Catacol

EPS for @Scott Inghamday still looking good…!

Nope. I wouldn't put GFS ahead of EC or UKMO at any stage within 192h. It is erratic, and by the NOAA's own published data it has declined in accuracy while they have been playing around with it over the last few years. GEM now outperforms it at 120h though personally I think GFS is a bit better than GEM for our north atlantic sector particularly. That is purely an anecdotal view but not much in it. UKMO and ECM are a long way ahead in general for our region. My point is that each run is going to place this break differently....so those waking up and feeling morose need to understand the reality of medium range modelling, especially in circumstances where a surge of momentum is about to throw a segment of the sub tropical high up ahead of it. Micro analysing model data at any point over 72h is a fool's game. Look for the broad pattern only - which right now is good to very good, detail to be decided.

Back a little further north This see sawing will continue for several days because a wave break is very hard to model. Roll with it. Note John Holmes' post on the NOAA perspective.

Sounds like its time to begin reading up on hemispheric drivers. Only then will NWP make sense, and allow the scientific approach you seek. Start here. Berry GSDM.pdf

Evaporated? What - you mean like this rather interesting chart for 96 which will interest anyone on high ground or this one for 24 hours later which will interest those in the far SE or this one 24 hours later which may interest anyone on the east coast then providing a 2 day window of hard frosts and falling temperatures before something rather interesting develops at 240h? I really think perspective is required. We just don't get wall to wall white nirvana very often. In 3 days time the weather is going to turn colder and plenty of options are on the table. I'd take this morning's ECM run above 95% of runs from the last 15 years. Yes - yesterday was better and occasionally we have seen better over that time but anyone wanting more is going to end up with a very sore head, banging it against the wall so repeatedly!

Hmmmm - now what's going on here? Lordy lordy.

Best EPS in a long time - since 2018? And trending in the right direction rather than the wrong one. Gulp...

No - it will be a minor warming now...but it is looking as though the impact of that minor warming is leaving the strat vulnerable to the current momentum spike and high GWO orbit meaning a Greenland High can be properly established via a trop led split. We will need to see just how big the wave break will be in the context of a weakened vortex but someone earlier mentioned Dec 2010 style synoptics and I am wondering whether we might head roughly in that direction if things click. Not the pattern I had expected going into this month as I thought cold would more likely come from the east over time, but instead we may get @Scott Ingham's north to north easterly as a more likely candidate. I wonder whether that kind of extreme amplification is what the MetO have been seeing in their crystal ball as the low probability severe cold option all along. They have certainly stuck to their guns despite the failed SSW. I had assumed it was SSW linked. Matters not. We live and learn. Someone will do an analysis of why the SSW bombed but right now we are learning plenty about the potential that lies within high amplitude pacific impacts and the mechanics of GSDM wind flows. The SSW has failed, but the weakened vortex certainly looks as though it will help magnify that pacific potential as we move forward. The Wave Break is coming.... Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4991322

No - it will be a minor warming now...but it is looking as though the impact of that minor warming is leaving the strat vulnerable to the current momentum spike and high GWO orbit meaning a Greenland High can be properly established via a trop led split. We will need to see just how big the wave break will be in the context of a weakened vortex but someone earlier mentioned Dec 2010 style synoptics and I am wondering whether we might head roughly in that direction if things click. Not the pattern I had expected going into this month as I thought cold would more likely come from the east over time, but instead we may get @Scott Ingham's north to north easterly as a more likely candidate. I wonder whether that kind of extreme amplification is what the MetO have been seeing in their crystal ball as the low probability severe cold option all along. They have certainly stuck to their guns despite the failed SSW. I had assumed it was SSW linked. Matters not. We live and learn. Someone will do an analysis of why the SSW bombed but right now we are learning plenty about the potential that lies within high amplitude pacific impacts and the mechanics of GSDM wind flows. The SSW has failed, but the weakened vortex certainly looks as though it will help magnify that pacific potential as we move forward. The Wave Break is coming....

No. I wonder what went wrong. Good feed from Amy Butler yesterday reminding us all that SSWs are normally under modelled and that it is unclear what has caused the models to go over the top on this one. Recovery now looks likely and then we will have to see what transpires are we approach February. I'm not sure we have sufficient trop precursors going forward to suggest a SSW in Jan or first half of February. Wonder whether this will be one of those few eQBO/Nino seasons where we don't get one now. Currently though winter is being saved by the pacific. Very high GLAAM and amplification signal, sufficient to make the most of the weakened vortex state perhaps. I might have to eat the words of my theory this year, that proper cold in the UK is no longer reliably achievable without a SSW. I would be delighted to do so by the way!

