Derecho

Morning here is my interpretation of the 06z GFS and 00z ECM and UKMO Only interpretations upto T240 are made Upto T120 little change in the output, Thursday still the coldest day with the chance of snow showers on eastern coasts. However at T144 we come to a very important point: GFS UKMO ECM To keep the potential for cold in the medium time range it is very important that the two highs in the North Atlantic join together and the Azores low is kept stationary. It also limits milder air coming in from the NW next weekend The GFS and ECM both achieve this but the UKMO is worrying because at T144 (showing yesterdays chart for some reason) there is some energy between the northern part of the two high pressure cells. This could disrupt the link and send the high just to the West of the UK south. The next step at T192 is to keep the high pressure to the NW and avoid any sinking into Spain. The ECM strikes the sweet spot, GFS is getting there on the 06z but the WAA towards Greenland isn’t as strong. The low around NE Canada also needs to be stationary and not interfere. GFS ECM If that happens then we end up with a northerly reload aka ECM T240. GFS not too far away with a chilly-ish North-westerly and sustained heights to our west but lacking the ECM's immediate potential from this point. GFS ECM Overall ratings UKMO 5/10 snowflakes – Potential spanner in the works at T144 GFS 6.5/10 snowflakes – More prolonged colder temperatures though messier thereafter ECM 7.5-8/10 snowflakes – Best of the bunch and sustained northern blocking

OK quick comment on the 18z, not too bad, again cold in the middle of next week but some milder air threatens to filter through as the high sinks, will still be on the chilly side though. That high to the NW at T240 is mighty impressive but we must not get height rises over Italy otherwise It'll be no use, it could encourage cold air to fall to our west unless the high gets shunted away. Nevertheless that's way into FI. Upto T240 I give the 18z 6.5 out of 10 snowflakes

Hello, thought I'd interpret the charts at different time steps and add my own personal rating to the output , I'll do this for the 00z and 12z when I've got time. So for the 12z output today, no big changes at T96 but at T120 we have a consensus emerging that the block won't stick to Greenland and will sink back south. Not a huge surprise as it has been hinted at but I would like it to take a bit longer to pump more warm air further north into the arctic. ECM T120 GFS T120 Normally the high would just sink and we would go to cold and dry before an eventual return to westerlies but the Azores low means that the jet will remain quiet. ECM T168 GFS T168 By T168 we end up with a slack northerly and there is actually pretty good agreement here between the GFS and ECM. It will be pretty cold at the surface during the early part of December, hopefully the UKMO will improve more. ECM T240 GFS T240 By T240 we see that the high pressure slips away or breaks into two. The GFS shows a part sinking to Spain and the other moving towards Greenland. The jet coming out of Canada is absent at this point which is good. But we see high pressure to our South which is bad because that would stop cold from advancing towards the UK or leave the potential for a west based NAO. ECM however is a good deal better. Overall next week is still looking cold but not particularly exciting either, some exposed areas could see wintry weather on Thursday and there will be air frosts. Though to get something greater will be more difficult due to the lack of retrogression in the medium term which is frustrating...but its still early days. Nothing amazing but pretty seasonal. Overall here are my overall ratings for the 12zs upto T240 UKMO - 5/10 - We get some cold but not to the extent of the others, block sinks quicker and colder air to the NW is more organised so retrogression from here looks more difficult. GFS - 7/10 - Pretty good, cold throughout next week and a clean northerly on Thursday. Good build of pressure around NE Canada but I don't like the build of pressure over Spain ECM - 7.5-8/10 - The best of the bunch, heights build over Scandi allowing cold to last longer, has potential in later frames for another northerly. For context here is my scaling 1/10 - Bah! Ghastly relentless zonality or Bartlett's e.g. January 1989 2/10 - Warmer then average and little hope of that changing e.g December 2016 3/10 - Mix between mild and cold zonality not much prospect of blocking e.g. December 2014 4/10 - Some colder days at times and hints of potential on the horizon 5/10 - Cold weather likely but longer term prospects of sustained cold lacking e.g January 2017 6/10 - Some decent cold in the outlook though nothing exciting e.g. February 2013 7/10 - Potential for lowland snow in favoured spots though not up there with the classics e.g. Early December 2012 8/10 - Good cold spell that will last for quite a while e.g. January 2013 9/10 - Very impressive, will be talked about for quite a while e.g. March 2013, February 1996 10/10 - Absolute classic sequence of weather! Big snow events and long lasting cold!! E.g. December 2010, January 1963, January 1987

Looks like another build of SLP around Greenland, the Azores low means that I can't see a return to zonal anytime soon.

Interesting read Steve, I hope your onto something re. the zonal wind reversal, its a shame we only have a small sample size with the QBO.Easterly phases definitely seem to be associated with a weak polar vortex and the weaker jet due to low arctic sea ice extent also has a physical basis. Also important I think are SSTs the North Atlantic cold blob has been a regular feature in the last few winters but seems weaker this year.Then we have variations in ocean circulation and in the stratosphere which are less well understood as well as solar activity.The NAO has been pretty volatile in recent years, is it becoming more sensitive to external influences?Compared to the last few years things appear to be a lot more favourable for a colder winter this year, the QBO especially.

