plunge of unseasonably chill NW and N'ly winds as the LP sets up for the best part of the coming week in the North Sea. ECM 12Z OP has the North Sea LP phasing with the next Atlantic LP and prolonging the unsettled, cool and wet theme - GFS and other models don't have the phasing and the HP builds in to provide a few calmer days at the beginning of next month.

Very bad in Puerto Rico though the flooding seems the worst aspect - supposedly heading for the Turks & Caicos next.

GFS 06Z OP continues to play the familiar "Hunt the Autumnal Pattern" game. Indeed, as ex-Hurricane Fiona moves up over Nova Scotia and then turns NW to merge with the trough over NE Canada, a big push of WAA moves up over Greenland encouraging height rises over the continent. The heights in turn push the jet back south: Once again, the pattern is more reminiscent of a post-SSW April rather than the onset of autumn.

Does a storm as severe as this have a wider impact on the Northern Hemisphere? It seems to be disrupting the jet over North America which, you'd think, would have considerable downstream influences on the North Atlantic and Europe and perhaps on the progression of a storm like Fiona or does it have very little impact beyond Alaska?

Evening all 12Z GFS OP offers a direct hit from ex-Hurricane Fiona which utilises the very warm Atlantic SSTs to maintain its intensity even as it curves away from the Eastern Seaboard. With a more southerly track it reaches southern Britain as a small but vigorous feature: Obviously one option among very many - Control sends Fiona north and it dissipates over southern Greenland.

Evening all Once again the far FI reaches of GFS 12Z OP show warmer air and heights returning to Greenland. Many options of course with the possible Hurricane Fiona including the Gulf of Mexico but GFS takes the storm up the eastern seaboard and north into Canada where it is absorbed into a trough over NE Canada which sends a nice dose of WAA over Greenland. Will be watching this next possible hurricane with interest.

Evening all Once again, GFS 12Z OP is more April than September - a classic early spring post-SSW evolution with heights in Greenland, the trough over Scandinavia and the cold air mass approaching the British Isles from the North and Northeast to end the month. What do you expect with the Atlantic still moribund and little or nothing happening in the tropics?

Evening all May be something, may be nothing but I just noted on the GFS 12Z OP in far FI a new push of warmer air and heights back into Greenland. It does look as though the summer pattern is hanging around - once again, after Danielle and Earl, nothing much coming from the tropics. 10 HPA temperatures falling and the winds starting to pick up but nothing dramatic as yet.

Evening all Thought a diversion to the models might be in order. I'm struck immediately by the predisposition to northern blocking - the 12Z suite looks more like a post-SSW late spring than an early autumn. For most of the runs, a a moribund Atlantic with only extra-tropical systems giving a little energy to move things downstream. The earlier hints at the colder airmasses pushing into Greenland and NE Canada and kick-starting the process of cyclogenesis in the western Atlantic seem more muted tonight with HP building less from the south west via the Azores and more from the north or north west which again seems unusual. After Earl, we may have another Atlantic lull - I just wonder if October will be an active month. In the absence of the early autumn jet we may well luck out with an anticyclonic evolution but it may not draw up the warm S'lies for which some will hope but an E'ly. All very interesting.

Evening all I suspect the lack of comment is symptomatic of the variability of solutions being offered by the models. Three broad thoughts: 1) Danielle - looks as though she will go nowhere slowly in mid ocean before finally heading towards Iberia. 2) Earl - will be briefy a major hurricane but it looks now it will go north which will put some energy into the area south of Greenland 3) Having thought autumn was on the way last night, 12Z GEM and JMA both keep the more summer pattern in place with a warmer air mass and heights over Greenland. This leads to a huge variation with GFS OP which goes into a more classic autumn profile of LP near Iceland and a weak ridge over southern Britain/northern France. 12Z GEM OP at T+186: Again, just worth noting how unusual Danielle has been in terms of its longevity - a long way north and east of what we'd normally expect. I've seen comment on warmer North Atlantic SSTs and what that might mean for ex-tropical features. Might be worth considering as we move into October.

Evening all What strikes me on the GFS, ECM and GEM charts is the rapid transition from the summer to the autumn patters. Any remaining warmer air over Greenland is quickly displaced and the colder air mass takes over (850s down to -20 by T+384) spreading south west from the Pole suggesting the formation of the PV underway and signs also of a strengthening Atlantic jet into FI. At this stage the jet isn't strong enough to prevent HP forming in the vicinity of the British Isles and GFS OP ends very anticyclonic, Control much less so. Clear signs of increasing 10 HPA winds and falling temperatures in the stratosphere:

Evening all Have to say looking at the GFS 12Z OP I had to remind myself it was September and not a post-SSW March. Indeed, in some ways a dream winter chart with LP to the south and south west, heights to the north and in far FI retrogression to Greenland - "if only"... Control goes for a Scandinavian HP so that's both your winter dream scenarios. The truth is we've not anywhere resolved what ex-Hurricane Danielle will do let alone Earl which is forecast to be a serious hurricane by T+120. For now, trying to work out much in the medium term isn't easy.

