Joe Levy

http://www.meteociel.fr/cartes_obs/gens_display.php?ext=1&x=305&y=139 (South East England) http://www.meteociel.fr/cartes_obs/gens_display.php?ext=1&x=190&y=173 (South West England) http://www.meteociel.fr/cartes_obs/gens_display.php?ext=1&x=265&y=1 (Northern England) For the most part, fairly unremarkable uppers on the 18z GEFS, however that is only half the story. There is a consensus amongst the members of a lull in precipitation amounts for a period of a week or so, after which time the spikes return. Additionally, though the uppers are rarely very low, they are still cold enough for snow, especially under low thicknesses. So this is a visually deceptive set and has a lot more potential than is at first obvious from looking at the graphs.

This chart at the end of high res on the 18z GFS is extraordinarily complicated! With cold surface and upper air from previous days, snow would be feasible in this instance as the low pressure hits the block, but all in FI territory and part of the whirlwind tour of every option known to man which we have been subject to!

Many in the South and South West and any other flood hit areas would be pleased if this chart came to fruition - not only is it cold, but it's dry too! Not that it is very likely to happen mind you - one run out of many and over 5 days away!

Well the GFS has settling snow in the sea off the coast of North Devon... EURO4 is in line with what is being modelled for Sunday night into Monday morning with wintry showers across South-West and Central Southern England. The areas where settling snow is shown in Southern England are Dartmoor, the Dorset Downs and Salisbury Plain

These are my tips to avoid disappointment this week when we have some cold and wintry weather lined up in the output! 1. Do not over-personify the models or blame them when they don't accurately predict the forecast for your area - mostly they get trends right, but specifics are so much more difficult. Remember that they do not control the weather and they are only computers! They don't hate you! They don't know you exist! 2. Don't over-analyse specific details until two days at the earliest before a weather event. Again, it's all about trends! 3. Even if you do not get snow this week, there is still potential further in the future as it can snow in April (a white Easter is more likely than a white Christmas). For that I refer you to Tamara's post earlier on and remind you that February is a traditionally cold month in meteorological history in this country. 4. Lastly, use the radar on the day! Don't overuse the models on the actual day of a weather event. The radar is more accurate and more specific. You can also find tables on weatheronline amongst other sites that show dewpoints as well as various other indicators of snow potential. Hopefully this post is useful - I don't pretend to know everything, but it's always worth reminding ourselves of good model watching etiquette when cold weather is approaching and nerves are fraying!

At 120 hours on the ECM, the uppers over the UK are colder and with an easterly flow and, I would imagine, low dewpoints, this would be an interesting day for many indeed, should it come to fruition

I am sure many will point out how much better the ECM 96 hours is, but there is one particular thing that caught my eye and that is the depth of cold over Eastern Europe which is being advected westwards. I doubt that it will reach us, but it could prove interesting in terms of how resilient the block is

Whilst the specifics of such an event are so finely balanced, betting on small changes is risky. However, the GFS 12z is considerably further west than the 6z and is exactly the sort of correction we need to see, as well as greater consensus between the models. My three 'c's: correction, consensus, cold

The BBC forecast is not the METO, it is the public representation of the METO, a very different thing altogether. IF quite clearly said yesterday that the Met is holding fire on being too specific to the public about these matters as there is still too much uncertainty and snow showers are hardly the priority given the current flooding situation. In particular, the region most affected by the flooding is also the region most likely to see wintry showers from the wraparound effect, so you can imagine which issue they are going to focus on! With regards to the 'snow and stay' comment, well in the South West that is rare in itself, so seeing any snow is going to be pleasing for many. The short range models (EURO4) still indicate the possibility of wintry precipitation over the region and into Central Southern England, and that is good enough for me!

Suit yourself, but coming from a long term snow starved region, I call that a success in computer modelling! The signal for unusually cold uppers from a westerly was shown some days ago now, and has been maintained to a degree ever since. You will generally only be disappointed by model output more than a couple of days away if you focus on specifics. Trends are more important and the trend is still being fulfilled.

Exactly - it is a finely balanced situation and so any statements otherwise are misguided! This is probably now becoming a situation where the computer models can no longer help us properly and in fact we must wait and see what happens tomorrow evening, allowing us to focus on the bigger developments that are developing because of that low pressure system that is sweeping in tomorrow.