Great to see the ECM deliver the goods tonight. Not convinced we know where this wave break will end up yet, but aside from the GFS with its tendency to power up the northern arm of the jet in these scenarios we have a growing sense that the break will be far enough north to give that first wave of interest before the next pulse hopefully shifts it northwards again. Not sure we will get a third pulse out of this momentum phase unless my interpretation of lag is way off target, so we need to get the block where we need it by mid month in order to sustain the cold. The only word of caution would be that we have seen the EC over amplify before, so plenty of road be travelled yet. But a cold minimum 7 days looks about as nailed on a medium range forecast as you can get with the latitude of the high and its orientation dictating any snow chances. A hell of a lot better than most Januarys in the last dozen or so years. No reason for any gloom whatsoever.

On a gloomier note - how much cold air is there these days in mid winter? Not much. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990240

It is possible for a vortex split to occur without a reversal or technical SSW. This is where the latest EC run is. We had one of these last year in December which gave us the very cold spell which was dry for many but still featured some severe frosts. The issue with splits like this is they are very different to the one we got in 2018 where the split was representative of complete vortex disintegration. If we get one in 7-10 days it will be with the vortex still intact but certainly weakened. That means no nirvana synoptics as we got back then, but still very possible for things to align with the block dragging cold in from Scandy…but we will need a dose of good fortune. And we could do with a swift MJO passage through mid Jan in order to sustain any such cold. The MetO are keeping their options open on this one - clearly their super computer is suggesting that the block might possibly entrench and then be reinforced though they see that option as low probability. For amateurs like us, keep checking the strat forecasts, keep a beady eye on the MJO and we will cross fingers that the dip in GLAAM isn’t too long lived. We have falling momentum now (well - 29/12 anyway) and the first sign we will get of a move back towards reinvigorated support will be when the gradient on this frictional torque graph turns positive. Bear in mind all of this discussion is impacted by lag. Add 10-14 days approx for impacts near us, though I have never quite been able to nail down the lag period with any precision. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990455

It is possible for a vortex split to occur without a reversal or technical SSW. This is where the latest EC run is. We had one of these last year in December which gave us the very cold spell which was dry for many but still featured some severe frosts. The issue with splits like this is they are very different to the one we got in 2018 where the split was representative of complete vortex disintegration. If we get one in 7-10 days it will be with the vortex still intact but certainly weakened. That means no nirvana synoptics as we got back then, but still very possible for things to align with the block dragging cold in from Scandy…but we will need a dose of good fortune. And we could do with a swift MJO passage through mid Jan in order to sustain any such cold. The MetO are keeping their options open on this one - clearly their super computer is suggesting that the block might possibly entrench and then be reinforced though they see that option as low probability. For amateurs like us, keep checking the strat forecasts, keep a beady eye on the MJO and we will cross fingers that the dip in GLAAM isn’t too long lived. We have falling momentum now (well - 29/12 anyway) and the first sign we will get of a move back towards reinvigorated support will be when the gradient on this frictional torque graph turns positive. Bear in mind all of this discussion is impacted by lag. Add 10-14 days approx for impacts near us, though I have never quite been able to nail down the lag period with any precision.

Looks like GFS is late to the party, but turning up nonetheless, bottle in hand. With the EPS reinforcing positive anomalies over Greenland and extending the low anomalies further in a belt to the south we are looking good at present. Some of the recent GFS op runs have been substantially at odds with the weight of evidence on the table. Not going to trot out the same teleconnective evidence here - it has all been said for the coming phase. Check out Tamara’s last post if you want the holistic, GSDM summary. ECM strat forecasts today are spotting a split all the way up to 10hpa. May not last long - may not happen at all - but it is a clear sign that the vortex is weak, stretched and entering a period where its influence will wane. That EC split centres around Greenland and if it were to land then it will be easier for the forthcoming wave break to gain high latitude. The game remains well poised, but right now the bench for the good guys looks strong. I really want this block to get far enough north to drag in cold and secure a proper undercut. We will need this for any battleground snowfall later in the month when falling momentum leads to a weakening of the block and , I suspect, retrogression westwards. We then get the next momentum surge - can it arrive in time to re enhance the blocking signal? Tasty times. I hope everyone had a great holiday. Back to work for me on Wednesday, punctuated of course by checking the data at least twice daily!

