WhiteFox

Temp 2.1 dp 0. Rain/snow mix.

Temperature now 3.6, dp 1o, so 0.5 degree drop in 13 minutes. Sorry I don't have a fancy graph like Mapantz!

Temperature 4.1 and dropping. Dp 2. Rain rate 1mm/h total rain 0.8mm. Reading (for Timbland).

Temp 5.7, dp 2. Nothing here yet near Reading but would expect drizzle very soon.

Yes, 6z ECM had more in way of liquid PPN in S/SE at least initially as the front pushes in. The angle of approach makes a big difference.

True enough. It's a fallacy to say that the SSW has not affected out weather: it has, just not in the way that some of us were forecasting and others were hoping. We head into to February with a weak PV at a time when we would normally start to see more blocked scenarios increase in likelihood anyway. As @TEITSsays, there's no reason why we cannot see a swing to cold in the medium range, especially given we don't have a very organised PV centered over the Arctic. This is not to say that one run or another will verify and we will see widespread freezing weather because of it; but, the setup does not seem to favour a long term (i.e. 10 days plus) pattern of mild W to SW flow. We may end up with some more slider scenarios, or we may end up with a more prolonged cold spell, but I do not think that we are looking at a prolonged above average period of weather before the end of winter. I would be surprised if the upcoming snow this week is the last we see before next winter.

I think still about five or six days as it has been one of those winters where small adjustments make a big difference! A warm up looks likely to me; the question is how mild and for how long? At this time of year the PV would normally be waning anyway and February tends to be more blocked. As with the winter to date, it's about what happens to that lobe of PV over Canada....

Agreed. Looking back, the stratosphere and troposphere seemed to be decoupled at the start of the season, so probably needs factoring in to any future forecasts. In addition, more attention paid to state of QBO and why this particular SSW left such a persistent vortex over Canada. These are all lessons to be learnt for future reference. Science is about testing and refining theories. Unfortunately, meteorology is a science which does not allow experiments in controlled environments, so all we can do is try and use previous observations. It's all about learning. Some seem to simply enjoy seeing others fail in their efforts; this has been the case through history in every area of science, so it will never change, but without trying theories we would never get anywhere!

It's a different setup, but I recall the same claims being made about storm Emma last year; the GFS was consistently pushing it further north than the other models and at that time I remember many people saying the the GFS was over-egging it. Not saying the GFS is correct of course, but it doesn't always get it wrong!

With no blocking in place you'd usually expect it to spin past the north of Scotland, by which time a lot of milder air has been sucked from the southern side. It would probably lose intensity as it made its way across. But who knows this winter?

The very cold airmass over the Mid West is forecast to lift out very quickly - Chicago goes from a low of around -34 on Wednesday morning to a high of 4 by the weekend. Could be a reaction to that extremely cold airmass pushing east and interacting with warmer airmass coming from south. Certainly potential for a storm I guess, but probably overdoing it somewhat!

It does a little. But, looking at the Northern Hemisphere view, could be a transitory milder period: seeing an Aleutian Low set up and draining of lower heights over Eastern Canada. Last hurrah for the Atlantic? Academic of course at this range, but I don't think the ECM day 10 chart necessarily represents a flat and mild pattern for any great length of time.

A very interesting chart for many reasons. Occluded front by south coast and an ESE draw so dewpoints probably ok. Also the system coming out of North America looking like it could disrupt SE as well.

Definitely can't rule anything out in the medium to longer range. Forecasting is a mug's game if people categorically state it will definitely be mild or definitely cold...

But they weren't. They've been showing a broadly NW flow for quite some time and that is what has been happening. Bear in mind that a higher anomaly towards Greenland does not mean a high pressure; heights are generally low there in January, so it means higher than average at the 500mb level. In recent cases this has been enough to help with the steer from NW to SE. Not to be confused with SLP anomalies or absolute SLP heights!

NWP models may be showing building of heights further south towards the end of output, but also a trend for heights to the north at 500mb. I'd tend to trust the 500mb output over surface level charts at that sort of range. It's not a case of dismissing a possible mild outcome at that range, it's recognising the fact that NWP models are really struggling to get the surface pattern correct at a relatively short range right now. As mentioned before, the ECM (yesterday or Friday?) Had the deep Atlantic system filling in situ over the UK by the end of this week with a mild SW air stream deep into Western Europe; that is no longer forecast. In these sorts of situations, models tend to try and push the Atlantic in too quickly. At the same time, we appear to be seeing a shift towards northern blocking appearing. Nothing is guaranteed of course, but I don't think comments about when the Met Office are going to change their outlook to fit in with an 8 day NWP chart really show much analysis...

Yes they are, and what do the 500mb anomalies tell us? A growing trend towards northern blocking. Perfectly consistent with approaching low pressure systems being pushed further south. The anomalies do not show an increasing signal for a milder pattern.

This is a key point. We are well aware that a small difference in data has a huge impact on the SLP further down the line; even without an SSW we often see big variations because of chaos theory. Now, throw into the mix a downwelling SSW which has caused forecasting issues in the upper atmosphere. What you get is an even greater level of error as you progress through time on an NWP model. What are we seeing at day 8 on the ECM? Something that is very different from what was shown on the day 10 two days ago. We do not know what the Met Office are seeing in their models, but they clearly have an indication that either the Arctic profile will shift towards blocking in our part of the world, and/or tropical forcing helping this along. As with all medium to longer term forecasts, this is based on an element of probability and no doubt there is still a probability that we will not see the cold outcome. There is absolutely nothing run with commenting on models and their output, but it is very clear that solutions are subject to large amounts of uncertainty beyond four or five days. If the Met Office and those of us who have/are saying that there is still a good chance of a colder outbreak are wrong, then so be it. However, it is all about learning where we went wrong this time and incorporating the lessons into future forecasts. That is science, and that is how we make progress...

