weatherguy

Hi all, taken a few days off to avoid model fatigue and to actually enjoy the coldspell that has taken hold - some lovely scenery around up here the last dew days! Anywho, let's take a look at what the models are indicating in the medium term, using +72h as the starting point... The GFS 72/96/120/144/168/192/216/240 The cold air established over the UK starts to see its influence wane as heights slowly build over the continent, and due to the angle of the incoming LP from the west (caused by the absence of a significant block) we establish a S/SW flow going into Sunday. Interestingly, the warm uppers don't last long as, while we appear to enter a more zonal phase, the atlantic is very low-power for the time of year due to the core of the PV remaining on the Asian side and the emergence of an Aleutian HP system. This means that we are subjected to a period of "cold zonality" with shots of PM air from the north-west as we move through the +144-+240 time period. With a cut-off HP effectively acting as sticking a rod into a bike wheel, we can again see signs of more northerly blocking across the NH profile as we move into the more long-range period. I won't go into huge detail with the ECM as the 12z will be out soon, but we see a similar outcome here. ECM +144/192/216/240 Again, S/SW flow established into Sunday, followed by the introduction of PM NW airflow (interspersed with TM SW airflows). We also see the development of a combined Aleutian cutoff high and some renewed ridging towards Greenland. Something to keep an eye on is the state of the sPV, given the stress that the tPV is still under. This +168h chart shows the sPV is quite elongated after sustaining some mild wave 2 activity. No signs of an SSW just yet, but this is not looking like the typical raging December PV, and this can only be good news for longer term cold prospects. Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765058

Yes agreed, nothing in the forecast to suggest we'll see one any time soon. More pointing out that the sPV is looking a bit unsure of itself and this combined with the tPV state is quite unusual for December - should encourage further high latitude block development in the medium term.

Hi all, taken a few days off to avoid model fatigue and to actually enjoy the coldspell that has taken hold - some lovely scenery around up here the last dew days! Anywho, let's take a look at what the models are indicating in the medium term, using +72h as the starting point... The GFS 72/96/120/144/168/192/216/240 The cold air established over the UK starts to see its influence wane as heights slowly build over the continent, and due to the angle of the incoming LP from the west (caused by the absence of a significant block) we establish a S/SW flow going into Sunday. Interestingly, the warm uppers don't last long as, while we appear to enter a more zonal phase, the atlantic is very low-power for the time of year due to the core of the PV remaining on the Asian side and the emergence of an Aleutian HP system. This means that we are subjected to a period of "cold zonality" with shots of PM air from the north-west as we move through the +144-+240 time period. With a cut-off HP effectively acting as sticking a rod into a bike wheel, we can again see signs of more northerly blocking across the NH profile as we move into the more long-range period. I won't go into huge detail with the ECM as the 12z will be out soon, but we see a similar outcome here. ECM +144/192/216/240 Again, S/SW flow established into Sunday, followed by the introduction of PM NW airflow (interspersed with TM SW airflows). We also see the development of a combined Aleutian cutoff high and some renewed ridging towards Greenland. Something to keep an eye on is the state of the sPV, given the stress that the tPV is still under. This +168h chart shows the sPV is quite elongated after sustaining some mild wave 2 activity. No signs of an SSW just yet, but this is not looking like the typical raging December PV, and this can only be good news for longer term cold prospects.

We'll take 72h as the starting point here, GFS/ECM both show the cold trough over us, with a Griceland high and MAR blocking off the atlantic, while that storm tracks eastwards gradually. Also worth mentioning that we can expect some snowfall anytime from Thurs-Sat due to features popping up in the unstable air - nothing particularly significant showing up as of yet though. By 120h: The GFS edges the trough westwards, while the LP stalls and starts to disrupt SE. The ECM is similar, but extends the trough further westwards. Another notable difference is the energy present over western Greenland on the GFS, which nudges the Griceland high further to the east. This has consequences further down the line. At 168h: Due to the placement of the high to our north, the GFS blocks the route of a "cold reload" from the north and instead builds a new scandy high, while the ECM allows the cold to come down the NE with the HP instead renewing itself around Greenland. Throughout, both models show the LP disrupting against the block and there would be cold conditions with a chance of some major snowfall. It is clear that we need the forecast time to come down to resolve these differences - namely where the energy may go both over Greenland and in terms of the approaching LP to our west (how exactly it interacts with the cold dense air as it approaches, and how exactly it will disrupt), but what is encouraging is that "all roads lead to cold" at the moment. The GFS shows a riskier evolution, but with the potential for more major battleground scenarios (and followed by a lovely easterly I must add), while the ECM shows perhaps less PPN but a brutally cold setup within which snow would surely come. The tPV is not in a healthy state into the medium term, that's for sure! Take your pick, enjoy what's on offer!

