Altostratus

Heavy snow here Temp has dropped to 0.5C and dew point now -1.9C. Snow has already settled on wet surfaces.

All rain here - melting the little snow we had. Temp is 1.1C and dew point is -1.3C.

Another morning and another set of very good runs. Despite numerous exciting prospects this week, you can’t help but notice the pattern starting to emerge in the medium to long term. As many of us expected, the cold spell looks to be extended and it is increasingly looking likely that the big day itself will be cold. For the cold spell to extend beyond Christmas, we need the lows to start undercutting into Europe. This will help to maintain us in the trough and help to shunt the ridge further eastwards. Any warm air would then be advected along the flanks of the ridge up into Eastern Europe, away from us. The other thing we don’t really want is for any approaching lows to rapidly deepen and amplify the trough and subsequent ridge. The models look to be moving away from this now, so we are heading in the right direction at least. Regarding the update from the MetO, they always underplay things until they are absolutely sure. I wouldn’t worry about any BBC forecasts either. Best thing to do now is look out the window.

The 18Z is a thing of beauty. That really would be somethng special if that happened. It's nice to see this crop up in the models as it has been mentioned as a possibility my both myself and others. Probably unlikely, but the fact it is even showing up on the models is good news. Lots to enjoy before then anyhow.

It seems that everyone is in good spirits this morning and rightly so! It has been a tiring and anxious last few days but it seems we have turned a corner today, with cold and snow looking virtually guaranteed in places. It is time to relax, sit back and see how it all unfolds. Regarding the question of snowfall distribution, it is, as many others have said, still far too early to say - this will likely come down to radar watching a few hours beforehand. A prime example of this was last February when Devon and Somerset were hammered even though the snow was not forecast there. My assessment of the situation at present is that favoured areas in the east look good for the initial easterly and then as we move into the weekend, the emphasis shifts to the north as that bitter northerly kicks in. I would imagine most parts of the country would receive at least some snow from the northerly at some point. Polar lows are a distinct possibility too. It’s exciting as there seem to be so many options for snow formation once the cold air has established. Too many times in the past we have lacked one of the two necessary ingredients: cold air and instability. Both are present in abundance here so anything could happen – don’t be surprised to see some surprise snowfalls if the northerly kicks in as currently shown. Regarding a breakdown, my hunch is that the majority of models are being too progressive. Expect the cold air to last longer. I will be surprised if it has completely disappeared by Xmas. Anyway, let’s see what variation the 06Z will deliver…

Interesting. GFS is showing a potential snow event on weds. Thickness and dew points are good but will the temperature be low enough? Looks like it might well be on this run. Something to keep an eye on anyway.

There is no reason to be despondent this morning. The ECM and UKMO are excellent, with snow over many parts of the country. Could even be some heavy falls in the south were these runs to verify. The details of this will invariably change between now and then, but it will certainly be turning much colder and it is looking increasingly likely that at least some snow will fall later this week IMHO. Too early to say where exactly, but if eastern parts of the country receive nothing out of this cold spell I will be very surprised. You have to side with the ECM and UKMO over the GFS in this situation. It would take a very brave person to side with the GFS. The GFS looks to be back to its usual trick of being over-progressive IMO. I could be proved wrong; if that’s the case, kudos to the GFS.

Another set of good runs this morning. There is still a large amount of variation at T72. We can now confidently say it will turn cold or very cold, but where the snow falls is still very much up for debate in the medium term. The ECM is fanatastic in the long run and looks like a further pressure rise to the NE could establish itself - a reload after a reload! My only concern the with that run is whether or not enough cold air can be entrained prior to the onslaught of the northerly. The GEM is also fantastic in the long run as is the GFS (not quite as good though). In summary, a slight downgrade in the short run, but better agreement on a sustained period of cold weather IMHO. All this will change with the next set of runs, no doubt!

Much better run for those in the south. Starting to look very exciting indeed. I arrive at Heathrow on thurs lunchtime - I couldn't have picked a better day and time - perfection!

