chionomaniac

It's a big game of spin that wheel to determine where the Arctic heights will end up at. With a bit more amplification from the Azores high and a bit less jet streaky and with the Scandi heights not totally vanquished it could be game on in 10 - 15 days.

Not too far from reversal at 1hPa.......

I can confirm that it did get dark here tonight and that when it occurred I am sure that it was a tiny bit later than last night.

That proposed Aleutian low/ Siberian high could well be feeding back into the strat with the latest wave breaking warming the best signalled yet. From what I have seen from the runs over the last few days, Feb 5th could be an important stratospheric day this winter.

I think that you are being unnecessarily critical, cc, or have unrealistic expectations of strat FI charts. In fact Tim, there has been a warming of the stratosphere around now - yes, not as severe as the charts initially forecast - but perhaps more in line with others pre winter expectations for around this time. And without this stratospheric warming that has resulted in the split in the PV, we would still be talking about the next bout of severe storms hitting the west coast. Instead, we now see the first signs of a Scandinavian high influencing our weather pattern. We may not see the easterly that was initially programmed, but we have seen a big change in the pattern in our sector that will assist for the rest of winter. It appeared earlier this week that we has rolled a 6 at the first throw, but now (in line with earlier expectations ) we see that that is not the case. But, as Tamara has pointed out earlier today, we still will probably have more throws of that dice that will possibly drag that Siberian high further westwards as winter progresses. Hopefully. Added below is current 10 hPa chart showing minor strat warming from wave 2 forecast that has induced split.

That was tongue in cheek, Ben, as it is far too early to even speculate! It was just that the ensemble warming towards the end of the month would fit in nicely with pre-winter thoughts.

"Mr Warming, we have been expecting you!"

I think that when you look at the potential vorticity charts of the whole vortex (1hPa >100hPa) at past events, you will see that there is no set guidance at to where a displacement that resides at 1hPa will reside and affect the vortex at 100 hPa. Any displacement can often curl around the vortex rather like a helter skelter, so that anomalies at the tropospheric level can be difficult to predict. A split vortex tends to have far less of this effect and can be a little more predictable in terms of tropospheric anomalies.

Jokingly, I am tempted to say that the 18Z is a lot better if you prefer your rain slightly colder- but I wont.....

Perhaps you two could carry on this debate by pm and spare the rest of us. The fact is that the UKMO will have changed by the 12Z making this snow /rain debate at T+144 a complete irrelevence!

That's always the problem with whiteout runs. That then becomes the minimum expectation rather than a blend of all the output. Even some of us old timers on here forget that - I know that I always use the best run as the benchmark but at least I have learnt to temper any disappointment with realism.

Would definitely agree here Kriss. The overall pattern hasn't changed - over than normal variations expected. I'm just happy to see the Atlantic stop for a week or too and hopefully whilst it does, we see further attacks on the vortex.

The signs have been growing over the last few runs that wave 1 is going to pick up in intensity - I can't quite work out yet how this will affect the tropospheric pattern - it will very much depend where the vortex displaces too. I much prefer a split in that regard because they tend to be Atlantic/Pacific based.

possibly because the Azores high cuts off the cold feed before it becomes truly established...... I do think that this would be the outsider in the short term though.

I think that the more repeated and stronger the wave activity rebounding into the stratosphere, the greater the chance of cold outbreaks. To get a split vortex in the right position to produce an easterly at the first attempt is a bonus indeed - especially as the vortex isn't destroyed as it would be if a SSW were to occur. Normally one would expect a few rolls of the dice before rolling a 6 - but this could potentially be a 6 on the first roll. (Without the wave activity we are not even rolling the dice and have no chance of disrupting the vortex). I still expect another roll towards the end of the month btw.

It is the wave activity wot dun it! Great to see these charts - and a couple of weeks earlier than expected as well. Bring it on!

I think that I may have mentioned this last year, but fundamentally it does not matter how strong a jet streak we see coming out of east America across the Atlantic if there is a split vortex in the way, the jet streak will dissipate and not cross the split. A classic example can be seen on today's 12Z GFS at with the strong jet exiting the US at day 2 and yet it seems to hit an invisible brick wall. But that brick wall is not so invisible when we look at the 70hpa charts - we can see the day 4 split vortex that creates the barrier - this will disrupt even the strongest jet streak! The GFS has flirted with how long it keeps this split over the last few runs - the longer the better!

Better! East QBO for a start!

Just with a quick glance there is no surprise to see that a lot of those peaks coincide with wQBO years.....

