I usually start these off with a recap of last winter, but this year I covered that in a separate blog, so here's one I made earlier:
Moving onto this winter, there are a number of factors that make this a really fascinating one to watch. I'll go through each of these in turn, explaining what they're likely to do and how that's likely to affect us, before going on to look briefly at the methodology of the forecast, and, finally, getting to the fun bit, where I pull all of this together to make a (still wildly speculative) detailed winter forecast.
Factors to Consider
The most influential factor globally at the moment has to be El Nino - temperature anomalies in the central ENSO region are almost 3C above average http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf putting this easily in the top 3 strongest El Ninos on record. The impact in many regions is acute, with well above average Sea Surface Temperatures in the Indian Ocean contributing to an unprecedented two cyclones hitting Yemen in the space of a week.
However, for Northwest Europe the impact is far more subtle and indirect, with its main contribution being that Sudden Stratospheric Warmings (more on that later) increase in frequency in El Nino years http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00207.1 . Therefore, in isolation, a strong El Nino marginally favours a colder than average winter, although nothing compared to the hype of the tabloids (and even the Sunday Post) who seem to think it guarantees an oncoming ice age. Of course, it's more complex than that, particularly when you consider that there are essentially two types of El Nino - a 'traditional' east based El Nino e.g. the giant red pincer that was the last huge El Nino event back in 1997, the precursor for a mild winter in 1997/98:
and an El Nino Modoki, which is a more central-west Pacific based event e.g. in the exceptionally cold winter of 2009/10:
This winter's El Nino doesn't quite match either of those, with much weaker warming in the eastern Pacific compared to 1997 and in the central/Western Pacific compared to 2010:
This suggests that, while there will be an increased chance of a Stratospheric warming as a result of El Nino, the idea of a winter similar to 2009/10 isn't a firm favourite on the basis of this, at least. However, the sample size with El Ninos of this magnitude is relatively small,
My model of analogue years weights El Nino in a number of ways: firstly, by weighing the Multivariate ENSO Index to the current value, secondly by weighing towards neutral El Nino Modoki years, and thirdly by considering the joint impact of El Nino and the QBO which I'll get onto in the next part.
The Quasi Biennial Oscillation is a measure of the stratospheric wind near the equator. It runs in an approximately 2 year (hence quasi-biennial) cycle, where it reverses from easterly to westerly and back. This winter, we're looking at a very strong westerly QBO bottom graph:
which in isolation would suggest a more westerly dominated winter.
Last winter saw the strongest easterly QBO in the recorded dataset, which was one of the reasons many people were leaning towards a colder than average winter - an easterly QBO in isolation slows the westerlies at higher latitudes and hence weakens the vortex, leading to increased high latitude blocking and generally colder winters. However, as last winter's failure made clear, taking the QBO in isolation is deeply flawed - in particular, its interaction with both ENSO and the solar cycle complicate matters. In the former case, both El Nino and and East QBO increase the wave activity acting on the vortex, with the East QBO acting to advance the onset of the El Nino forcing on the stratosphere earlier in winter while the West QBO delays this signal, though the difference between the two QBO states in the mid-lower strat. seems to disappear almost entirely by late winter: Calvo paper This would suggest that we're likely to see a strong vortex developing early, but with the potential for more blocking as winter progresses. Thus far, in spite of this weekend's early cold snap, we've seen the stratospheric vortex behave in a manner consistent with this, with stronger than average westerly winds and near record low heights at the pole:
As (hopefully) an improvement to last winter's forecasting method, the QBO itself is not weighted but rather than ENSO/QBO and Solar/QBO interactions (more on these in the next passage) are, and in addition years with similarly strong early stratospheric vortices are factored in too. These can work both ways - on one hand in the short to medium term they tend to be associated with more zonal, westerly conditions prevailing, but can also increase the likelihood of an SSW further down the line,
This is certainly one of the 'hotter' topics in medium/long range forecasting discussions, and has been since the out of the blue very cold winter of 2009/10 coincided with the deepest solar minimum in nearly a century. The current solar cycle was unusual in two ways: firstly, it was one of the shallowest in a long record, and secondly it had a secondary peak in activity which was stronger than the initial (and what was assumed to be at the time the main) peak. As you can see if you read the recap of last winter's forecast, this secondary peak's emergence during last winter appeared to be one of the main reasons the vortex didn't behave as expected. Anyway, what of this winter? Well, it looks as though we're finally heading back towards solar minimum values (approximately <100 sfu) and if the next cycle plays out as expected it may not get much above that mark for the next 15-20 years:
The issue we now have with the solar/QBO relationship this year is that, while last winter solar max conditions acted along with the negative QBO to enhance westerlies, this winter the QBO flip means we're once again at a relative disadvantage when looking for vortex disruption, compared to if only one of these variables had flipped. Nonetheless, the lack of moderate/low solar, WQBO strong ENSO winters in the dataset means we don't have that much to go on, so confidence is pretty low, and low solar activity might at least inhibit subtropical heights from developing too strongly.
