Catacol

Yes. The goose has done a runner with the golden egg scenario, namely the split SSW that would have produced the potential for a long term cold setup, but we have 2 distinct period of opportunity despite that failed split. The first is the tropospheric driven blocking starting around 7 January which should persist for a week to 10 days, with breakdown events possible as you say. Then - given the particularly swift MJO passage this season a second bite at the tropospheric cherry into early February. The unknown bit is how significant the impacts of strat stress are going to be. A technical SSW now looks out of reach, at least for the first half of January, and the tropospheric signals for the kind of forcing required to create one are on the wane. We are moving towards a North Pacific ridge and low pressure around the Urals, and this will serve to halt the warm assault on the strat as well as enhance the sharp decrease in GLAAM from mid month. So… what will the impacts be of the weakened, stretched and potentially wobbly vortex at the time we get that reduction in GLAAM? Way too early to know. If - as appears to be the case - the vortex retains its core then we want that to stay around Siberia and not trot across to Canada. I suspect the low percentage scenario in the Met long term text is built around this possibility because a weakened vortex sitting over Siberia mid month might possibly allow blocking to our north to hang on. Knife edge stuff. Lots to contemplate over the next month or so, but for the first 10 days of January we have an evolving blocking scenario to follow and snow chances for some…maybe many! Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987772

Yes. The goose has done a runner with the golden egg scenario, namely the split SSW that would have produced the potential for a long term cold setup, but we have 2 distinct period of opportunity despite that failed split. The first is the tropospheric driven blocking starting around 7 January which should persist for a week to 10 days, with breakdown events possible as you say. Then - given the particularly swift MJO passage this season a second bite at the tropospheric cherry into early February. The unknown bit is how significant the impacts of strat stress are going to be. A technical SSW now looks out of reach, at least for the first half of January, and the tropospheric signals for the kind of forcing required to create one are on the wane. We are moving towards a North Pacific ridge and low pressure around the Urals, and this will serve to halt the warm assault on the strat as well as enhance the sharp decrease in GLAAM from mid month. So… what will the impacts be of the weakened, stretched and potentially wobbly vortex at the time we get that reduction in GLAAM? Way too early to know. If - as appears to be the case - the vortex retains its core then we want that to stay around Siberia and not trot across to Canada. I suspect the low percentage scenario in the Met long term text is built around this possibility because a weakened vortex sitting over Siberia mid month might possibly allow blocking to our north to hang on. Knife edge stuff. Lots to contemplate over the next month or so, but for the first 10 days of January we have an evolving blocking scenario to follow and snow chances for some…maybe many!

Lots of churn. A reminder that - at 120h - the models verify in the following order: ECM, UKMO, GEM, GFS. The GFS has actually declined in accuracy since 2018 according to NOAA stats. We use GFS a lot because it runs out to distance and is 4 times per day, but once we are inside the 240h envelope it drops in relevance. ECM/GEM solution fits the drivers best. It's a pity we don't get a UKMO solution out to 240h but we have the Met text for a bit of that. All good this morning.

6/7 Jan is where the fun begins, with a significant atlantic wave break creating the change. This is an MJO/GWO/+EAMT product. See ECM last night for the 7th: GEM lunchtime today for the 6th: GFS lunchtime today also for the 7th: Any further analysis at this stage is a waste of time. A wave break like this is not easily modelled, and how the jet interacts with it to the south, and how the break aligns, will be crucial to any low pressure undercutting through Scandy or indeed from the atlantic. ECM and GEM offer very tasty, if different, mid term solutions to this at 240h, GFS doesn't get the break quite as high. Given both ECM and GEM are outperforming GFS in 120h stats at the moment I'd go with then over the GFS....but time will tell. The next few days of model watching are going to be some of the best of the season so far. Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4987040

6/7 Jan is where the fun begins, with a significant atlantic wave break creating the change. This is an MJO/GWO/+EAMT product. See ECM last night for the 7th: GEM lunchtime today for the 6th: GFS lunchtime today also for the 7th: Any further analysis at this stage is a waste of time. A wave break like this is not easily modelled, and how the jet interacts with it to the south, and how the break aligns, will be crucial to any low pressure undercutting through Scandy or indeed from the atlantic. ECM and GEM offer very tasty, if different, mid term solutions to this at 240h, GFS doesn't get the break quite as high. Given both ECM and GEM are outperforming GFS in 120h stats at the moment I'd go with then over the GFS....but time will tell. The next few days of model watching are going to be some of the best of the season so far.

