Singularity

An interesting feature that keeps appearing in the scenarios for next week is an exceptionally strong yet zonally-elongated Azores High. That means a lot of very warm air heads generally eastward across the N. Atlantic & there's a decent change that high is large enough to nose across the UK at time when usually we'd stay in a changeable, at times cool regime. If it does so, the air could be unusually warm for a WNW to NW airflow, such that temps range from near average in the northwest to a few degrees C above in the southeast. All this will be very sensitive to how zonally elongated (i.e. stretched west-east) the Azores High is. The 00z was at the high end of the spread for that, while the ECM 00z was more middle-ground and the GFS 06z has moved someway toward that. That middle ground looks a bit drab for the northwest with a lot of cloud and temps near or a little below average, while the south and southeast experience near to slightly above average temps with sunny spells. Lots of water to pass under the bridge before then, though. It's hard to sit comfortably when the UK looks to be on the periphery of high pressure as opposed to right underneath it (that will become more likely during the 2nd week of July).

The usual question of progressiveness arises - with the deterministic model runs, we often see a new direction of travel brought on a bit too fast at first. ECM 00z felt like a good effort, with the slack ridge next weekend before a more emphatic one arrives on D10, though I’d not surprised if it happened a day or two later in reality with LP staying closer to the NE beforehand. At this time, the 2nd & perhaps 3rd weeks of July looks to me to hold highest potential for a lengthy very warm or hot (but potentially very hot at times, which is a concern) spell of weather affecting more than just a little of the UK.

EPS produce more interference with the MJO signal in the RMM measure used by Hendon & Wheeler, probably related to the probable existence of at least one tropical cyclone in the tropical Atlantic / Caribbean, along with unusually strong tropical waves for the time of year. GFS/GEFS have been underestimating those features, leading to less interference. Regardless, the actual MJO forcing exists & as the ECM 00z shows, maybe strong enough to force an onset of extension of the Azores High across the UK in early July.

I believe there’s a combo of bias (low in 4/5 for EPS) & some misrepresentation by the HW method of plotting in play. That said - the event is not a given, as noted in my post.

We have growing support for an MJO/CCKW propagation across the Maritime Continent (Indonesia) in early July. When the MJO is propagating east across these phases (4-5), AAM tendency has historically been at its most positive. When AAM climbs toward neutral from a very negative starting point in July, there tends to be a shift of anomalously strong high pressure eastward from the Azores to western Europe. It typically weakens SLP-wise once overland due to surface heating (rising air lowering SLP) but remain robust higher up, as evidenced by high 500 mb geopotential heights. As Tamara described, this is how a more sustained heat build can manifest. That's how the MJO should really be used as a forecasting tool, rather than relying on composites for particular phases that don't account for whether its an actual MJO/CCKW event (i.e. propagating east) or some other noise (not propagating). I'm actually worried about what manner of heat build is likely should this MJO/CCKW event manifest. There's a severe marine heatwave affecting the western Mediterranean, while the eastern North Atlantic is mostly warmer than average, so overland temperatures will be able to rise quickly and far, much as seen over western mainland Europe last Fri-Sat. Unusually dry soils are also widespread, further increasing the potential daytime maximums. This 'preconditioning' is quite similar to that which was in place prior to the 2003 heatwave. Whatever heat build does occur, its longevity will depend on just how high AAM climbs. It feels unlikely that it will make it far above neutral, but you never know - a strong MJO would be able to force that, at least briefly. Once AAM drops back down, it will come down to the AO & summer NAO; if they are positive then trailing ridges may keep a hot pattern going for maybe a week or so. Current extended ECMWF modelling is suggesting that outcome and it's been consistent for a few updates now - notable given it predicts temperature anomalies 1-3°C across nearly all of Europe, including the UK, for 2nd half July.

AAM has fallen a great deal, the trouble is some people correlate that directly with unsettled UK weather, when actually it also depends on how fast & far AAM falls & how much resistance to the usual response pattern (mid-Atlantic high & low over W or NW Europe) there is from the North Atlantic SST pattern (via influence on NAO) and Arctic Oscillation. Lately, those two have resisted a great deal, favouring a +NAO setup with north-shifted jet stream. This has been enough to keep low pressure visits to the UK transient with finer interludes in between. Signs are, that may continue through late June, though the low looks to stick around a bit more this time as -AAM is at its peak. Into early July, there remains expectation for a CCKW or perhaps MJO event to take shape and transit the tropical Pacific. That will drive AAM upward & promote some longer spells of dry, warm or hot weather across Western Europe - but to what extent remains to be resolved, as AAM might only rise to near neutral which would not suggest a prolonged pattern shift.