Yes. Certainly seems to be backed up by the 8-14 day 500mb height anomalies (which as mentioned in the other thread were given a 5/5 confidence score by forecaster yesterday). Still a gap in above average heights between Greenland and Azores High, so certainly suggestive of energy squeezing across the Attlantic and running into the above average heights to our north east. Certainly seems to back up what the Met Office were saying yesterday.

Wise words, but I'd expect that from someone with a beard such as yours! In terms of forecasting beyond the short range, to me I think it's about guidance rather than pronouncing an exact setup. Past the 4 day range synoptic SLP charts can vary; not necessarily enormously on a hemisphere scale, but small differences in SLP have a big impact locally. That is why looking at the 500mb charts is always a good idea as it gives an idea as to the overall pattern and helps to ascertain probabilities for a particular outcome at a lower level. If we see a deep low pressure system developing on SLP charts, then do the 500mb charts across other models and previous runs back up the signal? Of course, 500mb charts are also subject to change, but much less than their SLP brethren. Going even further out, it's a case of trying to figure out what drives the pattern higher up which is where analysis of teleconnections and the stratosphere come into play. This is really at the edge of understanding at the moment (I think!) and the best we can do is to estimate general patterns going forward. We're a long way from being able to nail down SLP at a range greater than 8-10 days (as the CFS clearly demonstrates), but we can at least give probabilities on overall patterns which drive outcomes. Unfortunately, many people seem to think that longer range forecasting should be like short-term forecasts with details such as PPN type and temperatures accurately predicted. Anybody who takes the time to study the subject more closely knows that this is simply not possible and the best that can be said is 'likely' or 'probable'. Overall it's a fascinating area of interest and I enjoy being able to discuss the weather in detail with people who aren't simply making polite conversation. I particularly appreciate the efforts made by those who go into great detail about their thoughts and analysis and help others such as myself to continually learn new things!

It's a difficult one to call. Obviously a deeper low would naturally track further north and east, buy in these situtations a difference of 100 miles is enormous interns of PPN type and global models at this range will struggle with the details. Once this gets into high resolution model range we can really start to analyse in more detail. Just a quick note on the overall pattern; I noted that yesterday in the 500mb CPC discussion they gave a confidence of 5/5 for both the 6-10 and 8-15 day 500mb pressure anomaly charts. US forecasters clearly think that the pattern shown is well represented across models, but of course we have to expect variations in the surface level analysis. It is these variations to which we now turn our attention with respect to snow possibilities in the shorter term: eyes down for a snow chase!

It's an interesting question. If you look at this plot from earlier: You can see that the reversal is forecast to reach the surface by end of January/start of February. The 12z GFS does show this, but it's in the wrong part of the Arctic to promote blocking in our region and serves to push lower heights to areas unfavourable to development to our north: Of course, it would not be static and would move around (were the downwelling forecast to verify) so the positioning could change to a more favourable location. A negative AO only tells us that the average heights over the Arctic are higher than the average heights around 37-45o North. It would seem that the Met Office are seeing something in their models which favours blocking towards our side, so it's something to watch. Obviously nothing really showing on the NWP models that we can see right now...

Indeed. Has Christmas been cancelled? All this furore over what is a tiny difference in a run at a time frame that is nearly a week away. As for background signals, they are all still there. As many people seem to like to anthropomorphise the models, put it this way: cold has been dealt a strong hand. The problem is that it appears that not-so-cold (not mild I may add) has also been dealt a strong hand. It may seem that not-so-cold has the upper hand now, but does cold have a trump card up its sleeve? We're entering the period of winter when the PV would naturally be waning anyway having passed the peak period. We also have a more favourable phase of MJO upcoming, plus further downwelling yet to (possibly) impact further on the AO. All this adds up only to probabilities, not guarantees, but conditions are only becoming more favourable for cold, not less. In the meantime, it may be worth looking to closer time periods to see what may be coming sooner rather than later.

Some decent vorticity there too; fair bit of PPN on that I would suspect? Just goes to show that there are some good chances of some snow coming up. Dare I utter the horrible cliche about it being reminiscent of eighties winters? I think I just have... Still a lot of vorticity to spin out before we reach that point though; let's not make ourselves dizzy obsessing over it (sorry for the dodgy puns)!

Which brings up a further question I think: If the AO does move strongly negative, that obviously implies considerably higher heights over the Arctic than at the mid-latitudes right? Firstly, the AO is a measure of the differential between the heights over the Arctic (I can't find the exact definition of the Latitude used for the Arctic measurement) and the average heights at around 37-45N; clearly that can mask areas of significantly below average heights in some parts of the mid-latitudes with above average heights in other parts. Perfectly possible to have below average heights over a large area of Siberia/East Asia say and above average towards the mid-Atlantic. The question is that if the AO moves down to around -3 or -4, does that imply that you could see the lower heights pushed that bit further south, leading to what we see forecast with a large trough across Western Europe? So although heights near the Azores may be at or even above average, we see a steep gradient appear on a NW/SE axis from Canada towards Southern Europe? The more negative the AO, the steeper the gradient, therefore keeping the UK on the colder side of the jet? And then, of course, you have the trough digging more deeply over Central North America, and the returning wave pushing further North thus strengthening the higher anomaly towards the mid-Atlantic? Again, all musings!