Other SSTs (Atlantic and the PDO)
The most notable factor this summer for many of us was the Atlantic cold pool, which contributed to a rather wet and cold winter overall, moreso the further north you went:
This cold pool remains in place, albeit somewhat less anomalously cold and flanked by record warm tropical waters, and is expected to remain for the rest of winter, again prompting some less scrupulous sources to start ramping about exceptionally cold winters. But is that really the case? Certainly, you would expect Arctic sourced westerlies that are usually heavily modified to be less so, which could potentially mean more cold zonality type outbreaks like we saw last winter. But looking at the area where the cold anomaly was strongest over the winter (15-45 degrees west and 40-55 degrees north) the correlation between SSTs and Scotland's winter temperatures is weak, and in fact weakly negative, although with the coldest SST winters seeing generally below average temperatures, suggesting that it does perhaps have some effect. The main reason for the weak (or perhaps negative) correlation for this region is that it overlaps very strongly with the northernmost of the NAO tripoles - this is the signature SST pattern which indicates the prevailing pressure pattern over the Atlantic.For a -ve NAO signature you'd be looking for colder waters further south, with anomalously warm waters north of 50 degrees e.g. the opposite of this:
Therefore, this is potentially a signal for a +ve NAO/ Low pressure dominating at around our latitude (for early winter at least), but also for those westerlies to be colder than they otherwise would be.
I've also weighted for the PDO, which has a more significant effect over the US but is another indirect factor, particularly since one of the key features of the SSW loading pattern appears to be the Aleutian Low, which tends to be associated with a +ve PDO:
Coming into this winter we have a positive PDO pattern, although not quite a classical one - the warm anomalies in the Central Pacific are atypical for a +ve PDO set up. While most studies have shown that El Nino, +PDO winters tend to be more associated with displacement SSWs i.e. ones where the stratospheric vortex is displaced off the pole rather than being split in two, the Cohen precursors for each type suggest that this warmer pool, which is likely to encourage higher pressure in the mid latitudes, would be more consistent with a split type SSW later on than a displacement.
Snow Advance Index + Arctic Sea Ice
These aren't linked as such but for the purposes of this can be thought of in similar ways. The Snow Advance Index, developed a few years ago by Judah Cohen ( https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiY6Y_aqKTJAhWFgQ8KHTGMC4YQFgggMAA&url=http%3A%2F%2Fwww.aer.com%2Fnews-events%2Fblog%2Fsnow-advance-index-new-tool-predicting-winter%25E2%2580%2599s-severity&usg=AFQjCNF2cjqp4wFUVmmZdXpWEsShHJTBsA&sig2=BvbNGHy0fC49jfGPhAAGoA
is a measure of the advance of snow cover during October above 60 degrees north, which has been shown to correlate inversely with the Arctic Oscillation, primarily because strong Eurasian snow cover advance in autumn encourages the growth of the Siberian High which is another key feature in the precursor for SSWs, and hence one of the key factors driving vortex disruption. This winter, as in almost all winters I've been on the forum for, this index is strongly positive, hence indicative of a negative Arctic Oscillation. However, it's worth remembering that last winter's SAI was also strongly positive and yet we ended up with an above average Arctic Oscillation.
Arctic Sea Ice is important because it has been shown that low ice extent, as again has been common in the last few winters for obvious reasons, is correlated with a weaker polar vortex and hence negative Arctic Oscillation. Therefore, weighting for these increases the likelihood of a colder, more blocked winter.
These are also factors which makes it a bit more difficult to find decent analogue years - both 1998 and 1983 had negative SAI values, and neither had sea ice extent as low as this one either.
Long Range Models
[A quick look at what the long range models are showing, given they did pretty well last winter.
