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This forecast will attempt to predict trends in the weather patterns for December, January and February.
I am using a host of different variables & pattern matching to create this forecast. I have not used any seasonal forecast model data (NWP) to compile this forecast (EG. CFS, EUROSIP, UKMO etc)
This is going to be quite technical, if you want to skip the detail, please CLICK HERE to the actual forecast at the bottom.
UK Winter Forecast 2014/15
Last winter was dominated by a very strong west QBO (section 1.1 below) which aided development of a powerful stratospheric vortex (see section 1 below). In fact it was observed to be one of the strongest and coldest we have seen for many years. It was therefore relatively easy to suggest a largely snow-less start to winter 13/14 with a small chance of something colder developing later in Jan-Feb depending on how much disruption to the vortex was seen in mid-late winter (the extremes that we saw were not so easy to predict)! As it turned out, there was enormous pressure put on the vortex through wave breaking and subsequent warming, bringing us very close to a SSW (section 1), which during a winter that featured such a cold/strong vortex, was quite remarkable to observe. But alas, we became entrenched in a very stubborn pattern with a strong ridge over NW USA forcing cold air into central and eastern parts of north america, amplifying the jet stream and throwing an army of deep low pressure systems across the Atlantic to the UK. The vortex disruption later in winter certainly did increase northern blocking, but the blocks were not favourable and essentially all they managed to do was lock that cold air in north america and lock the UK in our dismal winter. The vortex was also able to recover and regroup fairly rapidly following any disruption and those strong westerly winds dominating the middle stratosphere continued to flow, largely unabated.
This is demonstrated on the image below, showing the upper air pattern (anomaly) for Dec-Feb 2013-14.
This serves as a good example as to why I don’t believe we will see a repeat of last winter. You will see in the information and forecast below that the pattern looks very different, particularly as winter progresses. Before we get into that, here’s an example of why northern blocking, particularly when centred towards s Greenland, offers the best prospects of sustained UK cold weather, using an example from our infamous December 2010 cold spell.
The aim of this forecast is to lay down the various drivers that i believe will be most pertinent to the coming winter period, briefly explain the drivers and suggest how they might influence the winter period. Then the forecast follows at the end. If you want to avoid the technical discussion, just scroll down to the forecast.
After some feedback from last winter’s forecast, at the end of each section, I have included a “score”. This is like a penalty shootout between Winter and Anti Winter. Mainly just for fun but also so that less technical readers can, at a glance, see what potential implications on UK winter might be.
Like last year, I have not used any seasonal forecast model data (NWP) to compile this forecast (EG. CFS, EUROSIP, UKMO, JAMSTEC etc).
I am looking for trends and the general winter picture. No specifics are possible. In terms of snowfall I will attempt to predict the most likely periods of winter when colder than average conditions may prevail and in turn an increased risk of snowfall. This is complex and quite experimental.
1. The stratosphere (polar vortex), QBO, solar activity & ozone
Before we get into this, I will briefly explain the role of the stratosphere on the troposphere below it. Each winter the stratosphere cools significantly. The difference in temperatures between the Arctic and warmer latitudes further south results in the formation of a strong area of low pressure, called the stratospheric polar vortex. Below this is the tropospheric polar vortex (the area of low pressure located around the north pole that we see on weather charts). The colder the stratosphere is, the tighter/stronger the stratospheric vortex becomes and consequently, the tropospheric vortex too. The strength/position of the tropospheric vortex influences the AO (Arctic Oscillation), which is a measure of pressure between the north pole and the Azores, as shown in the image below, courtesy of NASA.
A stronger vortex can result in a positive Arctic Oscillation which, for Europe, equates to a less cold winter. The opposite is the case with a weaker vortex.
Sometimes, during winter, a strong vortex is put under pressure, warming it and displacing cold air from the pole down to the mid latitudes. Rossby/planetary waves circumnavigate the globe and during winter, when a powerful large wave encounters a mountain range (eg. Himalayas, Rockies, Andes), if the wave is large enough, some energy is deflected poleward (mountain torque). It needs to be a sizeable deflection to achieve this, but these waves can penetrate into the stratosphere, creating a warming disruption to the otherwise usually cold and stable wintertime stratospheric environment (think of the sea-shore where waves break all the time but only the strongest will push more inland resulting in coastal erosion). There are two main types of disruption to the vortex via this process; a displaced vortex, where wave breaking and consequent warming moves the core of the vortex away from the pole; or a split vortex where the vortex is put under even more pressure and is split in two. These events are often referred to as sudden stratospheric warmings (SSW) where the zonal winds at 60N/10HPA are reversed from westerly to easterly. In both cases, the warming and movement of the vortex, pushes cold air into the middle latitudes as higher pressure builds over more northern latitudes. Generally speaking the number of the wave (1,2,3) refers to the number of waves at that time. Wave 1 usually displaces, and strong wave 2 can cause splits. Following a displacement, if wave activity subsides, it is common for the vortex to fairly rapidly regroup and cool. Following a split, if wave activity wanes, it can take much longer for the vortex to recover and regroup. So a split vortex is the ideal scenario we are looking for and historically the UK has benefited more in terms of prolonged cold weather from splits rather than displaces. Split vortexes can also lead to faster response at the surface.