You can breathe easy…

GFS06 around the 10 day mark reading the script being written by upstream drivers. ECM the same. Whether it will snow or not is a useless and irrelevant argument at this stage, but it looks as though it will become cold enough to do so. Snow requires luck (or some would say bad luck!) on a micro level and that means waiting until 72h realistically to get any broad sense of frontal or convective activity.

Icelandic ridge emerging...

CMC top of the pops on 3 consecutive days here at 120h ...and now better than GFS at 120h?

Hehe - you've not been watching NWP output long enough if the current ECM looks dismal to you, and for the northern half of the country it is close to spectacular. This would be a bit closer to dismal! A chart like this one at only 96 hours is very good for some

ooooo - yummy

…but no getting away from the disappointing nature of this update and the trend it represents. 2018 redux in the heart of winter looking a long shot now which is a great shame. Plenty of flux hitting the vortex at the outer reaches of reliable modelling so maybe chances come later.

Whatever happens week 1 of January this story is not over. Significant poleward flux forecast on GFS today for end of that week. And plenty of members still seeing reversal potential. Lots of time for this tale to develop further.

Hehe - I like that question. Headscratcher. In a nutshell, as we know, we need a block over Greenland, Iceland, Scandinavia or a combination of these. How do we get it? The answer perhaps is split into two: either upstream forcings encourage the ridging of the semi permanent sub tropical high (Azores) with the jet dropping in underneath to sustain it or we get a top down forcing caused by vortex disintegration and a resultant -AO that also allows for a block to drop into place. It’s worth remembering - and we see ir clearly in summer when there is no polar vortex - that high pressure does not naturally evacuate these regions. It is forced out by the pressure of the vortex and then the strength/latitude of the jet. So we look for two developments. 1. Upstream amplification of the jet caused by an injection of momentum. Think of a pole vault. The pole is straight until significant force is applied and this causes it to compress and bend. Same with the jet when momentum is injected - and this creates a wavy long wave pattern. Combine this with a Rossby pattern driven by the MJO in phases 7/8/1 and we tend to get the bend in the right place to drop the jet southwards in our North Atlantic sector and and eddy to form to the north = a block. Once in place, even after a reduction of momentum occurs (because it doesn’t stay enhanced for long under normal circumstances) and the block can be sustained via retrogression and ongoing jet angle. Nino conditions enhance momentum spikes which is why Nino is generally better than Nina for midwinter blocking. 2. Disruption to the vortex. If the vortex is weakened it follows automatically that the conditions for blocking to return to higher latitudes, as occurs in summer, improve. And if the stratospheric vortex is shifted away from the Atlantic sector then so much the better. What weakens the vortex? Warmth. Warm air fired into it slows it down, acting like a spanner in the spokes of a wheel. The dynamics are complex, but warm air bounced off Rockies or Urals plays a part as does ridging driven by tropospheric factors described in 1. above. There is therefore a clear link between tropospheric drivers that promote ridging and stratospheric impacts down the line. This is why many of us are not unhappy to see a Euro high in November if it encourages a trough over Scandy and ridge over the Urals downstream, and why I have personally been celebrating the Nino-driven North Pacific trough. American cold lovers are hating it right now, but it has allowed some significant poleward heat off the Rockies over time. Right now all is good. We have a bending jet courtesy of high momentum, promoting northerly ridging in our sector. We have a distressed vortex, SSW or not (though SSW would be better.) We have the MJO in the right phases. There are other factors not discussed here, the most important perhaps being the QBO. It is an umbrella feature and it is in the right easterly phase also to encourage an SSW. Sit back and wait. NWP will come into line. A cold spell in January will happen. Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986134

Yup. Something like this is where we are heading.

