Singularity

The increased interest from models in raising heights to the N of Scandinavia in 8-10 days time has caught my eye. One of the difficulties before any such development is how far NE the main trough sets up over Scandinavia / NW Asia. We’re very peripheral in that sense. A high to its north encouraging that trough to ‘string out’ would really aid driving of a deep cold airmass toward NW Europe during 3rd week Jan. Worth looking out for any increased ensemble support for it in the next few days.

I wish we had error measures for each of the major regimes plus +AO / -AO. For all we know ECM could be excellent at the much more common +AO but not top dog for -AO. Same with the NAO.

A nudge toward the UKM 00z by GFS in terms of the NE Pacific. There, the UKM was good from a cold UK perspective because it prevented the linkup, meaning the HLB has nowhere to go & would remain across the Arctic Ocean & Greenland, such that we’d likely see the cold airmass reach the Channel sometime on 15th. ECM remains more open to the idea of that Pacific linkup, alas I’d need to carry out considerable research to be able to say whether that’s likely down to erroneous bias as opposed to higher model skill versus others.

As far as the mid-Jan cold spell goes, it bugs me that it’s ECMWF raising questions at a time that AAM looks to be starting a new climb. Usually it’s GEFS that’s poor at resolving the impacts of that via underestimating the climb. Trouble is, I don’t know what bias ECMWF has AAM-wise. What if it’s a positive bias leading to an overly strong amplification of the Pacific pattern, leading to too much refocusing of high latitude high pressure to that side? As it is, I’d be a lot more questioning of the mid-month cold spell of UKM wasn’t so markedly different to ECMWF for a week’s time. I’ll be watching both those models with high interest tomorrow.

Pays to bear in mind that we don’t have a major SSW, clearly supportive MJO combo to make a strongly -NAO almost certain & give us that 2018 style reel-in of a very cold/potentially snowy outcome for the UK. Instead, a minor SSW with messier results & an MJO that may or may not be helping out depending on which analysis of its current position you look at (stalled in phase 3 aids -NAO during +ENSO but in phase 4 it encourages +NAO instead - a fine line!). Over the years I’ve noticed that ECMWF’s models have more of a tendency to strongly side with an outcome only to suddenly scatter before either reverting or coalescing around a new outcome. The wide scattering episodes usually last 2-4 runs so we really ought to see the model start to make more of a decision one way or the other by the 12z if tomorrow, IMO.

ECMWF’s being very aggressive with connecting a Pacific ridge to the cut off high & essentially tethering it to that side of the hemisphere, leading to a refocus well away from the UK. Seems not a lot of the ensemble runs are matching that, most at least being more like GEM which makes the Pacific connection but allows the high to hold on a day or so longer across Greenland, which is long enough to get the cold airmass across the UK prior to the polar jet attempting to shift back north. GFS is barely interested in the Pacific connection, hence the blocking remains not far to our NW for yet longer. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4998460

ECMWF’s being very aggressive with connecting a Pacific ridge to the cut off high & essentially tethering it to that side of the hemisphere, leading to a refocus well away from the UK. Seems not a lot of the ensemble runs are matching that, most at least being more like GEM which makes the Pacific connection but allows the high to hold on a day or so longer across Greenland, which is long enough to get the cold airmass across the UK prior to the polar jet attempting to shift back north. GFS is barely interested in the Pacific connection, hence the blocking remains not far to our NW for yet longer.

Important to view the Sun-Tue ‘might be’ easterly as independent from the -NAO driven one being signalled for mid-month. By comparison it’s a lot more fragile, so to speak, due to it depending on precisely how the high shapes up and orientates. Still feasible that it could do so in a manner that allows a decent cut-back of low 850s across S UK; we may as well pick options out of a hat regarding such a highly sensitive detail. As far as I can deduce, the subsequent retrogression signal is connected to a combo of the minor SSW and lagged response to MJO phase 3 coupled with AAM not falling very low due to El Niño. At this lead time the signals from those will dominate how the modelling behaves so yes, as always, well run the ‘finer details gauntlet’ nearer the time - but it’ll take a more substantial spanner in the works to totally derail it, compared to this coming Sun-Tue. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4992534

Important to view the Sun-Tue ‘might be’ easterly as independent from the -NAO driven one being signalled for mid-month. By comparison it’s a lot more fragile, so to speak, due to it depending on precisely how the high shapes up and orientates. Still feasible that it could do so in a manner that allows a decent cut-back of low 850s across S UK; we may as well pick options out of a hat regarding such a highly sensitive detail. As far as I can deduce, the subsequent retrogression signal is connected to a combo of the minor SSW and lagged response to MJO phase 3 coupled with AAM not falling very low due to El Niño. At this lead time the signals from those will dominate how the modelling behaves so yes, as always, well run the ‘finer details gauntlet’ nearer the time - but it’ll take a more substantial spanner in the works to totally derail it, compared to this coming Sun-Tue.

A minor one most likely, which is where the 60N mean zonal wind at 10 hPa reduces greatly & quickly to low values but doesn’t reverse. There can still be -NAM to propagate down but it’s less clear cut, with more chance of wave breaks not being in the right place to drive that process, compared to a major SSW. The tropospheric pattern is also important & we have interest there on this occasion - no rampant +AO to resist cooperation.

So GFS decided to give us a run finish to the day. When the MJO crosses phase 3 (eastern Indian Ocean) during an El Niño event, historical composites suggest a window of opportunity for a -NAO pattern to establish soon after. Seems reasonable to presume that the SSW, even if only minor, is helping to realise that potential. Not saying it’s absolutely going to happen on approach to mid-Jan, but I can see how it could.