Studying the ensembles, the main question for the rest of June and into early July is: Will the AO remain positive enough that troughs tend to stay more toward Scandinavia than further south in Europe, such that the hot air over western and central parts of Europe is largely left in place, with the UK peripheral, often warmer than average in the south, near or a bit below in the north? Or will we transition to a more typical low AAM pattern of cool NW/N flows with mainly showers, sometimes longer spells of rain, by next Thursday? The runs are almost 50/50 on which way it goes. That the persistence possibility exists at all seems to be down to some heat dome characteristics coupled with a sort of MJO-Monsoon interaction over the western Indian Ocean which shows historical correlation with anomalous ridges centred over both the North Atlantic and Central Europe. For early July, extended modelling has long suggested that the MJO will set off eastward and drive a big AAM increase. As Tamara noted, this would give us a more clear cut warm/hot & dry weather pattern. There’s yet to be much sign of it in the outer reaches of the mid-range guidance, though.

For the southern 3rd of the UK, the UKV 03z has the warmest, driest outcome for Father’s Day that I’ve seen so far outside of the ensemble runs, reaching the low to mid 20s. On the other hand, it’s among the coolest and cloudiest futures north with low-mid teens maximums. ECM has things further south and GFS a little further still, so fortunes are almost reversed (it’s just not as warm in the north as it is in the south in the UKV scenario). Clearly we can’t all ‘win’ on either day this weekend… and road trips are more expensive than ever! Just have to cross your fingers and hope for some luck. Beyond the weekend, really interesting how often we’re seeing runs that find a way to extend high pressure right across the UK from the west via ‘propping it up’ using a low over Western Europe. It’s pretty much the only way to achieve a particularly warm setup during a low AAM phase. The positive AO and NAO is a crucial player - some GFS runs have shown us what usually happens otherwise.

Basically not worth analysing the Fri-Sun details of any one model run at the moment, except for the fun of it with something like the UKM 12z which has a fascinating sequence of events Sat-Mon. Models are often trading solitons, such that there is no discernible trend, just options picked out of a hat. Confidence is actually a little higher for what happens afterward; ridge nosing back in from the west due to the heat plume taking on heat dome characteristics, modifying the surrounding environment (air rises within the very hot zone and sinks around it). It can take a strong kick from the jet stream to collapse those things… and they’re very dangerous to be underneath, as seen in 2003 for example. It’s a worrying outlook for much of western and Central Europe. The low AAM background may facilitate the jet stream kick needed, but the persistent low heights across the Arctic won’t help with directing it where it needs to go (straight into Europe). We saw in 2018 how they can keep the polar jet much further north even with very low AAM, though that was an extreme example also involving an exceptionally positive NAO, which we’re not seeing much sign of at the moment - so I’d not go hoping for something like that summer. The driest (but probably not totally dry) and warmest outcome I can see on the table is variably slightly cool to a bit warmer than average as high pressure focuses just west of the UK but periodically extends a weak ridge across.

It stands out to me that the modelling is generally not keen on developing a full -NAO with HLB in play. Instead a zonally orientated, highly inflated ridge is now favoured, with the surface high able to focus further east relative to the upper ridge than would be seen otherwise. This eastward offset between 500 hPa and surface nearly always exists to some extent - yet is often overlooked in the analysis I see on social media. The outlook past the potential plume is more akin to June 2010 than June 2012 in that regard.

The 18z GFS reminds me of the strange 850s record-setting June 2019 event with the unusual means to limit surface heat in the UK; an unusually strong ridge building across from the west. Doesn’t do so as much on this run as back then though. That time was bonkers with a NE flow keeping max temps mainly mid-high 20s when 850s were widely low 20s. Curious to see the Atlantic pattern adjusting so much flatter. AAM drop being toned down, perhaps?

Actually GFS has jumped to the ECM ship with the small cut-off low by Iberia stalling further west & for longer, leading to a bigger heat build that peaks Fri-Sat rather than Wed-Thu. With UKM & GEM also in support, there is now a serious chance of exceptionally high temps affecting the UK for at least one day late next week. Still only a chance, mind. We’ve almost been here before - just not with quite such strong model support for a top tier event at 7 days lead time. Often a seemingly innocuous change at 3-4 days lead time causes an abrupt removal of the plume scenario, so I’d give it a couple more days before hitting the panic / celebrate button (delete as applicable).

