Altostratus

The last few days have certainly illustrated why so many of us advise not reading too much into the outputs outside the 96hrs timeframe. Having said that, even I am surprised how the output has changed significantly over the last couple of days – the magnitude of the shifts in output is extraordinary. We haven’t quite had a complete flip within 96-120hrs, but it isn’t far off. It is an extremely delicate balancing act that is proving difficult to resolve for even the highest resolution models. It is certainly a good insight into how small features and subtle adjustments in energy distribution can equate to large deviations in all model outputs. Who knows what the ECM will bring tonight...

As expected, huge variations beyond T+96. This is the key period and has a huge effect on later developments. It is such a fine balancing act that it could go either way. Little chance of a middle ground IMHO – it will either turn very cold or very mild. Given the situation, it would be hard to break out of either of these patterns once they’ve set in. Looking over this evening’s outputs you’d have to say odds are growing for a mild future as opposed to cold. We are approaching T+72 now, so time is also running out for complete flips. If the GFS is showing something similar to the UKMO or ECM tomorrow, then unfortunately it’s probably game over.

It's a high resolution model used by the UKMet. You can view it here: http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=euro&MODELL=nae&MODELLTYP=1&VAR=prec&HH=0&BASE=-&WMO=&ZOOM=0 I think you can get better charts than that but can't remember where from. If anyone knows of this please post the link so that people can bookmark it. Edit: Barb beat me to it.

It’s an interesting time to be model watching isn’t it. We’ve certainly been spoiled this winter. Today’s outputs have shown growing support for a cold spell, but given the evolution to cold occurs well out in FI, confidence still remains low. It’s not worth getting excited just yet. In these situations I tend to use T+96 as safe marker. In normal circumstances I’d be pretty confident up to T+120 but as we all know, these are abnormal circumstances. For the reliable time frame, the GFS serves as useful guide, however the MOGREPS NAE is meant to be better for short range forecasts.

The GFS has just produced one of the best runs in history. It just shows the potential of this event. With those huge blocking highs you'd expect something good to come out of this eventually.

The ensembles are better than I was expecting to be honest. A lot of scatter as one would expect beyond T+120 however. With the mean below -5C throughout, a cold outlook is favoured at the moment. Just how cold, remains to be seen. It is interesting to note that the operational and control run follow a similar route with the cold being delayed a few days. Let's just hope the cold doesn't keep getting pushed back.

Horrible 6Z - it would be very frustrating if that verifies. The jet loop running between Iceland and Norway never dies down and this prevents pressure from building into the key area. With the whole pattern stopped in its tracks, only eastern (possibly south eastern) parts of Europe would benefit from any CAA along the underside of the block. Without pressure building up into GL, the GFS scenario for energy reinvigorating over the block is quite likely. As I said last night, the really interesting developments are still in FI so plenty of time for change. Several days yet before the correct evolution is known I would have thought. The potential is still there, but patience may be needed.

Yes there are some pretty special runs in that ensemble suite – quite a few go for full undercutting resulting in blizzards. A long way to go yet, but nice eye candy nonetheless.

You raise and important point. People often miss things at close range when concentrating on things far out. -10C uppers at the 850hPa level aren't always exciting though; maybe if you like cold weather - but it is certainly no guarantee of snow. Yes there might be a bit of transitory snow from the northerly, but the set up is not conducive to deep long lasting snowfall i.e. something that is worth getting excited about.

Shaun, The GFS was poor for the pre-xmas cold spell if memory serves - the ECM on the other hand, was fantastic. My comment re cold and dry was merely an interpretation of current outputs. As I alluded to, this is an evolving situation and is subject to change. Your comment about features being picked up at the last minute is correct. I also agree with the classic features you list, however in order for these to be realised, an awful lot needs to happen first.

Well, there is still nothing to get overly excited about in the reliable time frame. Unfortunately, the exciting stuff is well out in FI at the moment – déjà vu anyone? With that huge Siberian block forecast, it is hard not to envisage it exerting an effect on the UK at some point – what this effect might be, is anyone’s guess right now. Taking this evening's output, it looks like it will be cold and mostly dry as opposed to very cold and snowy. There will no doubt be ups and downs as we close in on the event. Certainly looks promosing from a cold perspective at the moment though - sure beats seeing endless zonality IMHO!

I’d agree with you about the holy grail being the GL/Siberia link-up. It has only happened a few times in the last 100 years. Feb 47 is the classic example, as you say. I think it may have also happened in Jan 1940 - the Thames froze over that year I believe. To get a repeat of Feb 47, we’d need the block to become completely cut off from the main circulation, with all energy going south, resulting in full undercutting – this isn’t being shown just yet, but the building blocks are in place with that elongated omega block. Make no bones about it, if this output verifies, a sustained period of very cold weather would ensue. It would certainly be a great help with getting that low Feb CET.

This is like something you see in the archives from those historic winters: http://www.wetterzentrale.de/pics/Recm2161.gif

Yes, it is an encouraging trend so far. The usual caveats apply though. In terms of modelling, we saw similar things being forecast in the medium term, for the previous failed easterly attempt. The models will only really be able to pin this down until within T+96. The ECM correctly predicted the last failed easterly, while the GFS and UKMO underestimated the strength of the northern branch. It is certainly a good sign that the ECM likes this easterly evolution - for now, anyway.