Split all the way up to 10hpa on this run. GFS has a split also, not quite as early and not quite as high, but still there We have missed the opportunity of the SSW, but we may yet get a trop led split that will aid any -NAO going forward. Knife edge stuff.

Happy New Year everyone. Top 2 models going for the higher latitude option this morning - a good way to start the year. Not much more to be said right now. The scene is set, we know which drivers are in play for this upcoming phase and we wait for the models to move towards consensus. Longer term relative AAM has started to reverse but the GWO orbit is high and overall GLAAM is very high. We therefore have a cushion to work with from around mid month, but chances of longer term cold I think do hinge on the extent to which the block can get onto the northern side of the jet. The chance is there, the MetO text alludes to it - and EC and UKMO this morning look onboard with that idea. Going to be a close run thing and we dont have a split SSW to fall back on. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4990236

On a gloomier note - how much cold air is there these days in mid winter? Not much.

Happy New Year everyone. Top 2 models going for the higher latitude option this morning - a good way to start the year. Not much more to be said right now. The scene is set, we know which drivers are in play for this upcoming phase and we wait for the models to move towards consensus. Longer term relative AAM has started to reverse but the GWO orbit is high and overall GLAAM is very high. We therefore have a cushion to work with from around mid month, but chances of longer term cold I think do hinge on the extent to which the block can get onto the northern side of the jet. The chance is there, the MetO text alludes to it - and EC and UKMO this morning look onboard with that idea. Going to be a close run thing and we dont have a split SSW to fall back on.

I’d describe it as a diagnostic tool probably more than a forecasting tool. The fact the MetO don’t use it illustrates its one weakness, namely that the GSDM approach is itself the product of other forecasts. So…if pressure patterns don’t develop to allow a mountain torque spike, or if the pacific wave is derailed or lowered in amplitude then GSDM forecasts, like any other, can bust. The ENSO profile can suggest the likely direction of the GWO orbit, but - as we have seen in recent years - the atmosphere can fall into a pattern that disconnects from this forcing and perhaps most significantly the GSDM approach produces a shape of wind flows to the hemisphere and this doesn’t factor in locally relevant features that can overtake it on a local level. However, what it does do is provide the context within which NWP forecasting sits. So….we can be pretty confident in a period of low momentum and coupled Nina season that amplification will be significantly less likely to occur than a higher momentum profile coupled to a Nino ENSO. We know that momentum spikes produce meridional responses and the phases of the MJO help give a clue as to where the peak/trough pattern will setup. So GSDM diagnostics help provide an understanding of higher or lower probabilities of events occurring. Finally - to show its value the GSDM approach correctly “forecast” the cold at the end of Nov/early Dec. It also correctly forecast the flatter period through the middle of the month. It didn’t quite get the latter third of December because the MJO didn’t play ball…so 2 from 3 there. And now we have a period of blocking on the horizon that GSDM analysis diagnosed as likely a fair while ago. 3 from 4? Useful. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989213

Fascinating model watching ahead. Long term trends are good, becoming a medium range model stand off. 144h is where it begins to get really interesting. Taking the top 4 performing models in order: ECM Atlantic approach angled steeply SE, wave break forced up ahead, low pressure previously sat over Scotland has dropped over Europe as pressure rises over Iceland. UKMO Less amplified over the Atlantic, different orientation of the Euro low and softer profile over Scandy. GEM Similar Atlantic shape to the ECM but again a different take on the Euro low and developments over Scandy. GFS 00z (taking from same data point) Similar wave break profile to ECM and GEM but another different take on the Euro low and profile over Scandy. In short - we have cross model agreement on the wave break (no surprise) but quite different outcomes to what occurs downstream of it and, indeed, the angle and composition of the momentum upstream causing it. Consequently anything beyond 144h is in Disneyland and we won’t know for a while yet just how much northerly traction the block will get nor the extent and angle of any undercut. We don’t get many of these knife edge, high potential scenarios in our part of the winter world so here’s hoping folk can enjoy the ride rather than play the doomsayer “told you so” card that often gets trotted out if a run shows less snow/cold than the previous one. There is a fine line between realism and pessimism, so let’s get it said now: yes…cold options in the U.K. are rare and often downgrade rather than upgrade. There is no wisdom or kudos for pointing this out ad infinitum. But when there is a chance - and there is a chance over the medium term - then let’s keep it optimistic. If it stays dry or westerlies win out then snowhunters can commiserate together (hopefully without the doomsayers playing that predictable card once again) but fingers crossed for some luck, some favourable alignment and some snow by the middle of the month. Remember Scotland has seen some good falls already, and the South got lucky in parts in early December. This has already been a better winter than many so fingers crossed it can continue. Broad scale drivers are favourable. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4989158

