Met4Cast

You're talking about the MJO & AAM as if they are two separate things, but they're not. AAM is simply a measurement of how much westerly momentum is in the atmosphere, the MJO influences this via frictional & mountain torques, i.e driving positive MT increases westerly momentum. More westerly momemum tends to = a more perturbed jet stream and thus, a higher likelihood of high latitude blocking but it's equally important to see which direction those anomalies are travelling. The MJO is taken in isolation too much in this forum & more generally, it's just 1 variable of the global windflow budget so it's no real surprise that looking at the MJO, checking the composite & expecting the atmosphere to respond and look like the composite isn't really working, the whole point is to view the MJO as part of the windflow budget, the MJO is incorporated into the GWO. The AAM, or GSDM isn't supposed to be used to determine regional specific weather but to help diagnose global weather patterns, it's essentially the best tool we have for medium-extended range forecasting. GSDM (which incorporates processes like the MJO) > Global NWP modelling > High res modelling The GSDM provides context to NWP modelling in the same way NWP modelling provides context to more localised high resolution modelling. You're right, the MJO is currently in a phase which if you view in isolation and look at the composites, does favour high latitude blocking, but that's not the entire story. AAM is positive at the moment indicating a good link between the +ve ENSO state & the atmosphere, however as discussed here in recent days fluxing of positive (and negative) anomalies are and have been equatorward rather than poleward, this serves to inflate the Iberian/S Euro high, whereas poleward fluxing (see early Jan) helps to amplify high pressure into the Arctic regions. The current state of AAM fluxing helps explain the current patterns we've been seeing despite, on the face of it, a favourable MJO. I do wonder if the very warm SErn Atlantic waters has perhaps helped to extend the Hadley cell this winter and thus inflate the Iberian high regardless of other forcings such as the AAM, it seems very much to have been the default, base state through much of this winter despite seemingly (and often) favourable teleconnective forcing. Climate change is without doubt changing the dynamics. I don't think we should just ditch the MJO, GSDM etc just because it hasn't performed with pre-determined outcomes. If anything, this winter has been a fantastic learning point on this, but it is important not to take them in isolation or out of context. Original post: https://community.netweather.tv/topic/99814-model-output-discussion-22nd-jan-2024-onwards/?do=findComment&comment=5035355

You're talking about the MJO & AAM as if they are two separate things, but they're not. AAM is simply a measurement of how much westerly momentum is in the atmosphere, the MJO influences this via frictional & mountain torques, i.e driving positive MT increases westerly momentum. More westerly momemum tends to = a more perturbed jet stream and thus, a higher likelihood of high latitude blocking but it's equally important to see which direction those anomalies are travelling. The MJO is taken in isolation too much in this forum & more generally, it's just 1 variable of the global windflow budget so it's no real surprise that looking at the MJO, checking the composite & expecting the atmosphere to respond and look like the composite isn't really working, the whole point is to view the MJO as part of the windflow budget, the MJO is incorporated into the GWO. The AAM, or GSDM isn't supposed to be used to determine regional specific weather but to help diagnose global weather patterns, it's essentially the best tool we have for medium-extended range forecasting. GSDM (which incorporates processes like the MJO) > Global NWP modelling > High res modelling The GSDM provides context to NWP modelling in the same way NWP modelling provides context to more localised high resolution modelling. You're right, the MJO is currently in a phase which if you view in isolation and look at the composites, does favour high latitude blocking, but that's not the entire story. AAM is positive at the moment indicating a good link between the +ve ENSO state & the atmosphere, however as discussed here in recent days fluxing of positive (and negative) anomalies are and have been equatorward rather than poleward, this serves to inflate the Iberian/S Euro high, whereas poleward fluxing (see early Jan) helps to amplify high pressure into the Arctic regions. The current state of AAM fluxing helps explain the current patterns we've been seeing despite, on the face of it, a favourable MJO. I do wonder if the very warm SErn Atlantic waters has perhaps helped to extend the Hadley cell this winter and thus inflate the Iberian high regardless of other forcings such as the AAM, it seems very much to have been the default, base state through much of this winter despite seemingly (and often) favourable teleconnective forcing. Climate change is without doubt changing the dynamics. I don't think we should just ditch the MJO, GSDM etc just because it hasn't performed with pre-determined outcomes. If anything, this winter has been a fantastic learning point on this, but it is important not to take them in isolation or out of context.