It’s become evident to me that ECM/EPS has some of GFS’ tendency to lower AAM too far, which has lately led to the very negative AAM pattern being expected too soon. I imagine this has to do with shorter timescale interference with the La Niña base state being picked up on more nearer the time. It makes sense for AAM to drop again later next week before a probable rebound, but perhaps not far enough to force a full-on -NAO, NW Euro trough pattern. With which in mind, in the 10-16 day range, I feel that yesterday’s GFS 18z is a more useful guide than today’s GFS 00z.

The MJO has almost finished its trans-Pacific journey and AAM has peaked in the neutral range. This week it should fall and the model are showing the typical response pattern, despite ex-Alex complicating matters later in the week: Most unsettled and cool in the north, most settled in the south where dry and warm weather outweighs cool and wet. Thereafter, a typical progression for a La Niña base state would be a further fall in AAM, leading to a southward shift in the polar jet with high pressure tending to locate west of the UK. This, it seems, extended modelling has been assuming… until today, perhaps. There are emerging signs that the MJO might sufficiently have altered the eastern tropical Pacific sea surface temps to enable establishment of a standing wave there to compete with the La Nina one. If this comes to fruition, it will promote an unstable AAM profile - one featuring substantial rises and falls in AAM which mean it’s not always in the Nina-like stages. In that setup, the rises facilitate ridges over Western Europe which at times extend far north enough for the UK to get involved with hot weather - usually for a few days at a time. The falls counter with cooler, sometimes unsettled spells (whether unsettled depends how far down AAM drops). With warm SSTs around the UK this pattern could well give us an above normal CET & UK mean temp but with near to above average rainfall for most.

It’s a bit odd how we keep seeing runs here and there (most prominently todays GFS12z & yesterday’s ECM 12z) that behave as if the MJO suddenly and rapidly collapsed in phase 7, despite all projections continuing it through at least phase 8 before weakening much. For Mon-Sun. UKM & ICON fit to the phase 7-8 progression pretty well, as did the 06z GFS. The alternative pathway featuring a second build of a ridge to our west up toward Greenland would be very surprising in the circumstances… but I daren't rule it out.

We might finally be seeing a more coherent shift toward the phase 8 MJO response in the modelling, setting in during next weekend. It supports high pressure extension across the northwest of Europe. Tricky part is how that interplays with the low that looks to be there beforehand. Does it boot that low out of the way or build across to its north? In the midst of this, southern UK could find itself on the cool/hot boundary Thu-Fri with a lot of rain around. Or there might be narrow escape. Both options are on the table. Tense for the Jubilee!

It’s now hard to say what’s most likely to occur this weekend, as it’s too soon for the AAM change to be a likely reason for GFS’ huge adjustments to the weekend evolution. Possibly it just handles the Atlantic low differently due to how it’s dynamics now resolve the situation, which is obviously different to the other models, though they have budged slightly. This in turn lowers confidence in next week’s pattern. A renewed AAM surge should mean movement toward establishing a high across Western Europe and southern UK and GFS does that, but it happens to soon and the pattern actually drifts the wrong way later in the week. ECM has the right direction of travel until Thu but then stutters unexpectedly. It sort of restarts the drift on D10 buts it’s a bit half-hearted with the UK in a nomansland between a weak west Euro high & one to the northwest. To my understanding, the best fit to the AAM sequence would be a slack N flow Sun-Mon followed by temps steadily climbing as high pressure slowly becomes focused over W Europe while the high latitude blocking high moves fully away from us to form a sort of west based negative NAO pattern.

A more typical rapid shift to west-based negative NAO from the GFS 06z (it doesn’t only hold truth in winter!). Its persistence in initiating the -NAO is noteworthy, as is ECM’s return to that ship. UKM is still a notable naysayer, though, with the cutoff low setting up much further southwest and supporting a mid-Atlantic high with eastward extension at least as far as Ireland. In winter it’s often said that cold spells are unlikely when UKM isn’t supporting the initial setup stage. Could the same apply now, late in spring?

One cutoff low near the Azores sets the GFS 06z on its -NAO path. Probably overdoing the duration of the setup but still, it’s a major fork in the road point whether that low cuts off or not. Statistics favour it not; the models often do it too readily in the 6+ day range. Yet with ECM having explored it a lot Thu-Fri, it would be daring to dismiss it, despite the model having steadily drifted away from it until fully switching tracks this morning. If it doesn’t cut off then the ridge amplification - driven by tropical Kelvin wave action - should focus over NW Europe instead. This is the same sort of situation as we saw back in late December when a -NAO could have kicked off big time but instead we saw an exceptionally mild high over W Europe.