Since mid October, two main features, low pressure over the north pacific and high pressure over eastern & northern Europe and Eurasia have been the main drivers for this enhanced wave driving. These patterns are signatures of both an El Nino atmosphere and tropospheric response to high October snow cover (see ENSO and snow cover sections for more).
1.1 The QBO (Quasi-biennial oscillation)
The QBO is a measure of wind flow across the equator high up in the stratosphere (measured at 30mb). There are two phases of the QBO, east and west – referring to the direction of those winds. These cycles or phases last roughly 18 months or so. The QBO has a significant effect on the state of the polar stratosphere during wintertime, and is therefore of much interest for the winter forecast. In late Spring/early Summer, we moved out of the west QBO that dominated winter 2013/14 as easterly winds started to dominate the equatorial stratosphere. Since May the East QBO has continued to strengthen and Octobers mean value came in at -23.86 (m/s), indicative of a strong east QBO firmly in charge. I’ve seen some talk of the QBO having already peaked and possibly entering a west phase by January but this makes no sense at all given the current strength and the usual length of time east phases last. We may see the strength remain fairly consistent over the winter or it may strengthen a little more, but a steady and strong east QBO for the remainder of winter is very likely/or even pretty much guaranteed. To demonstrate how much influence the QBO has on stratospheric temperatures/strength of vortex, below are some composites. The top image is west QBO years, the bottom image east QBO.
The two images say it all really and depict nicely how E QBO years usually feature warmer temperatures over the polar stratosphere and colder than usual temperatures across the tropical stratosphere, indicative of an enhanced BDC (see ozone section for more on this).
Possible implications of E QBO for UK winter 2014/15?
In isolation, this makes a winter featuring a weakened polar vortex and a negative AO more likely during the upcoming winter and therefore it’s a goal to winter. (Winter 1 Anti winter 0)
1.2 Solar activity
We currently have low (ish) solar output. Influences of solar activity on our climate are still not fully understood but in combination with our other background signals (mainly the e QBO), low solar output will increase our chances of seeing a SSW this winter. Labitzke et al have published several papers alluding to the 10.7cm solar flux’s relationship with the stratosphere and northern hemisphere winters and specifically have drawn attention to an increased risk of SSW if the 10.7cm flux is at or below 110 units during an E QBO winter. The image below (Labitzke & Kunze) shows this relationship between solar flux, the qbo and SSW’s. In the sample of 65 years (1942-2007), only 5 SSW’s occurred with 10.7cm solar flux higher than 110 units, whilst there were 10 SSW’s recorded with solar flux at or below 110. So it’s not out of the question, but at or below 110 is preferable (currently and recently has been hovering around the 160 mark).
We saw quite a large spike in activity in early – mid Autumn, which has since declined and solar activity is expected to continue to bump along low (ish) levels during winter. There will be further spikes, albeit, not as large as recently seen. The 10.7cm flux will need monitoring throughout winter.
Implications of solar activity on UK winter 14/15?
A sustained increase in solar activity does have the potential to scupper cold prospects for the UK. It is not forecast to do so, but all the same I have to give anti winter a tentative, cautious goal here. (Winter 1 Anti winter 1)
1.3 Ozone/BDC (Brewer-Dobson Circulation)
Levels of ozone concentrations are also known to have an influence on stratospheric temperatures. As mentioned in the QBO composites above, during east phases, the tropical stratosphere is cooler than average and the polar stratosphere warmer, because higher levels of ozone release heat into the surrounding air, and less ozone will result in less heat release/flux. Based on this distribution of temperatures, we can assume that E QBO winters see higher concentrations of ozone over the pole and smaller concentrations over the tropics. The BDC refers to the transport of ozone from tropics to the pole during Autumn into the winter. Keeping an eye on stratospheric temperatures during Autumn can give an idea as to how active the BDC is. The image below is the temperature of the tropical stratosphere.