Evening all The key development today is the forecast intensification of Earl from midweek onward. Having looked as though it might be a minor player, Earl is now forecast to be a major hurricane at T+120. By T+216 (on the ECM12Z), it is out in the Atlantic and seemingly heading toward us. I have to say the ECM 12Z isn't much of a run for those wanting sunshine and warmth GFS 12Z OP at T+216 is a shade different though the position of Earl isn't dissimilar to ECM. GFS then sends Earl to the Azores where it lingers and then gets absorbed into the broader Atlantic trough and runs rapidly NE to the British Isles as a secondary LP which would have a lot of rain but not so much in the way of wind as we often see with these ex-tropical features. Right out at the end of GFS FI we see the first signs of lowering 10 HPA temperatures but the PV isn't in place and the Scandinavian HP can form without too much trouble at this stage so perhaps after a fairly wet spell the second half of the month will see more of the traditional "mists and mellow fruitfulness" so to speak.

Yes, GFS 12Z brings the remnants of ex-Hurricane Danielle across the British Isles as quite a deep and complex LP for early September with a lot of rain for southern and western parts in particular. The following ex-hurricane takes a more traditional northward path and is absorbed into the main Atlantic trough.

One of the reasons why the ECM 12Z develops the way it does is what happens with Hurricane Danielle. As it becomes an extra-tropical feature, it moves north and elongates into a broad trough By T+240 it has settled in the Atlantic but it's not a typical zonal feature by any means - look at the absence of cold air over Greenland for example. The typical cold North Atlantic jet hasn't fired up yet but the residual energy from Danielle looks as though it will encourage cyclogenesis and allow a possibly vigorous secondary LP to develop and run NE towards the British Isles. In turn, the Azores HP ridges positively but is well to the south. Plenty of northern blocking on that chart - certainly NOT autumnal.

Evening It looks as though Hurricane (as she soon will be) Danielle is going to meander round in the middle of the Atlantic for a few days before being able to engage with a trough and give the LP over the British Iles enough of a kick to push it east and allow heights to rise. Some models project a second storm to come out into the Atlantic this time next week but very little agreement on its course which of course leads to all manner of FI synoptics which need plenty of caution at this time. We need to see what Danielle does once she reaches cooler water and some wind shear - I'd still expect some form of heights by the end of next week but not the traditional Azores ridging north east but a separate cell of HP perhaps over or slightly to the north east of the British Isles.

Once again, the problem of forecasting these tropical or ex-tropical features has significant downstream impacts on the European synoptics. In general terms, if your storms stay well away from the British Isles, the chances are you will get HP resulting as the storms force the Azores HP to migrate NE. When a storm traditionally recurves and becomes absorbed in the Atlantic jet, it brings energy to the jet and the storm track so pressure rises are usually confined to the south and east of the British Isles. In a period with such a weak jet and continued northern blocking, the storms amble pointlessly around mid-ocean until either becoming absorbed into a larger trough or decaying in situ - that's what makes the mid range forecasting so difficult. After next week's unsettled weather, it's possible a strong ex-tropical feature will kick the shallow LP to the east and allow heights to build at the end of the weekend after next but that's just one option.

The GFS 12Z OP illustrates the impact of a significant tropical storm/hurricane. It takes quite a southerly route which sends the Azores HP up over the British Isles in response. Control takes the storm on a more traditional route curving NE until absorbed into the broader North Atlantic trough. Nearly every other model does something different - as we know, forecasting these ex-tropical features at 10 days or more remains problematic. Interestingly and of little relevance to Europe, it remains active over the Eastern Pacific - GFS 12Z OP sends a significant hurricane up between Baja California and the Mexican coast so presumably the monsoon type airmass over the south-west of the US would be re-invigorated meaning more rains for southern California, Nevada and Arizona.

