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!