You can see that during the Autumn so far temperatures have been colder than average in the tropical stratosphere. At times it has been close to the coldest temperatures in the 1979-2013 mean. That’s a great sign and indicative of an enhanced BDC. This is confirmed by looking at total global ozone levels, below:
Implications of this for the uk in winter 14/15?
The more active BDC should mean there will be higher concentrations of heat releasing ozone over the pole during winter, weakening the thermal gradient across the hemisphere aiding in a weakened polar vortex.
(Winter 2 Anti winter 1).
2. ENSO (El Nino Southern Oscillation)
Enso refers to the warm and cold phases of the waters along the equatorial Pacific. Warm is El Nino, cold phases La Nina.
Whilst climate influences of the warm and cold phases of ENSO have more pronounced implications on weather in the tropical regions, they have been shown to influence temperatures and precipitation in the northern hemisphere, especially during winter. Furthermore, regions closer to the equatorial pacific are seen to experience a tropospheric response to different phases of ENSO, whilst in Eurasia, the influences take a more stratospheric pathway. That’s not to say a tropospheric led influence does not exist for parts of Europe, indeed, a strong El Nino, for example will amplify the STJ with downstream implications in the form of unsettled and mild (ish) weather for western + northern Europe for winter as a whole, albeit, January’s in the UK can often see cold spells during El Nino.
In terms of that stratospheric pathway, put simply, an enhanced risk of sudden stratospheric warming is present with either phase of ENSO (El Nino or La Nina). During El Nino phases, the polar vortex is observed to be weaker as a result of enhanced poleward planetary wave breaking and increased ozone transport, resulting in warmer stratospheric temperatures, particularly by mid-late winter.
This idea of tropospheric vs stratospheric pathways is evidenced by A H Butler et al in the following image. The middle (top) image shows the sum of el Nino+la Nina winters where a SSW was observed and the subsequent surface temperature. Below it (middle bottom), is the geopotential heights at 500mb – notable features being an extensive cold pool over Europe, high pressure centred towards southern Greenland, and extensive Atlantic trough stretching into western Europe, suppressing Azores high pressure. On the top right, we see El Nino and La Nina winters where no SSW was observed, so essentially just the tropospheric response to ENSO. Much of Europe is milder than average and looking at the bottom right image, its clear to see why. Northern blocking is evident but centred further west towards Canada (west based negative north Atlantic oscillation), relative absence of Atlantic trough, allowing Azores high pressure to ridge north.
So our hopes of turning the largest swathe of Europe colder than average are pinned on us seeing a SSW. Which is usually the case in any winter really, but nice to see evidenced here.
Cautionary note that despite either ENSO phase seeing an enhanced chance of SSW, El Nino winters seem to feature more in the way of wave 1 activity compared to La Nina winters where wave 2 tends to be more dominant. As mentioned above, it’s strong wave 2 activity (allied with wave 1) that offers the best prospects for a prolonged cold spell in the UK. This Autumn has seen plenty of wave 1 and i hope into December to see wave 2 pick up vigorously.
ENSO impacts on UK winter 14/15?
How will El Nino develop into the winter? My best guess is for a continued slow strengthening but remaining in the weak-moderate bracket. The +PDO is likely to work with El Nino and warmer water in NE Pacific as also along the equator are expected for the remainder/bulk of winter. Despite the cautionary note, a weak-moderate El Nino combined with other factors seems to lead to an increased risk of major stratospheric disruption (SSW) and cold outbreaks in the mid latitudes. (Winter 3 Anti Winter 1)
2.1 SST (Sea surface temperature)
The distribution of sea temperatures in late autumn can give an insight into how winter might evolve, particularly in terms of the possible state of the AO/NAO.
Reanalysis of previous years with similar SST distribution paints a familiar story into the winter period. With a strong signal for high pressure stretching from southern Greenland to Iceland, a southerly displaced jet stream and significant trough in the Atlantic stretching into Europe.
Possible implications on UK 14/15 winter?