More evidence of a battleground:UK scenario today. Bring it on!

It's ridiculously chaotic, but good at 216h

Well, that ECM certainly caught me off guard! No getting around the fact that it's poor in the medium-term and a hell of a downgrade. However, no need to throw the toys out of the pram over one run. Especially when the GFS/UKMO/GEM are all still on board, to varying extents, for a lot of snow potential as LP systems track to the south of the UK. I do understand there will be a certain amount of IMBYisms for such a setup, mind. Just for fun though, a look at that ECM +240h chart: The tPV is still under severe pressure and, were this to come off exactly (barely a chance in hell, in my opinion) at T0, I'm sure there'd be another spell to be chasing in the not too distant future.

With the trop PV in its current state, and the usually-ever-present Canadian PV lobe conspicuous by its absence, the Atlantic jet simply doesn't have the strength for LPs to make much of an inroad into the cold established over NW Europe. As a result, it's logical that those LP systems slide south. This is good for those wanting snow a chance at more significant snowfall, as it introduces moisture into the area - we don't have raging convective easterlies showing just yet in the reliable timeframe, though of course that may yet be the end result here. We see this on the UKMO at +144 (to a lesser extent though), GFS at various times (+210 shown), and on the ECM 0z at 216h

Yes, in that part I was referring to the net result at that timeframe ie. what would "currently" be felt on the ground at that time. I believe I alluded to the excellent NH pattern within that post and, if not made clear there, then certainly in others! Anyhow, the more the models play around with a scenario of LPs tracking south, the more we introduce continental air with low dew-points at which stage any potential lack of "deep" cold uppers becomes more of a moot point. Plenty of sliders on the 18z!

ECM 72/96/120/144 GFS There is quite remarkable consistency WRT the longwave pattern out to D7 between the big 2. The trough dropping over us, HP holding out both to the North-west and building to an extent to our NNE. We're kept in very cold conditions, a mostly dry set-up but you never know with disturbances in the flow potentially developing at short-notice. I am still leaning towards a snow event Thurs-Fri. It is at +168h where we start to see a big deviation in terms of impacts upon our small corner of the world - ECM/GFS: The GFS at this timescale ejects a shortwave south of Greenland, which prevents another cold blast coming around the eastern flank of Greenland and also results in less stalling of the LP to our west. The result is that by +216h we don't get a "reload" from the north in the GFS model, whereas we're opened up to whatever the opposite of the blowtorch southerly is... The GFS is still very cold, and there is still ample opportunity for snow as that LP tracks south. In both, the longer-term outlook is for a continuation of cold into at least the medium term, with the main segment of the PV shunted to the east and any atlantic development being stalled and shunted southwards. We need more time to determine exactly how the tropical depression will interact with the block and how energy is resolved around Greenland, as this is still quite far out. Happy with today's output, let's see where it takes us in the future.

Good grief what a chart!

I'll likely save a more meaty post for later when the ECM comes out, but just wanted to point out this chart at +78h from the GFS: There's not too much ppn showing up, strangely, but that kink combined with the unstable air in the trough really ought to see some decent accumulations of snow in places as we move through Thurs-Fri.

The shortwave in question is this feature, I believe

As "bad" as things are looking compared to past output, I think it's important to keep some perspective. ECM 12z uppers shown for +48h to +120h, with the +120h NH profile for reference. We have some pretty deep cold entrenching itself, with associated snow risk, and this is only modelled to recede by +216h. This is, of course, way out in FI at this stage as it's very unlikely to have nailed exactly how the "Atlantic Hurricane" will interact with the cold air mass. It's a high-risk potential setup, but not worth getting stressed over at this stage. In any case, the 240h charts for the ECM/GFS (shown below) are wildly different from each other, and both still have plenty to interest us with the NH profile as a whole - there will be further bites of the cherry in all likelihood.