The ECM det was in cluster 4 so not overly supported within the EPS. Cluster 1 does look a little more favourable for cold.

I can’t say i’m surprised by this evenings ECM in all honesty, although little point in analysing individual deterministic runs when we’ve got ridiculous amounts of ensemble spread, it’s virtually impossible to pick out a route forward using NWP modelling alone. Teleconnections aren’t providing much in the way of clarity either, equatorward fluxing of AAM anomalies are I suspect, helping to strengthen/maintain heights to the south of the UK whilst MJO through phases 7 & likely into weak amplitude 8 suggests blocking of the higher latitude type, two evidently opposing signals & an occasion when the tropics & sub tropics are not aligned towards the same potential outcomes, i.e opposing signals playing havoc with both NWP modelling & our ability to forecast in the medium-extended range with any clarity or confidence. A potential SSW in the mix too only further complicates the picture. I think getting cold into the UK is going to be difficult, particularly further south and particularly during February. It’s a case of waiting & seeing how things transpire at the moment, not much hope can be offered in terms of cold outcomes though.

As good as the GFS det was in the extended it doesn't have much support at all.

It has been like pulling teeth, hasn't it? Particularly given for the majority of this winter we've had a weak stratospheric vortex and it hasn't been driving the patterns, the troposphere has very much been in charge. We were in a very favourable position in terms of teleconnections in mid January for a continuation of cold/blocked patterns but then we saw a SSW (not a major one) reflect back, reshuffling the pattern away from something favourable and instead we ended up very mild and stormy. A fascinating winter from a stratosphere & teleconnective point of view at least, if not from a cold/snow point of view.

The GEFS 00z run has backed away from the idea of an SSW, looking very different from the previous runs. 61% of members still suggest a reversal but this is down from 80%+ yesterday.

It’s perhaps quite telling that despite a huge amount of spread on the EPS, only 1 or 2 ensembles suggest the -10C isotherm. There is a complete lack of a signal for deeper cold within the model suites.

A Quick Trop Response (QTR) simply means a rapid response to a SSW. I.e impacts from the SSW reach the trop faster than what would ordinarily be expected. But.. I’d argue a QTR cannot be instantaneous, i.e the same day an SSW happens. Imo you’re looking at about a week, rather than hours. As Simon Lee said though, the idea of a QTR doesn’t appear in scientific literature on SSW’s so the actual definition is open to interpretation. Might be helpful if we agree on a definition for future use of the term though..

johnholmes Quick Trop Response.

Trying to diagnose a potential way forward is proving impossible. The aforementioned equatorward fluxing of +AAM anomalies suggests a strengthening Iberian/S European high however the MJO likely transitioning into a weak amplitude phase 8 combined with an increasingly unstable polar profile (SSW) suggests a likelihood of high latitude blocking. Two rather opposing signals fighting for dominance, no wonder modelling is struggling to the degree that it is. From a forecasting point of view, the mid-extended range at the moment is nigh on impossible. Getting cold into the UK with an Iberian high in the game though really will be a struggle, the ECM shows this reasonably well with both HLB and an Iberian heigh. @Kasim Awan might have to change his name again.

Daniel* It seems a "QTR" isn't something that appears in literature so it's difficult to actually define. In my mind a QTR simply refers to rapid impacts from downwelling -ve SSW anomalies, something which still takes time & therefore cannot be instantaneous.

Whilst pointless analysing beyond day 5, the GFS is not interested in the idea of a wedge of heights to the north and if anything the 18z has flattened the pattern more than the already flat 12z.

You can't see a trop response to an SSW on the same day the SSW is happening, that is impossible. The trop could already be in a position where blocking is happening during an SSW but that would be unrelated to the downwelling impacts of the SSW. Complicated, but I don't think this can be classed as a QTR to be honest. Surely a QTR is a direct response to downwelling easterlies? Pre-primed trop patterns that happened to produce blocking as an SSW occurs, whilst you could argue is loosely related to the SSW happening in the first place, isn't a response to the SSW in of itself. Either way. Given the huge amount of spread I think it's largely irrelevant what any det run is showing at day 10 currently.