Hehe - I like that question. Headscratcher. In a nutshell, as we know, we need a block over Greenland, Iceland, Scandinavia or a combination of these. How do we get it? The answer perhaps is split into two: either upstream forcings encourage the ridging of the semi permanent sub tropical high (Azores) with the jet dropping in underneath to sustain it or we get a top down forcing caused by vortex disintegration and a resultant -AO that also allows for a block to drop into place. It’s worth remembering - and we see ir clearly in summer when there is no polar vortex - that high pressure does not naturally evacuate these regions. It is forced out by the pressure of the vortex and then the strength/latitude of the jet. So we look for two developments. 1. Upstream amplification of the jet caused by an injection of momentum. Think of a pole vault. The pole is straight until significant force is applied and this causes it to compress and bend. Same with the jet when momentum is injected - and this creates a wavy long wave pattern. Combine this with a Rossby pattern driven by the MJO in phases 7/8/1 and we tend to get the bend in the right place to drop the jet southwards in our North Atlantic sector and and eddy to form to the north = a block. Once in place, even after a reduction of momentum occurs (because it doesn’t stay enhanced for long under normal circumstances) and the block can be sustained via retrogression and ongoing jet angle. Nino conditions enhance momentum spikes which is why Nino is generally better than Nina for midwinter blocking. 2. Disruption to the vortex. If the vortex is weakened it follows automatically that the conditions for blocking to return to higher latitudes, as occurs in summer, improve. And if the stratospheric vortex is shifted away from the Atlantic sector then so much the better. What weakens the vortex? Warmth. Warm air fired into it slows it down, acting like a spanner in the spokes of a wheel. The dynamics are complex, but warm air bounced off Rockies or Urals plays a part as does ridging driven by tropospheric factors described in 1. above. There is therefore a clear link between tropospheric drivers that promote ridging and stratospheric impacts down the line. This is why many of us are not unhappy to see a Euro high in November if it encourages a trough over Scandy and ridge over the Urals downstream, and why I have personally been celebrating the Nino-driven North Pacific trough. American cold lovers are hating it right now, but it has allowed some significant poleward heat off the Rockies over time. Right now all is good. We have a bending jet courtesy of high momentum, promoting northerly ridging in our sector. We have a distressed vortex, SSW or not (though SSW would be better.) We have the MJO in the right phases. There are other factors not discussed here, the most important perhaps being the QBO. It is an umbrella feature and it is in the right easterly phase also to encourage an SSW. Sit back and wait. NWP will come into line. A cold spell in January will happen.

It is time to take a step back and wait for model chaos to disperse. I have shown twice in the last two days how modelling gets to the 6/7 day mark and then doesn’t have a scooby… and I think until we see what happens post Hogmanay storm (that is certainly coming) the rest is like a basket of odd socks. I’ll trust the drivers as I try to do all the time because they tend to deliver more often than not - though once again it is worth acknowledging we play a probability game when trying to forecast the future and nothing is ever certain. And those drivers, semi predictable though they are, are also apt to change. I think many of the big guns out there are waiting to see what happens with the next 2 or 3 rounds of strat modelling. Quiet has descended and the nature of the strat profile is going to have a huge say in how the majority of the rest of winter unfolds. In the near term we are continuing to see a signal for height rises as we move beyond New Year but timing is unclear and the extent to which we get a block rather than a wedge - and how the Atlantic interacts with this is still up in the air. But a zonal profile for January this is not despite the claims of some earlier today. On the positive side of things trawling for opinion is a useful exercise. Cohen last night has come out and thrown a tentative hat into the ring for a split vortex SSW despite the uncertainty in the modelling. Webb has rather distanced himself for now from the SSW discussion but gone for persistent -NAO through Jan and Feb no matter the outcome of strat developments. And he has been quite bullish on this trajectory for a while. And the third interesting bit of browsing is this dataset. This is the shape of Jan and Feb in years where a displacement SSW occurred when ENSO was an El Niño, It makes for good viewing and matches the direction of travel currently of EPS anomalies. A hell of a lot changes in two weeks of weather watching. I have no doubt that, once we get to Thursday 11 Jan we will be looking at a very different picture and my money is on a blocked and cold pattern, either in place or soon to be in place. Enjoy the ride… Original post: https://community.netweather.tv/topic/99584-model-output-discussion-into-winter/?do=findComment&comment=4986053