I did think after I posted hang on that doesn't exactly fit my recollection oops, blame the Saturday night festivities Broad point still stands mind, as I recall the Dec 2010 spell started in November and did last a very long time with reloads, so fortunately my post wasn't full of complete gunk! I have edited my post to show the actual progression now!

Take a look at my profile pic, that was 2010 An outrageously strong GH with an entire segment of PV dropping straight onto us...Followed by a brutal easterly...Followed by another brutal northerly...Followed by another brutal easterly...Etc etc. **Edit: As I was a numpty and put the wrong years for the next 3 charts, the actual progression was as follows: Brutally cold easterly, which then dragged the block up to greenland, all the while drawing in more cold air from the east, which developed into the monster PV lobe dropping on us. ** I think in 11/12 years it's easy to forget just how special that was, absolutely mindblowing that we effectively spawned our own coldpool. Onto this modelled setup: We have a big shot from the north, due to a strong GH (block around greenland), followed by an easterly with potential undercutting LP systems. Long story short, very good yes, and the initial source is pretty similar, but not absolutely outrageous as 2010 was. 2010-2011 also was incredibly prolonged, and we are yet to see if this will have the same longevity - it hasn't begun yet! Don't get me wrong, I am absolutely ecstatic that this current cold spell seems to be on the way, and with the first cold coming into the very reliable period and other signals suggesting this could go on for a while, then things could indeed get very interesting!

May the snow be with you...

GFS/ECM +72h GFS/ECM +96h Wonderful, we effectively have a good blast of cold air locked in for us now. What happens next is the interesting bit, so let's zoom in a tad. GFS/ECM +120h Fairly good agreement on the trough dropping over us with plenty of instability which should lead to snow at low-lying levels, though of course by this timeframe it's hard to forecast in too much detail WRT who exactly will benefit. Longer term, GFS/ECM +240h Huge differences, due to the handling of the LP to the SW that appears around +120h. ECM much blockier and causes more a cold spell rather than the GFS' cold snap. However, even the GFS out in FI does reestablish a form of northerly blocking. Tied in with chiono's thoughts from the other day, we have good reason to believe that this isn't an all-or-nothing period of weather, and we will likely have further bites of the cherry. To sum up, we have cold air entrenching itself over the UK, with potential snow events for many in the latter part of next week. What happens after is up in the air, but plenty to be happy about with this very early-winter serving

Well well well... Pub run giving us some late-night eye candy so far...

If we were in the midst of the majority of our winters over the last 10 years, and this chart popped up at FI/+300h I'd guarantee there'd be some excitement over it. This, however, is at just +96h. Variations on a theme run-run of course, but still very excited.

Gentle reminder that after an initial blast of cold uppers from the Ne/E, it’s inevitable that there’ll be a mixing out of these to some extent when milder/wetter Atlantic air comes in. But, this isn’t a bad thing assuming we maintain low dew points (which we do, when LP systems are to the south) - you don’t need brutally cold uppers in such a setup. More eye candy this morning!

More exceptional viewing for us coldies today, really excited for this in a manner I haven't been for a number of years! Drastic differences even at just +120h between the two big-hitters, but the NH profile is so damn good that we have a rarely-seen large margin for error which still ends with us being locked in for a very noteworthy December spell. Fill your boots, guys and gals!

That high pressure up by greenland was attached to the general band of higher pressure to the south/east. But, it gets undercut from the west and separated to the east by the cold stuff coming down from the north/north west. Thus, it is "cut off". When cut off, due to the circulation of air around it, it has the potential to remain in situ and disrupt the flow of atlantic-based weather rather than being "sunk" southwards by pressure from the north. It can't "sink" if there is low pressure south of it.

It's....beautiful. Slightly suspicious of how "overly clean" it all is to get to that point, I'm sure we'll have some shortwave drama along the way but with the general NH profile being corroborated by the GFS at the same timeframe (below), I'm feeling pretty good about this one.

WOW Shades of "THAT ECM".