The current and recent SST patterns suggest an increased likelihood of a -AO/-NAO during winter. (Winter 4 Anti winter 1)
2.2 PDO (Pacific Decadal Oscillation)
The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. The phases of the PDO are known to influence global sea level pressure patterns, amongst others. Historically, the UK’s coldest winters have coincided with low solar activity and cool phase of the PDO. There are exceptions, but the general theme is consistent. The phases last roughly 20-30 years. Despite the main phases of the PDO, there are still cold PDO years with warm phases and vice versa. We are still currently in a cool phase, but since January the PDO has flipped positive and is likely to now remain that way for the remainder of winter, perhaps weakening somewhat. As mentioned earlier, +PDO and El Nino (both being warm water phenomena) can work with each other and this raises confidence that both El Nino & +PDO are here to stay for (the bulk of) winter 14/15.
Interestingly, my analogues seem to reveal very similar results in terms of broad upper patterns using either + or – PDO years. I would therefore conclude that this winter’s +PDO is likely not a major player for the UK. Though it should be noted that El Nino combined with +PDO has a propensity to favour more in the way of wave 1 activity over wave 2. Something to bear in mind I guess.
2.3 Tropical forcing (MJO)
The Madden-Julian Oscillation (MJO) is the eastward moving mean centre of cloud and convection/rainfall in the tropics.
Tropical activity this Autumn has been quite low, and for much of October and into early November has been weak and centred in the western hemisphere/Africa, tending to fizzle out as it moves into the Indian ocean. However, recently, we have seen tropical convection become stronger across the Indian ocean, moving east into the Pacific. This should occur more often into winter. On this occasion, there is some doubt as to how much eastward extent it makes across the Pacific Ocean before fizzling out. I’ve got a feeling we may need to wait until the next wave to see something stronger.
Interestingly, it has been observed (Garfinkel et al) that certain phases of the MJO seem to correlate with SSW events a number of days preceding such events, up to one month/30 days. There is currently some stratospheric warming in current extended modelling and the MJO moving into phase 2-3. These phases of the MJO, (tropical convection centred in the Indian ocean) have been identified as the “preferred” phases in the 25-36 day time frame leading up to a SSW. Bearing this in mind & combined with other background signals, the current stratospheric warming in extended modelling, you could argue that chances for a SSW in the December 20th-30th time frame are increased somewhat and certainly worth keeping an eye on. I will discuss this further in the December portion of the forecast below. Incidentally, for the time frame of 13-24 days before a SSW, Garfinkel et al identified phases 4, 6 & 7 with phase 6 correlating highest. In the 1-12 day timeframe prior to a SSW, phases 7 & 8 are preferred with phase 7 correlating highest. This is shown on the image below:
It will be interesting to monitor this over the coming weeks.
3. Angular momentum
This is, in essence, a measure of the net balance of winds across the planet. For some time now, angular momentum has hovered around neutral with a recent dip mid November. Typical El Nino atmospheric response is usually to add westerly momentum but it may not be until later in winter that we see this occur. We have pools of colder than average waters away from the equator in both the Pacific and Atlantic, which add easterly momentum, effectively holding back full atmospheric coupling of those warmer waters. I’d like to see increase in angular momentum into late December and January as this is likely to result in increased wave breaking. Reanalysis of similar years where global winds did not embrace El Nino (or at least took an age to do so) yield similar results to other teleconnections, however, a propensity for the core centre of heights to be west of Greenland (west based -NAO). That’s not the best pattern for the uk and is something to keep an eye on, albeit its not disastrous.
Possible impacts on UK winter 14/15?
For reasons above, this disconnect between El Nino and global winds highlights a small risk for the uk this winter. (Winter 4 Anti winter 2)
4. Snow cover
Judah Cohen et al’s work on snow cover/extent is now well documented/advertised. To summarise, he has linked the rate of Eurasian snow cover during October to upper and surface weather patterns during winter, particularly January. During Octobers where snow cover rate is higher than average, distinct feedback mechanisms are observed over the course of the following 3 months.
High snow cover increases diabatic cooling aiding formation of a strong Siberian high pressure cell, which fluxes energy poleward into lower stratosphere. This energy flux (see stratosphere and waves section for more), disrupts the stable stratosphere, causing strong stratospheric warming which then feeds back to the troposphere in the form of high pressure at northern latitudes, pushing the jet stream south. The high pressure/warmer air at higher latitudes effectively squeezes the cold air locked over the pole south towards mid latitudes. Resulting in a typical -AO pattern. The following infographic explains the processes and feedback mechanisms involved (Judah Cohen 2014):
Last year whilst snow cover was high, the actual daily rate in October was not very high, which led me to not give snow cover any weighting in my forecast (also, there were other signals indicating a +NAO/AO during winter 13/14 which I chose to go with).