Thanks for the kind word. I confess to being puzzled - we know extra-tropical energy feeds into the higher latitudes as the ex-hurricanes merge with Atlantic fronts or LPs and energises them. We also know the eventual temperature contrast as the north moves into winter darkness will help fire up the jet as warmer and colder airmasses meet and cyclogenesis occurs and that's a normal feature of autumn and the ending of the summer pattern. What we have now is a typical late spring post-final warming pattern with a lot of warm air over high latitudes. I can't see that lasting - basic physics tells me it can't but if we have a strong late season (storms into October or even November) could we be looking at a rapid pattern change to a very active autumnal jet with strong storms and high winds especially, you'd think, for northern and western Britain? I also note the SSTs in the Atlantic which would be conducive for rapid cyclogenesis.

Evening all Been away from the charts a few days so popped in to view tonight's 12Z output. To be honest, I thought it was April not August with lots of northern blocking - perhaps indicative of the absence of extra-tropical energy from hurricanes. The current HP to the north eventually recedes to the NE and the trough fills the gap as there is no energy from tropical activity to give the Azores HP a push to ridge NE so we get stuck under the LP for a while. One or two hints the hurricane season might finally be getting underway - some American forecasters continue to be bullish - but I'm struggling to see anything much. Could be quite a wet spell to begin September.

As we discussed last evening, however, with no real push of tropical energy in sight, the Azores HP oscillates listlessly occasionally throwing a ridge across but we don't get the full migration sometimes experienced as a result of that input of tropical energy. That means it never really shifts the core to the east allowing a renewed push of hot air from the south so, in a sense, it's always winter but never Christmas for heat fans - warm yes but no final heat burst. If the energy isn't going to come from the tropics, it'll come from the north over time and that may allow more ridging but it might also begin to suppress the Azores HP back south and west as north and north west Britain come more regularly under an Atlantic influence but it's still August and the summer pattern prevails for now. The other possibility is the hurricane season will be late but intense with frequent storms getting going from say mid September well into October or perhaps beyond. That would certainly give us an autumn of interest in terms of model watching.

I'm also struck by the continued lack of tropical activity and the impact that may have as we move into September, traditionally the heart of hurricane season. I'm told this is due to high dust levels coming off the Sahara suppressing convective activity. We know the path of these storms can cause the Azores HP to respond (often favourably and that's how we can get our decent autumns and "Indian Summers") but if we're in for a much quieter hurricane season, my presumption would be the cooling to the north would create a more traditional Atlantic flow and bring a cooler, wetter and windier start to autumn than other years.

Evening all Plenty of detail still to be resolved, but looking across the models at T+120, remarkable unanimity: GFS OP, ECM, JMA and UKMO from the 12Z suite All have the British Isles under a broad but slack trough of LP which suggests a growing likelihood for showers and thunderstorms though these will obviously vary on local conditions. The heat may indeed take a little more time to leach out than if we were dealing with an Atlantic frontal system moving in from the west but the net effect will be the same as the trough moves up from the south and fills the gap between the two HP centres to east and west. Hot until Sunday but from then a progressive return to normal temperatures by midweek next week and as others have said some much needed rain but nowhere near enough.

Evening all Have to say the GFS OP and Control in far FI of interest for all the wrong reasons - a powerful storm forms over Canada and stalls to the west of Greenland throwing some WAA over the ice sheet and promoting heights. This has impacts downstream in terms of dragging the jet back south and allowing LP to become established over the British Isles and NW Europe. One to watch... In the much more reliable, the ridge continues to align positively so it's always settled and dry over southern Britain with rain-bearing systems to the north and west. Hints of a more unsettled week next week but obviously a lot still to be resolved.

Evening all Once again, I'm left wondering if I'm seeing the same models as everyone else. A few last embers of warmth this weekend and then into the end of month and August a slow but general pattern change to something more "typical" for the time of year. We've enjoyed a positively aligned Azores HP for an extended period with the ridge almost permanently tilted NE towards Ireland - this has kept most parts settled and rain-bearing frontal systems are suppressed. At the same time, as the ridge is a little further north, it has allowed instability to the west of Iberia to pull the African heat north through France to the British Isles. This has been supported by an almost moribund Atlantic (perhaps a legacy of the sudden and dramatic collapse of the PV in the late spring) but as often happens in August the Atlantic starts to revive and a new spell of weak but broad troughs starts to develop and meander across to the north of the British Isles into Scandinavia - this in turn is aided by a relaxation and re-alignment of the Azores HP into a neutral or slightly negative alignment. This keeps the hot air bottled up over central France but much further south it allows the African disturbances which become tropical storms and hurricanes to come off the west coast and start propagating west along the line of the ridge across the Atlantic. For the British Isles initially, it means more of a west or wnw flow and more active frontal areas getting further south with time. To be fair, much of the south and east will remain mostly dry but warm rather than hot and perhaps with more cloud. As to how the hurricane season will play out, that remains very much to be seen.