Sorry, posted the previous message in the wrong thread! Back on topic, the 12z has a slightly negative tilt (108h) but pretty much identical 06z 12z

Seems to be more potential for a build of high pressure over Scandi on this run

GFS produces a stonking pub run and as others have mentioned the surface cold will be building with the slack winds. Then the ridging to Scandi keeps warmer air at bay before a northerly later on! There are many positives from this run: - The major plume of warm air heading to Greenland could have knock on effects later, ie reinforcing any northern blocking later on - The fact that 850s prior to next weeks forecast northerly keep nudging downwards to our north - Potential for a rise in SLP over Scandanavia - Drop in temperature over the nearby continent. Still feeling a bit cautious due to the UKMO so I don't want to get my hopes up...but if the 18z transpired I would be more then happy. The most important thing is to get everything out to T120 nailed down and send that warm air upto Greenland and build that high. Once that is achieved things could get really interesting but lets wait and see, fingers crossed

The uppers associated with the midweek northerly seem to be getting slightly colder with each run Just like the GFS 12Z so far.

Amazing positive temperature anomalies in the arctic at the moment, pretty much the entire basin! No wonder the zonal wind across the NH is set to decrease...

Lets hope this GFS is onto something, energy heading south and plenty of WAA to support a build of pressure over Greenland yet again. Fingers crossed for a good ECM later.

GFS 12Z 06Z Bit of an improvement with the GFS, slightly less energy going out of the E of Canada. May improve things later would be good to see the UKMO on board. That makes a much bigger deal out of the low... may chop and change for quite a while given this is a region where uncertainties are often higher.

More of a sinking high today rather then a Greenland high. The culprit is an area of low pressure exiting NE Canada at T72, it heads NNE and is forecast to intensify more then it did in yesterdays runs: UKMO ECM GFS The consequence of this is that the block is put under more pressure and the high doesn't really build over Greenland. Unfortunately it is a feature on all runs now so it's going to have a role. For things to improve there we need the build of pressure over Greenland to be stronger then forecast at T96. Alternatively a link with the arctic high, a later attempt at Greenland ridging or a build of heights over Scandi could prolong the colder air. All still possibilities at this point despite added short term complications. Though I can understand why some are a bit disappointed.

So it looks as though a little shortwave cuts off the link of heights to Greenland but that could easily disappear in the next run, upto T120 its fantastic The 18z also really increases the blocking over the arctic later on compared to the 12z at the same time. Here are two charts at T186: 18z 12z

What a NH view that is!, with an 1040mb high in the arctic too! Just need to send that cold air over Svalbard our way. With the N Atlantic flow cut off from the continents they should turn cold very quickly. Would be even better if energy went under the North Atlantic block

The GFS run is superb, a wee bit more energy under the Greenland high in the later frames and we would be getting some even colder air towards us eventually. Good to see -9C 850s already. Do models sometimes overestimate upper air temps? and could colder pockets appear closer to the time? Either way the good thing about this time of year is that the weak solar input means temperatures at the surface can drop very quickly especially in slack winds. During 18th-25th December 2010 for example uppers were at a similar level (maybe a tad colder). Probably won't be as cold as that this time round at the surface but it just illustrates how we develop our own cold pool. UKMO very similar to GFS earlier on but differ in the level of strength of the Greenland high. If we can get the GH to develop more on the UKMO then it may stay more securely to our NW. Either way its still a very good run.

Also someone mentioned the lack of cold uppers.... well at T144 we can see unstable air develop near Svalbard and uppers drop very quickly, major steps forward today

The 18z going along the right path this time, just 6h slower then the UKMO and ECM, here is a comparison with the 18z (left) and 12z (right) at the same time The 18z shows more support for heights to the NW, especially when that low moves south across the UK. Great stuff and a step in the right direction at close range too!

That ECM is very similar to the p8 run from the 12z GFS . More pressure to the W of the block but the WAA is strong enough to keep the high pressure to the NW, a very good run indeed so lets hope we get the pressure rise in Greenland we need.

A look through the GFS ens shows around 5-6 runs with the potential for serious WAA into the arctic at T144. P3 is the closest to UKMO But the operational isn't out on its own. P8 meanwhile takes the award for the best run IMO

Even though the GFS turns westerly based later on, high pressure over the pole is a very consistent theme, all the way out to T384 Which is good to see

Yup I still think the GFS is a good run and is actually fairly cold next week. Though its open to being toned down whereas the UKMO is great because it sends WAA deep into the Arctic. Good output all in all, just on the search for some more sustained deeper cold which is why the UKMO is of particular interest.

For some reason the incorrect chart is showing when we use the WZ link, T144 shows a very clean northerly and strong heights to the NW.

UKMO looks superb at T144 GFS meanwhile much messier and looks prone to toppling, very unclear picture at the moment unless there is a link with heights over Scandi

Not yet , that's for someone else I think thankfully . For my research I look at how atmospheric circulation impacts regional climate, quite of bit of my earlier knowledge prior came from reading these forums over the years before I got deep into the studying .