Out to T+48 and already there are some very minor changes. The block seems to be slightly further to the north west. We could do with a decent run just to settle the nerves and help us get a good night sleep

The ECM T+144 is a cracker! http://www.wetterzentrale.de/pics/Recm1441.gif

Yes, we got there eventually, but that was well out in FI. The key developments are circa T+120. A lot can change between now and then. It will be fascinating to see how this pans out regardless of the outcome.

Not quite as bad as I first thought but nothing is guranteed beyond T+96. There are even big changes at T+72 atm. I am becoming concerned that we end up with minimal pressure rise over GL.

This is looking like a bad run. Maybe too early to say, but there is surely far too much energy going up into GL. I can see a sinker coming.

They will almost certainly analyse historic data and check the performance of the models via simulations. If anyone knows more about this it would be interesting to hear. The only way I can see historic patterns being programmed into the models would be via the use of correction factors to overcome any known flaws in the modelling. I could see this being particularly useful for areas with minimal observation and subsequent data input. Based on the model simulation analysis I suppose it would be possible to produce calibration curves as well. I would expect some large error margins however, as no two patterns are ever identical. Regarding the AO and NAO forecasts, it is my understanding that these are based on the model outputs from the GFS. They are purely reactive. They are not based on an independent model or forecast, and as such, provide no additional support.

Looks like a middle ground has been found to me, with better agreement at T+96. It isn't a total backtrack by the ECM. Looks like there is a slight downgrade in the short term, but what happens after that is anyones guess at the moment.

What a chart! http://www.wetterzentrale.de/pics/Rtavn1381.png

Looks like a good run to me. At least it hasn't followed ECM. FI should be interesting. Roll on tomorrows runs...

I smile to myself when people on here comment that the ECM is struggling or argue that because it has been the least consistent model from run to run, it can be discounted. At the end of the day, they are just computer models built around the laws of physics. They show it how it is; although they are obviously only as good as the data fed in. Perhaps the ECM has been inconsistent because it is more sensitive and picks up smaller details that the other models miss – I’m guessing it has a better resolution. If so, the ECM is likely to be the more accurate model and may be on to something – it can certainly not be ignored. The downgrade trend has been evident for the last few runs now. There is still time for small changes, but as we get the nearer, the error margin decreases giving less room for manoeuvre. On the flip side, it is feasible that the ECM has been oversensitive and overdeveloped a small feature which has been magnified as the run progressed. If the input data was good and the run isn’t an outlier, this is the only straw I can see us clutching at present.

Stunning chart: http://www.wetterzentrale.de/pics/Rtavn1441.png That will take some shifting. Edit: Not enough forcing to advect the coldest air towards us, but nonetheless, synoptically fascinating. It isn't difficult to envisage subtle changes to the development and alignment of that low to the SW to have a huge impact on us. My hunch is that GFS is probably over doing it.

Looks like a upgrade in the short term. Need an injection of cold air from the NE to maintain the cold spell though. It certainly looks possible but will it happen...

A cold spell is nailed now. The worst case would be cold frosty nights with no precipitation at all due to the high pressure taking residence over the BI and slowly sinking back into Europe. The best case would be the Arctic high exerting a bigger influence with a scandi trough forcing through with fully undercutting lows to the south – much like some of the charts showed yesterday and the day before were showing. Somewhere between these two extremes is looking most likely at present, with northern and eastern areas receiving at least some snow. It has to be said, this is slightly disappointing given the potential, but at least it won’t be mild – Mr Bett will probably have the week off. We could really do with undercutting in this run, much like in the previous parallel run

I was nervous before - I'm even more nervous after that ECM run! Worryingly, it seems to fit well with the METO forecast. Let's hope that was an outlier.

ECM out to T+120: http://www.wetterzentrale.de/pics/Recm1201.gif Not a great trend here - looks like the 'real' cold might not make it - if it does, it will be delayed.

The ECM is rolling out. Nervous times. I'm not too keen on the GFS this morning. There is massive potential for a big snow event, but it looks marginal to me in the south.