The ECM isn't showing a QTR, best estimates place the SSW on the 18th possibly 19th February, that would be an instantaneous trop response which unfortunately, isn't possible. In any case - I remain unconvinced of anything prolonged or sustained, the SSW is certainly a wildcard but I think it'll be rather difficult to see anything proper in terms of cold before the SSW takes place, and even after then I'd be cautious on any possible impacts re: UK specific cold hopes. Notice how through December these anomalies were propagating poleward, this led to the early January cold spell. Compared to now these anomalies are moving subtly equatorward, i.e no HLB. The sudden change on todays UKMO & ECM towards wedges is an interesting one, but given the ridiculous spread within all ensemble suites at the moment it's not worth putting too much emphasis or hope into that just yet. It may not seem like it but I'm really trying to find a reason to be optimistic about cold, but I can't just see it currently. Post SSW might be a different story though.

Even with a decent NH profile we still manage to see the Iberian heights poking it’s nose in

Bank! If it’s not going to be overly cold we may as well get some early warmth in across the UK instead!

Blessed Weather Interesting, thanks for the link I'll give it a read later this afternoon! The SSW seems to be triggered (assuming we do indeed see one, here) by the MJO but also (perhaps more likely) by the recent strong +EAMT event increasing heatflux right up into the stratosphere. There certainly seems far more evidence supporting a SSW this time around compared with the previous attempts this winter but without a QTR (this is another uncertainty) we'd be looking towards late February/early March for any potential impacts. I'm not seeing anything within extended NWP modelling to suggest an overly exciting QTR, usually you'd expect to see at least a couple of outrageous runs from GEFS ensembles but they've been relatively muted, the same is true for the EPS. MJO forecasts are currently quite uniform across. modelling, slow meander through phase 7 before a low amplitude phase 8 follows, albeit these are of course not factoring in any potential impacts from the (again, assuming here) possible SSW. Certainly quite a complicated picture unfolding but in any case, I'm not sure many would be happy with winter finally arriving in March just when some much needed warmth is wanted..

feb1991blizzard The MJO runs on a cycle of roughly 60-90 days, it won’t be back into favourable cycles again until April (ish). AAM tendency likely won’t return in any meaningful way until late March (assuming a coherent MJO cycle). So in a word.. no.

This is just awful The SSW will likely come at a time of falling AAM as the MJO returns to the Indian Ocean, not particularly good timing for cold/blocking conditions to be a quick response..

I want to go back to this post from the 22nd January (the quote shows a different date, had to bodge the quote due to that thread being locked, original post here:) Unfortunately much of this has rung true. There remains a lack of poleward propagating +AAM anomalies hence, despite the huge surge in momentum in the tropics/sub tropics & despite a relatively favourable MJO signal there remains little HLB on offer. Now, we have lost the Iberian heights and we are seeing some brief cold weather thanks to cyclonic wave-breaking but this is temporary. Going forward, there is some evidence within NWP modelling for height rises to the NE but these will be coming up against a broader Atlantic trough. It's possible we could see energy undercutting this but given the current situation regarding background forcing I'm not overly optimistic, it seems plausible that the height rises to the NE will be too far east to really advect cold air into the UK & allow for significant trough disruption SEwards into southern Europe. A SSW seems likely later in the month so the polar field is likely to become increasingly unstable but in the short-mid term, if you're lucky enough to get snow in the coming couple of days I'd enjoy it whilst it lasts..

The GEFS were never all that interested but the EPS have massively backed away from cold now too.

The GEFS aren’t all that interested in colder weather in the extended with the mean comfortably above average. The EPS on the other hand are a complete dogs dinner, I don’t think i’ve ever seen such a large spread!

Paul I mean it's an improvement but it just feels like how it was before but with extra steps..

The main period of interest for me is later this month, 20th February as a broad timeframe for something more interesting in terms of winter weather. AAM/GWO is firmly routed in Nino attracter phases and the recent strong +AAM tendency is starting to show signs of propagating poleward through this month. At the same time, the MJO will be moving slowly through phase 7 and possibly into phase 8 giving a little more credence to blocking amplifying towards Greenland, GEFS ensembles are broadly pointing this way. Meanwhile the stratosphere is once again becoming an interesting topic of conversation, the GFS in particular is quite bullish on a major SSW, reflected by the strong heat flux (likely triggered by the recent and to some extent on-going strong +EAMT event) A major SSW at a time tropospheric patterns are aligning to produce patterns favourable for blocking/cold outbreaks, we've been here before this winter and saw how a SSW disrupted the pattern, perhaps we can get a bit lucky this time around and see a SSW super impose onto the trop amplifying colder patterns? Those hoping for an early spring come March might be in trouble.