I’m very interested in how 2nd week Jan plays out from a scientific perspective. We’ll have Nina-like forcing from the MJO but mediated by AAM starting out high so probably not falling much below neutral. Midwinter, that typically makes for a zonal setup over N Europe with a strong subtropical high BUT that’s with a typical strength polar vortex. With one that’s anomalously weak, on par with what we’d tend to see in Oct-Nov, might we see a pattern more fitting of that time of year, where Nina-like forcing encourages well-northward ridging in the North Atlantic? (The seasonal polar vortex intensification is the main reason why winters during La Niña events tend to be coldest Nov-Dec, mild Jan-Feb, excepting MJO/CCKW forced Nino-like interludes later on like we saw in 2018 for example)

OTOH there are low heights propping up the ridge over Scandinavia plus the next Atlantic low is on a trajectory that would likely maintain the current position of the parent trough. So to me it looks like one of those ‘thin Scandi high’ setups where S UK gets an easterly, north a westerly. A half success, you might say. Inherently precarious of course. Overall the modelling has drifted from a Nino like MJO phase 4-5-6 response (ridging W Europe to Siberia) toward a neutral ENSO one (Atlantic ridge, attempts to dig a trough into Scandi), but GFS more so than GEFS and still more so than ECM & it’s ensemble. A key matter that needs resolving. Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4970382

OTOH there are low heights propping up the ridge over Scandinavia plus the next Atlantic low is on a trajectory that would likely maintain the current position of the parent trough. So to me it looks like one of those ‘thin Scandi high’ setups where S UK gets an easterly, north a westerly. A half success, you might say. Inherently precarious of course. Overall the modelling has drifted from a Nino like MJO phase 4-5-6 response (ridging W Europe to Siberia) toward a neutral ENSO one (Atlantic ridge, attempts to dig a trough into Scandi), but GFS more so than GEFS and still more so than ECM & it’s ensemble. A key matter that needs resolving.

Lightning to my east - over the eastern New Forest & Southampton area - in the past 15 mins, quite frequent at times, a few discernible bolts, all C-C. Some of those long, grumbling examples of thunder.

Had an absolutely classic moment - I was videoing some of the rain in hope of further thunder but to no avail. So, I took a break as the rain eased off… at which point lightning flickered nearly overhead and was followed almost immediately by tremendous clap of thunder, too quick for me to capture. That proved be one of just three peels of thunder from the line and by far the most dramatic

That did the trick - thunder heard just as the torrential rain arrived.

Torrential rain is about 5 minutes away with lightning detected - yet I’ve not heard any thunder yet, quite peculiar.

Indeed. High dew point, high relative humidity air can prop up temps even when there’s little wind. On the other hand, if the setup becomes as slack as most modelling now indicates for Thu-Fri, some degree of inversion is likely to form overnight in relatively low lying areas, a process GFS tends to underestimate. So it could well be that we see high elevation weather stations recording some of the highest minima. It’s going to feel pretty strange after almost no such heat in all of Jul & Aug. When we saw similar heat in Sep 2021, it wasn’t as long since the last for many central & western parts due to a hot week in July.

ECM, UKM, & GEM all have ex-Franklin stall & then drop down into the low to the west of Europe early next week which prolongs the heat plume setup UK-wise. GFS instead sends Franklin NE to merge into a large Atlantic trough & start pushing the pattern eastward quills flattening it out a bit. This is a more common outcome but with the other three pointing the other way at just 3-4 days lead time, it’s really up against it. I wish this meant we could just disregard it but it’s never that easy!

Big adjustments - given the timeframe - by GFS & UKM toward ECM in the overnight runs. It appears that ECM has once again demonstrated why it’s the model most worth paying attention to for pattern evolution at the 2-5 day lead times. Not a done deal just yet but it’s close.

ECM somehow recreated its previous 12z including the trapping of Franklin… when this model comes back to an idea, even a wildcard one, at the 4-5 day lead times, it’s hard to play it down. We ought to know by tomorrow’s 12z runs whether it’s onto something or just on something.

Strong EPS support for the 12z det run’s settling down of N European weather by Sunday. UKM 12z was along the same lines too. Into next week, funnily enough the ECM det is in the 5th, smallest, cluster but the 1st, largest, one is very similar while the 2nd isn’t far off. Promising signs that the Atlantic TC action may (emphasis on may) bolster rather than interfere with the momentum fluxes associated with poleward propagating +AAM that Tamara outlined yesterday. We’ve seen some impressive warm, settled spells when such has occurred in Septembers past, so the potential has got my attention.

Curious to see both ECM & GFS exploring possibilities that feature a small low drifting around to the west of Iberia. It’s the sort of feature that deterministic runs may have the edge on over ensembles, though that’s less of a thing nowadays compared to even a couple of years ago. The last two times a cut off low has been explored, the reality has seen it lifted out swiftly. Will it be third time’s the charm from that perspective? Honestly feels like a guessing game for now; the outcome is too sensitive to slight differences in detail.

ECM 12z edged into the south that rarest of phenomena for our part of the world: a ‘heat dome’. Characterised by a stagnant hot airmass, no plume setup involved. Can be hard to shift once setup right over an area - something almost unheard of in the UK, Aug 2003 perhaps one of very few examples. The ECM 12z didn’t take it that far; S UK is only on the fringe, not right under it. Rarely makes it beyond France and you can get a sense of why with the UKM 12z which is very similar on the broad scale yet doesn’t look likely to advance the heat dome far enough to reach S UK, due to having a little less of a ridge in the vicinity of Belgium next Mon. Original post: https://community.netweather.tv/topic/99056-model-output-discussion-mid-summer-onwards/?do=findComment&comment=4905716