The Met Office's GloSEA model had looked pretty promising in its October update for late winter at least,but November's update is a bit more sobering - low heights to the north, higher heights to the south, indicative of a generally milder than average, and probably wetter than average, winter:
though again there is just a hint of a possibility of a bit of Atlantic/Greenland blocking developing later on:
The CFS has, for a while, been going for a significantly milder than average winter for almost all of Europe, but seems to have toned it down a bit, particularly for December, in its most recent update:
Hints also of a potentially more blocked February after an unsettled December:
The Japanese JAMSTEC model, on the other hand, is going for a colder than average, drier than average winter for the British Isles, which mostly looks to be driven by the SSTs:
The above factors were weighted and a index of the deviation from this winter was calculated (accounting for the lack of data for SAI and Sea Ice during the 50s and 60s), and used to weight the composites years, with La Nina winters disregarded.
The composite years generated are 1995 (x2), 1964 (x2), 2007, 2005, 2003, 1978, 1988, 1993, 1970, 1983, 1987, 1958 and 1998.
While not perfect (and understandably weaker due to the fact that we're averaging years), the analogue years do pick up some of the key features we're looking for from this November, both at the stratospheric and tropospheric levels:
The main exceptions to this are tropospherically upstream, where it has heights much lower over the US than they actually were, and around the Kara Sea in Northern Russia, where there's again a positive anomaly instead of a negative one. The latter is actually quite significant, as autumn blocking in the nearby Taymyr region is shown to correlate with a negative Arctic Oscillation( http://onlinelibrary.wiley.com/doi/10.1002/joc.3968/abstract ). Given this is linked to a high SAI and low Arctic Sea Ice, the lack of very recent analogues is probably the reason for this failing, and I will be taking that into account.
As for December, my analogues set up a somewhat drier month than might be expected given the current modelling. The low heights over Greenland remain, but with a positive height anomaly over Scandinavia there's at least some scope for drier, frostier interludes between more Atlantic driven spells.
Taking the Temperature and Precipitation data would point towards a month of around average rainfall, with temperatures around 0.5C-1C above average for Scotland, and similar for the CET zone.
Overall, I'd perhaps lean on it being a bit more Atlantic driven, with a bit more of a north south split temperature-wise, but certainly a signal there for a quieter period of weather at some point, which will come as a contrast to the last two Decembers.
As for the stratosphere, the general picture is of a still strong vortex being squeezed from East Asia:
For January, a more interesting month if you're interested in cold weather - signs of somewhat above average heights towards Greenland, with low heights to our east, hinting at the prospect of a more northerly driven regime.
However, there's definitely a lot going on there - firstly, the block has a chance of setting up too far west for us to benefit greatly. There's also some danger from the anomalous heights to the south - while the Labitzke QBO-solar linkage would suggest that the best chance of blocking heights would come from increased solar activity in the next month or so the flipside of this might be a rise in heights to the south, as we saw occur during 2012/13, somewhat hampering what could've been a very cold winter here. The main factor affecting temperature here though is SSWs - when these occured in January the mean Scottish temperature was 0.8C below average, but above average without.
Rainfall from these analogue years is again around average, but with some huge variance. The common factor in most of the wet months is a cold Atlantic, which suggests that perhaps we're more likely to be above average on rainfall this winter, particularly further south.
The composites suggest that any Stratospheric Warming is most likely to result in at least a chunk of the stratospheric Polar Vortex ending up on our side of the NH (and most likely from the analogues a displacement rather than a split) :
However, given some of the specific factors at play this winter I feel a split is on the table more than usual. My own feeling is that we'll come close to a proper split quite early, not quite achieve it but eventually end up with something close to a technical SSW but more likely a displacement, and likely to be a bit of a mess all round until we eventually get the vortex properly cleared out by late February/March.
For what it's worth, here are the February composites:
This comes out as the coldest month for Scotland with a mean of 2.3C, and as you might expect the driest month too, slightly drier than the average. These are the kinds of composites we were looking at for last winter, albeit a bit more toned down, but it does give the impression that blocking to the northwest may become a bit more central Greenland based the deeper into winter we go. On the other hand, the threats of both a west-based NAO and height rises from the south remain. Given all of that, there's definitely enough there to cheer a 'coldie', but it doesn't quite look like one of the classic deep freeze months either.
March could be interesting too though:
2015/16 Winter Forecast
Now we get to the fun bit - the actual forecast. Again, it's worth a health warning that the predictions in this section are pretty speculative.