This is why the word 'if' is so important! The end of next week is still a while away and is FI as far as I am concerned. The next 2-3 days of weather are crucial, so small changes can still have a big impact. In particular, the low arriving tomorrow is one to watch, especially since there is still disagreement in the models.

There has been no downgrading of the prospect of -5 to -7 uppers coming from the west tomorrow night and into early Monday morning, with fairly good agreement from the models that the South-West, Southern Wales and Central Southern England will see heavy wintry showers on a strong WNW wind. IF has mentioned this several times now. Be careful about using the word "always", or some may hold you to that

Hence IF's mention of wintry showers moving across South-West England and penetrating inland on Sunday night into Monday as the cold air wraps around the low. Perhaps this is one of the few instances where a cold PV over Canada can be favourable for wintry weather in parts of Britain

ECM at 120 hours As SM pointed out, in a slider scenario, low 850hPa temps are not required for snow. Here we have low thicknesses and, with a continental feed, low dewpoints potentially as well

I have only just woken up this morning and made the mistake of reading the forum before checking the models. I was confused and expecting to be disappointed based on a couple of posts, and then found the ECM ensemble charts with 850temps like these: How could anyone say that this is rubbish for cold?! Aside from that, some great posts this morning as well

I've told my family a heatwave is coming just kidding... it's more fun to keep the information from them until a day or so before, so that I can nod my head in a serious manner and say "I warned you" even though I didn't... Back on topic (I am the only one who went off it...) and for many in the Eastern counties of England and Scotland, there is a good chance of seeing something from this next week in the way on wintry weather, as well as a lower chance of flooding than elsewhere. I doubt this will change dramatically in the model output, and may perhaps improve. For the rest of us, the balance is much finer. We need to see westward corrections such as that of the ECM ens continue, and not just for cold 850s to advect westward, but also to bring drier air from the continent that might allow a respite for flood hit areas of the country, especially the Somerset levels where the situation is dire.

Thank you for this, very interesting. In a region where wintry weather is exciting no matter what the extent of it is, this possibility is certainly appreciated! It will be interesting to see how this develops

Are the snow showers being modelled as mostly on windward coasts or could they push further inland? Or is it too soon for that sort of detail to be picked up?

Now I am sure that most of them do not get that cold for us, but it certainly is a big swing in the ensembles and would suggest that there is a great deal of potential for more significant cold to be had. I would be interested to see the London ensembles as that will indicate better how many of the members transfer the cold over to the UK for such a period of time.

I live in Exeter, I'll just go over and check if their eyes are raised for you joking aside, I agree - we have often seen situations where the strength of the block has been underestimated by the models, allowing the pattern to back further west over time. However, we should be careful, as the Scandi high, the Azores high and the Canadian vortex hold a fine balance and one slight alteration and we are either in cold easterlies or cool northwesterlies! Or something else...

That's not actually true. There has been snowfall in lowland Britain in -2 uppers before, due to low dewpoints and/or low thicknesses. Last year, in fact, there was snow in the South West with uppers of around -2/-3 if I remember correctly

I never said that in any case, I would be careful about being decisive about a situation that has yet to happen. The official forecast is certainly worth heeding, and currently a return to cool/cold zonality is the favoured option, but since we don't decide the weather, that is prone to changing and fluctuating. Where there is uncertainty, sometimes turning to previous examples can help a bit. If the low sinks south quick enough, then blocking to the north of us could strengthen. Cold air, as I'm sure you've heard hundreds of times, is denser than mild air, so it is harder to shift. For the time being, let's hope for a cold snap if that's what you're after, and see what happens.

Not really, it backs the pattern westwards and there's nothing wrong with that to be honest. It delays the arrival of the cold a bit, but gives us more room for error

I disagree, especially if a northwesterly comes into play. Even in lowland Exeter we can still get a few flakes in the heavier showers in such setups. Also, remember that chart IF posted a few days ago showing snow showers across the South West, South Wales and the M4 corridor? I don't know what the current situation on that is, but to say that there is no chance is a bit extreme! It's hard to know what the precise chances are to be honest for anywhere. Even in favoured areas, some will miss out and some will do well.