It is time to take a step back and wait for model chaos to disperse. I have shown twice in the last two days how modelling gets to the 6/7 day mark and then doesn’t have a scooby… and I think until we see what happens post Hogmanay storm (that is certainly coming) the rest is like a basket of odd socks. I’ll trust the drivers as I try to do all the time because they tend to deliver more often than not - though once again it is worth acknowledging we play a probability game when trying to forecast the future and nothing is ever certain. And those drivers, semi predictable though they are, are also apt to change. I think many of the big guns out there are waiting to see what happens with the next 2 or 3 rounds of strat modelling. Quiet has descended and the nature of the strat profile is going to have a huge say in how the majority of the rest of winter unfolds. In the near term we are continuing to see a signal for height rises as we move beyond New Year but timing is unclear and the extent to which we get a block rather than a wedge - and how the Atlantic interacts with this is still up in the air. But a zonal profile for January this is not despite the claims of some earlier today. On the positive side of things trawling for opinion is a useful exercise. Cohen last night has come out and thrown a tentative hat into the ring for a split vortex SSW despite the uncertainty in the modelling. Webb has rather distanced himself for now from the SSW discussion but gone for persistent -NAO through Jan and Feb no matter the outcome of strat developments. And he has been quite bullish on this trajectory for a while. And the third interesting bit of browsing is this dataset. This is the shape of Jan and Feb in years where a displacement SSW occurred when ENSO was an El Niño, It makes for good viewing and matches the direction of travel currently of EPS anomalies. A hell of a lot changes in two weeks of weather watching. I have no doubt that, once we get to Thursday 11 Jan we will be looking at a very different picture and my money is on a blocked and cold pattern, either in place or soon to be in place. Enjoy the ride…

Hard to see much at a trop level. Scouring the pacific jet and the poleward flux emerging from the Rockies region patterns look pretty much the same. Flux levels on the GFS likewise. I guess it would help to know the specific lag time between trop developments and impact at 10hpa but that is well beyond my understanding. For now I’ll just call it “inter run variability” and wait and see what transpires tomorrow.

Disappointing, but tomorrow is another day and things may shift again. Signal for significant weakening still there.

120h on latest GFS and the Atlantic train literally just stops. Buffers hit, energy sucked away. Reverse flow inbound… From here to here Great stuff.

Yes. Imagine if the world was populated only by the glass half full brigade. EPS at 10 days firming up on a pattern shift. Tick tock. Assuming folk are in here hoping for some great synoptics, rather than coming online to pick holes, they should indeed be buzzing.

Vertical levels are the same, different horizontal resolution. 2 distinct systems…which will come out on top? Interesting to follow. Implementation of IFS Cycle 48r1 - Forecast User - ECMWF Confluence Wiki CONFLUENCE.ECMWF.INT

Lovely. These high GWO orbits in mid winter do seem to be hard to land…or maybe that’s an artefact of the Nina sequence we have had and a hazy memory. Anyway very much into the Nino phases and since 22/12 we have had the MJO re-emerge into high amplitude beneficial phases. Once the Hogmanay storm passes through I think we can expect to see things develop rather well…

Today’s extended ECM isn’t out yet. Huffman is certainly attracting attention with a data source he isn’t prepared to reveal. Let’s wait for today’s data. Yesterday’s was the strongest yet and there will always be ups and downs in a model sequence. Perspective required.