The ECM is shaping up nicely isn't it. Looks like FI will be very good too - just need pressure to start rising to the north and a decent shot at an E'ly looks likely on this run.

The Feb 05 spell contained even better charts synoptically. The problem with that spell was that there were no cold pools to tap into. Pockets of cold air were present this time, however, this has mostly been mixed out now. Indeed, warmer air is starting to be drawn in from the Greece/Turkey region. Also, the curvature of the isobars hasn't been favourable from the outset, with a greater sea track and extended contact time. The resulting air mass gets modified to a greater extent as a result. This is no doubt a contributing factor; particularly as it is early in the winter and the sea temperatures are still cooling. How signifcant a factor it is, is difficult to say.

The ECM looks similar to the UKMO tonight. ECM goes on to develop a huge area of high pressure over Scandinavia and neighbouring Russia - shades of Feb 91 there. At the latter stages of the ECM, an extended ridge from the Siberian high also appears. This feature has been non-existent in recent winters; it used to be quite common. Good agreement on the cold continuing for some time yet. The mild breakdown is on hold for now it seems.

Fascinating outputs so far this evening. We are seeing an old school battle for the first time in the internet era and to use a clichéd phrase: ‘The models are struggling’. Well we knew it wouldn’t be easy! Both the UKMO and GFS seem to suggest the cold air hanging on for longer than this morning. The JMA shows no sign of a warm up all the way out T+192. The Atlantic low comes in and is shunted back by the block, only to come back for a second go that breaks down sending shortwaves underneath the block, followed by signs of pressure building into GL. There are a number of possibilities and the outcome, as ever, is not very clear. We have all seen how difficult it is to accurately model small areas of low pressure from as little as a few hours away, let alone a few days. The prospect of snow makes this even more tricky as a forecaster.

I wouldn’t be so pessimistic. The showers have clearly shifted in direction. They were barely skimming Kent before moving into the channel, whereas now they are extending well inland and look to be moving eastwards. The showers are being driven from the winds above – these won’t be exactly the same as ground level winds.

Judging by the radar it looks as though more of an easterly wind component is kicking in. The showers are really stating to build over the sea and the latest radar shots resemble those from the States when lake effect snow develops. The only difference is that the showers do not appear to be very heavy yet. It looks squally and not very productive for building snow cover. Drifting and blowing snow must be making it quite hazardous in areas. It must be excellent fun watching these showers come and go. What size snow drifts can people report at the moment?

Just as support grew for extending the very cold spell, with additional support for undercutting, the ECM throws a spanner in the works. If the ECM operational isn’t an outlier amongst the ensemble suite, particularly for the early stages of the run, it is possible it could be on to something. It is uncharacteristic of the ECM to be more progressive than the GFS. I feel that the ensembles will be revealing tonight. It is important to bear in mind, even if this ECM run verifies, it will remain on the cold side. Although whether it remains cold enough for snow is a bit dubious.

Not too dissimilar to last night’s. The operational has better support from the control run this time though. Still a lot of scatter, so nothing is guaranteed yet. Tne GFS operational was also warmer than it's ensemble mean.

It just goes to show how important it is to have a blanket of snow across the country. The cold uppers certainly help, but without the snow blanket, these temperatures would not be possible. Across a standard day, the snow cover makes it easier for a net removal of heat across the boundary layer. This makes it possible for successive days to become colder and colder. Dec 81 is a good example of this. The uppers were never exceptionally cold like in Jan 87, but the good snow cover allowed heat to be removed and for temperatures to fall from day to day. That is not to say the uppers aren’t important, as they are; they will limit the extent of heat removal from the system. While we are discussing temperatures, tonight is a very good example of the GFS not predicting night time temperatures very well. Many people have commented on how the GFS has a tendency to calculate too high temperatures. Shown below is the forecast from the 18Z for first thing tomorrow morning: http://www.wetterzentrale.de/pics/Rtavn1217.png Certainly for southern areas, this looks to be very inaccurate. I guess the GFS isn't aware of the extensive snowcover.

The ECM ensembles are excellent. I’m a little surprised there hasn’t been more mention of them. Maybe it is because everyone is focusing their attention on tonight’s exceptionally low temperatures. The operational run is clearly warmer than average towards the end of the run. Bearing in mind that the operational run was cold throughout, the ensembles are painting an even colder outlook. The ECM has been outperforming the other models consistently this winter and I have found the ECM ensembles to be a particularly useful guide as to the general temperature trend. I have been watching them closely and they haven’t yet got the trend wrong. If this is to be believed, it looks like remaining very cold throughout the period – definitely no mild weather on the horizon yet. If anything, the average looks to dip again at the end of the period. Some very cold runs are starting to show up as well. This is usually an indication that something is afoot.

The ECM is nothing like the GFS, as expected. The GFS as per usual has overdeveloped a single area of low pressure. The ECM has smaller eddies spinning off from the main low thus allowing for more energy to flow beneath the block. In turn this allows the cold to hang on for longer and brings the risk of snow to some areas. This will only act to delay the push of mild air, however, the cold air is mixing out and becoming more dilute as time goes by. For a stab at a subzero CET, there needs to be a shift in the pattern with another injection of cold air being needed. I only really see this coming from the north at the moment. Edit: The ECM out to T+216 is the kind of thing we need to see happen re injection of cold air into the system.