So, little change from both sides of the divide this evening - GFS still taking the unsettled route for next week while ECM takes a far more settled - though not entirely stable until day 9 - one. Meanwhile UKM keeps bouncing between the two, having been with GFS last night but hanging out with ECM this afternoon. There are no allegiances among forecast models!

The models are really struggling with the conundrum I put forward a couple of days ago. Will it be the quiet version of an AAM drop (high pressure close to the west, ECM style), or unsettled one (low pressure systems moving through, GFs style but probably with a less blocked overall pattern)? Either way, the pattern should then tend gradually eastward toward early June, making for a drier trend, as we settle back into a low AAM regime. How warm will again depend on the NAO. It’s evolution via interaction with N Atlantic SSTs also seems to be giving the models a hard time at the moment.

As I said before, this isn't what -AAM directly corresponds to in late spring and early summer. An unsettled UK outcome with -AAM depends on there being at least a weakly negative NAO setup in play. Without that, the Azores High tends to extend at least a little way across the UK. While I tend not to take its predictions very seriously, GFS actually predicts slightly +AAM through to 20th. http://www.atmos.albany.edu/student/ventrice/real_time/AAM.png AAM has indeed climbed impressively in recent times, taking us out of what had been a well entrenched -AAM regime so far this spring. This change is evident in the Azores High being greatly diminished in the week ahead. Instead there's a broad low to our west that reaches quite far south, which is more of a Nino-like setup under a neutral or positive NAO. With the oceanic La Nina going from strength to strength, AAM is bound to tumble away again sooner or later. Then, the sNAO (summer NAO) state will be crucial for what happens in June. Most seasonal models are banking on ocean-atmosphere feedbacks over the N. Atlantic sustaining a +sNAO and forcing the re-inflated Azores High to extend well to the ENE or NE, resulting in an anomalously warm/hot and dry June for a large part of Europe, including southern UK. That's what happened in 2018 when AAM cascaded during the 2nd half of June. Should the feedback cycle instead break down, the Azores High will be more inclined to focus further west (which as demonstrated in June 2010, can be half decent in early summer, albeit mainly for southern parts of the UK, if you can tolerate chilly starts to the day) or, if the sNAO flips negative, northwest (June 2007 style). We've seen GFS display its usual enthusiasm for such a switch-flip in the later stages of a few recent runs but given how often we've seen a +sNAO self-reinforce in recent years, it seems the lowest likelihood outcome at this stage (versus neutral or positive sNAO). As the summer goes on, a +sNAO tends to force the Azores high to extend more E than ENE or NE, making a changeable westerly pattern more likely by August. In 2018 that shift occurred unusually late, during the 2nd half of Aug, perhaps owing to the exceptional strength of the +sNAO setup that season. This is why, very broadly, La Nina summers tend to slide gradually downhill (or climb uphill if you like it cool & unsettled) over time. An El Nino summer tends more settled instead, but while the early summer tends to be more unsettled, that's often with setups like we're now looking at next week, so plenty of warmth can occur and of course there's a lot of scope thunderstorms. Hence an El Nino is preferred for the summer season by the majority of the weather community. La Nina ones can be impressively hot and dry when the NAO is positive, but some find that a bit boring.

Sun-Tue for S England ECM: Two thunderstorm outbreaks, 2nd one focused in E, followed by brief frontal rain on Tue. Overall 5–15 mm widely, locally up to 35. UKM / UKV: A few well scattered showers in far south early Sun, MCS clipping SE Sun morn. That’s it. Locally 1-5 mm. About as big a contrast as you’ll ever see at this range.

So, ECM has adjusted a little toward UKM for the weekend which has a bigger subsequent impact next week. The overall outlook has been twisted a bit in recent days, as modelling has shifted ever-stronger with a fast moving MJO/Kelvin Wave that will cross the Pacific in the coming week or so at quite high amplitude. That will orchestrate another big shove of the atmosphere toward a more El Niño-like configuration, curtailing the re-strengthening of westerlies in the North Atlantic in the middle stages of next week. This could result in an drawn out regime of mainly southerly component winds across the UK, with scope for instability depending on just how close the high to our east is. Overall it’s a less resolutely dry configuration than a La Niña one at this time of year. I’m unsure now how long it will be until we return to that and whether it will be a sharp or more gradual process. The former has greater potential to bring unsettled westerlies for a week or so. Otherwise, we may simply see the Azores High take over again for the final third of the month.

Key difference begins as early as this weekend with the trajectory of an upper low that serves as the trigger for the heat low. GFS steers it right into southern UK while UKM almost does only to send it packing eastward. Given this is within the 6 day range I consider UKM to be a strong contender… but ECM stronger still so what it does later will be a big deal in my eyes.