December is likely to start off in familiar fashion - unsettled, with a familiar train of low pressure systems making their way past. Expect high winds and very heavy rainfall to be a bit less of a factor than the last two, but still quite a wet and windy opening week, with the potential for at least one transient snowfall for Scotland from a backedge cold front. High pressure will make multiple attempts to build in from the south, resulting in generally mild and less cold conditions for the south and southeast, and eventually I'd expect mid latitude heights to develop by around the 10th. The most settled conditions, and potentially the most frosty, will again be in the southeast of England, with low pressure not too far from the north of Scotland, but in between a mix of mild drizzly days and colder, clearer days, but overall temperatures are likely to be generally above average. However, this will see our first attempt at 'proper blocking', as heights build towards Scandinavia and a continental flow develops. I expect this to fail, and for a more cyclonic regime to assert itself by the 20th, but there's a small possibility things click into place by this point and we end up with a more substantial cold spell to end the month. Even without this, though, I'd still expect there to be a reasonable shot at a white Christmas Manchester northwards, with deep low pressure to the northwest combined with a bit more amplification developing upstream giving the potential for some cold zonal setups like we saw for much of last winter, with this type of pattern holding until the end of the year.
Temperature wise, I'd expect a CET of around 4C, with a Scottish temperature mean of 3-3.5C - a cold mid month for England being offset by a milder start and slightly above average finish, with temperatures for Scotland generally highest mid month, particularly further northwest. Rainfall will mostly be average but with big regional variations - quite a bit above for northwest Scotland and Northern Ireland but quite a dry month for the east coast and the southeast in particular. Snowfall is unlikely to feature for southern England (unless the easterly mid month can develop into something more substantial), will fall a few times but unlikely to lie in northern England and could be relatively frequent other than mid-month in Scotland but away 2from altitude it'll be pretty transient, though maybe an outside chance of a more prolonged spell of snow on the ground between Christmas and New Year.
Similar months you might remember: early December a bit like any of the last 3-4 Decembers but with less wind, mid month similar to Decembers 2002 and 2006, late December similar to Christmas 2004, last January.
A tough month to forecast, as ever, but even moreso this year. In the analogues we have some of the mildest Januaries on record, one of the coldest spells in the entire 350 year CET dataset, and a few average to moderately cold months thrown in for good measure too.
The month may well start in similar fashion to December - with a cool unsettled regime giving way to pressure rising from the south. This could be a particularly mild spell, with a Euro High and low pressure to the northwest giving a mild southwesterly regime, with significant rainfall for northern and western parts. Hemispherically, however, the signs of a big change should materialise around the same time, with the vortex being shunted off the pole, and by the latter 3rd of the month a genuinely cold north/northeasterly regime is likely to have set up. This pattern change is likely to dominate the rest of winter, however where the UK lies in relation to the main blocks is hard to pin down, but the prospect of at least one prolonged cold spell is pretty reasonable.
Overall, January is likely to sit around average temperature-wise, though perhaps a little above depending on just how long and mild the mild spell is (if it does occur at all of course) with the Scottish temperature likely to come in around 2.5C, the CET maybe more like 4C. Similar to December, a northwest/southeast precipitation split could be the main feature, with the first snow of winter potentially not until mid-late January for central/southern England. However, when it does arrive it does at least give the potential for some more significant falls, with slow moving troughs in the vicinity once cold air gets embedded.
Similar months: first half of month similar to very mild mid January 2005, second half a slightly toned down second half of December 2009.
As the forecast goes on it gets a bit sketchier, however hemispherically again there are decent reasons to believe that February gives the greatest potential for a more blocked pattern to become established.
Starting off from cold late January, we are likely to be very much on the edge of the cold for most of the month. Troughing is likely to cycle between Scandinavia and the northwest Atlantic, giving the potential for some big snowfalls inland, but also for quite a wet month, particularly for Wales and southwest England, with lows stuck out to the west for days at a time. By month's end high pressure is likely to made a reappearance in some form, which depending on the positioning could either herald an early spring or the first proper easterly of the winter, but with the potential for a cold March the former would likely be premature and short lived...
On balance, this is likely to be the coldest month of the winter, with Scottish temperatures of around 2.2C and a CET around 3.1C (though potentially significantly lower if the early spring doesn't materialise). Rainfall, above average in the south but below further north, with snow totals generally largest down the spine of the country from frontal events, but with the threat potentially shifting more towards northeastern areas at times.
Similar months: February 1978 (not that I'd remember that), February 2010, if we're particularly unlucky then February 2014.
I hope you've enjoyed the forecast, let me know if there's anything that needs clarifying, or if any of the images that seem like they should be there aren't.