I am sorry that I am a bit late looking at how December has fared, but it is worth seeing where we are regarding the forecast for December and whether there needs to be a significant change in the forecast for January and February. To recap my forecast suggested that December would be drier than average with average or slightly below average temperatures with an absence of high latitude blocking. Well we all know that that has not been the case regarding the the amount of rainfall especially regarding the last half of the month ( the first half was reasonably dry!) The temperature have been above average and there has been an absence of high latitude blocking. The main reason for difference between the forecast and the actual weather has been a gross underestimation of how cold the stratosphere would be and how this has led to one of the strongest polar vortex seen in recent years and associated Atlantic jet stream. When looking back at the December stratospheric forecast anomaly - the below average Canadian based forecast was suggested - but not the intensity. Forecast anomaly at 30 hPa: actual 30 hPa anomaly: The negative height anomaly is over three times greater than originally forecast (though positionally correct). It is pretty infuriating to correctly forecast the position of the anomaly but not its intensity. Before I looked at producing a winter forecast my gut instinct was for this type of pattern, but this was not backed up with my anolgue year composites (so I disregarded it and I guess I would do if having to produce the same forecast again). This has had a massive affect on the 500hPa pattern and this can be demonstrated when looking at the height anomalies at this level Forecast 500 hPa anomaly: Actual 500 hPa anomaly: A big, big difference when looking at the Atlantic sector with a large negative anomaly over the Atlantic ( though with the Pacific positive anomaly correctly forecast any USA forecast would have been correct!). And this has led to the empowered jet stream driving the deep Atlantic lows. So. I think it is pretty clear and yet another example of how important and destructive an exceptionally cold stratosphere is to the weather pattern that we receive here in the UK, whilst the US can benefit from the same cold strat. Looking forward, even though the stratosphere has been far colder than forecast for the first third of winter, I would not change the forecast at all. I have suggested that wave 2 activity would increase in the stratosphere in January and that this would put pressure on the polar vortex throughout the month and this does look like it is occurring - whether it is enough to knock out the vortex completely remains to be seen - but I guess the theory is sound here. To recap here are the forecasts for Jan and Feb: Jan forecast: January January continues the average to slightly below average towards the southern half of the country, temperature wise. Average precipitation is likely with the UK to be situated in between a negatively tilted Atlantic trough and Scandinavian ridge. This pattern is likely to become more dominant as the month progresses and stratospheric influences during this month will determine which air mass will eventually be dominant over the UK. There will be an increasing risk of intermittent snowfall in more northern areas as depressions will likely track in a NW to SE direction. Feb forecast: February February is the most difficult month to predict, as there are two likely outcomes. The first is another average to slightly above average month temperature wise, with again average precipitation. The second outcome is for a significantly colder month with below average temperatures and the risk of heavy snow and easterly winds. This very much depends on whether we see a SSW in the stratosphere. In the second scenario the ridge that has been building towards Scandinavia will dominate the UK, whereas in the first outcome the trough to the west would be likely to dominate the UK. Perhaps January and February can rescue this forecast - but so far it goes to show how underestimating the severely cold stratosphere has led to a stronger Atlantic jet stream and the subsequent stormy weather seen so far this winter.

I think that we can go back to what was suggested before winter started, ms. It will depend upon whether another surge of wave 2 activity can knock out the vortex or not. If not, then perhaps a more zonal outlook, though not as severe as the one just finishing, would be likely. I think that there is a good chance that the wave 2 activity will peak again around the end of the month though, and, if there is any return to more zonal conditions then this will be temporary.

The current pattern is driven by the peak in wave 2 activity that has been on the cards for sdome time now. We can see that this has created a split in the vortex from the troposphere upto beyond the middle stratosphere as noted on day 5. http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/ec.php?alert=1&level=all&forecast=f120&lng=eng The wave activity peaks around this time and wanes only slightly - so there is likely to be a possible stationary wave situation that could result in holding the Scandinavian weak height rises in position for as long as this continues. http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/eczm.php?alert=1&forecast=all&var=ha2&lng=eng This is critical to how quickly the Atlantic train will restore itself, but note even the latest GFS run holds the split in situ at day 10 at 70hPa: and this could prevent the Atlantic reasserting in itself. I suspect that this is why there isthe divergence of model outcome that Ian F in the model thread hints at. Whereas the wave 2 activity hasn't completely destroyed the vortex,it has upset it enough to create the most fascinating period of model watching since this winter began. We may see that the Scandi high waxes and wanes through January and that an increase in wave activity again is needed to achieve a full blown easterly by the end of the month. And I certainly wouldn't rule that out with the monkey nut vortex shape presently.

Right - let the team deal with any issues - please report posts and don't respond to what you may think may be trolling. Thanks.

Looking at the ECM composite anomaly charts over the last two runs we can see a trend in the right direction. 12Z Sat 00Z today stronger anomaly to our NE seen on this and this is highlighted when looking at the daily anomalies. We are seeing slight improvements with the NAEFS ensembles as well.

I guess that is when a little experience comes in handy as well. I would say that when the ensemble trends in the same direction that the MJO, GWO and strat data point to then one will have a far better chance of picking out that needle in a haystack. Also you will have a far better chance of picking a trend looking at all of the ensembles over all the output over a number of runs, rather than just looking at the deterministic runs. I guess that you are trying to say that the ensembles can occasionally lead you up a garden path, but more often than not they will spot a trend especially if one looks at the cumulative anomalies set over a number of days.