I’d describe it as a diagnostic tool probably more than a forecasting tool. The fact the MetO don’t use it illustrates its one weakness, namely that the GSDM approach is itself the product of other forecasts. So…if pressure patterns don’t develop to allow a mountain torque spike, or if the pacific wave is derailed or lowered in amplitude then GSDM forecasts, like any other, can bust. The ENSO profile can suggest the likely direction of the GWO orbit, but - as we have seen in recent years - the atmosphere can fall into a pattern that disconnects from this forcing and perhaps most significantly the GSDM approach produces a shape of wind flows to the hemisphere and this doesn’t factor in locally relevant features that can overtake it on a local level. However, what it does do is provide the context within which NWP forecasting sits. So….we can be pretty confident in a period of low momentum and coupled Nina season that amplification will be significantly less likely to occur than a higher momentum profile coupled to a Nino ENSO. We know that momentum spikes produce meridional responses and the phases of the MJO help give a clue as to where the peak/trough pattern will setup. So GSDM diagnostics help provide an understanding of higher or lower probabilities of events occurring. Finally - to show its value the GSDM approach correctly “forecast” the cold at the end of Nov/early Dec. It also correctly forecast the flatter period through the middle of the month. It didn’t quite get the latter third of December because the MJO didn’t play ball…so 2 from 3 there. And now we have a period of blocking on the horizon that GSDM analysis diagnosed as likely a fair while ago. 3 from 4? Useful.

Fascinating model watching ahead. Long term trends are good, becoming a medium range model stand off. 144h is where it begins to get really interesting. Taking the top 4 performing models in order: ECM Atlantic approach angled steeply SE, wave break forced up ahead, low pressure previously sat over Scotland has dropped over Europe as pressure rises over Iceland. UKMO Less amplified over the Atlantic, different orientation of the Euro low and softer profile over Scandy. GEM Similar Atlantic shape to the ECM but again a different take on the Euro low and developments over Scandy. GFS 00z (taking from same data point) Similar wave break profile to ECM and GEM but another different take on the Euro low and profile over Scandy. In short - we have cross model agreement on the wave break (no surprise) but quite different outcomes to what occurs downstream of it and, indeed, the angle and composition of the momentum upstream causing it. Consequently anything beyond 144h is in Disneyland and we won’t know for a while yet just how much northerly traction the block will get nor the extent and angle of any undercut. We don’t get many of these knife edge, high potential scenarios in our part of the winter world so here’s hoping folk can enjoy the ride rather than play the doomsayer “told you so” card that often gets trotted out if a run shows less snow/cold than the previous one. There is a fine line between realism and pessimism, so let’s get it said now: yes…cold options in the U.K. are rare and often downgrade rather than upgrade. There is no wisdom or kudos for pointing this out ad infinitum. But when there is a chance - and there is a chance over the medium term - then let’s keep it optimistic. If it stays dry or westerlies win out then snowhunters can commiserate together (hopefully without the doomsayers playing that predictable card once again) but fingers crossed for some luck, some favourable alignment and some snow by the middle of the month. Remember Scotland has seen some good falls already, and the South got lucky in parts in early December. This has already been a better winter than many so fingers crossed it can continue. Broad scale drivers are favourable.

Interesting argument from Webb today that the next cycle will slow down. Cooler water of eastern IO I can understand as a trigger for this. But Brewer-Dobson circulation impacts? I’ve not heard of this as a factor in MJO development before. He also sees a magnification of wave strength over the IO as a result. Usually I would be unhappy to see a slowed and magnified IO progression but by the time it kicks in we might have such a high GWO orbit the way things are going that it might not flatten the Atlantic much before the magnified signal hits 7. However….much general guesswork here.

Anything can happen in the world of possibilities, but this would run counter to EPS, GEFS, MJO amplitude, impact of high GWO orbit and enhanced GLAAM, ECM op, GEM op and U.K. Met text suggestion. That is not to say that the GFS op at such range cannot be accurate but the wealth of data points the other way. GEFS for 240 doesn’t suggest a sinker.