October 2014 has been quite different to last year. Eurasian snow cover really exploded during October as depicted (unscientifically) in the following two images:
Since October, the tropospheric response to October snow cover has been quite consistent with findings documented by Cohen et al. Reanalysis of high snow extent October years reveals high pressure at northern latitudes to be a distinct theme and January in particular showing a strong -AO, as shown below:
Possible implications on Uk winter 2014/15?
Another strong signal for blocking centred close to Greenland with a deep Euro trough. Negative AO. That’s verging towards a west based -NAO which again needs cautioning but it’s still a great pattern.
Conclusion
Winter 5 Anti Winter 2 (final score). It is clear reading the above that the biggest potential obstacles to cold weather in the UK this winter are a possible west based -NAO and spikes in solar activity. It is also clear that a -AO/-NAO is largely dependant on significant warming in the stratosphere (SSW) and subsequent weakening of the polar vortex. At the same time, signals for such an event appear to be strong, as do the signals pointing towards blocking centred around Greenland, which I have demonstrated as being amongst the best synoptic patterns in advecting sustained cold weather to UK shores.
OPI (October Pattern Index)
This new index is based on October’s atmospheric patterns and the creators claim it can predict the state of the AO during winter with a rather impressive 90% accuracy. I have not given the OPI any weighting in my winter forecast this year because I haven’t had the chance to look into it in any great detail. Most of the papers are written in Italian which doesnt help either. Reading this forecast it’s clear that I use several factors in determining the possible state of the AO during winter, I see these as tried and tested methods that i have some degree of confidence in. Long range forecasting is very complicated and you can never have too few tools at your disposal. Perhaps by next year i will incorporate the OPI into my forecast.
Incidentally, this years OPI is negative (indicative of negative AO during winter) and is very much in line with the rest of my forecast. I want to use this season to “test” the OPI and also do some reading up on it.
Stratosphere forecast Dec, Jan & Feb
Before we get onto the forecast nearer the surface this winter, i want to explain how i expect the stratosphere to behave over the next 3 months or so, in terms of general pattern & temperatures.
*Please note this forecast is for the stratosphere only (30mb mid stratosphere)*
December
December sees a strong positive anomaly over the pole, indicative of warming disruption in this area.
January
January sees a similar trend to December with strong positive anomaly indicative of continued stress put on the polar vortex.
February
February has been tricky to decide. Certain analogues suggested the vortex becoming stronger/more dominant during February. However, i have opted for continuation of the Dec/Jan theme. I’ll look at this again in mid January.
My Forecast
Winter 2014/15 UK. Bitterly cold spells in parts of Northern and Central Europe but will the UK tap in?
I have made my own temperature charts this year. A blend of analogues (-QBO, weak-moderate El Nino, low(ish) solar flux, +AMO, high october SAI) and gut form the basis of this forecast.
December 2014
The problem is trying to figure out when we see the NAO flip negative. We can rule out the first 15 days of December, albeit the odd cold shot (northerly incursion) is possible. As already identified, the NAO flipping negative will be as a result of significant stresses put on the polar vortex via wave activity and subsequent warming and ultimately a SSW towards the end of December or into January. 10 day wave 2 forecast is still quite weak. The vortex is forecast to strengthen into early December. Most SSW’s are preceded by a period of vortex cooling/intensification. I’d suggest the earliest point to see a SSW would be Christmas to New Year, with subsequent cold into January. That leaves December a mixed bag. The first half likely dominated by intermittent periods of high pressure further south, with frost/fog at times and more unsettled further north with hill snow at times. A small chance of fleeting cold shots in the north bringing some snow to lower levels is not ruled out during early December, but remains an outside chance. From mid month and for the second half of December, high pressure is likely to build over northern latitudes. Depending how much residual energy remains in the vicinity of Greenland, will have a large bearing on how December 20th – 31st pans out. I will put the chance of seeing a widespread cold/snowy spell at 20-30%. Should this occur, it will, of course, have a bearing on the December temperature forecast, but I’m sticking with average to slightly below for December as a whole.
January 2015
Looks like a cold month overall. If no SSW end of December, then early January looks very likely. High pressure around Greenland should keep the Atlantic at bay, deflecting the jet stream south. The Uk should tap into a deep cold pool across Europe on a few occasions. Northern and eastern parts of the UK look coldest, being closer to the source of cold air. As mentioned previously, a small risk as January progresses that high pressure over Greenland may migrate further west resulting in a west based -NAO, leaving the UK on the ‘wrong side of cold’. But should this occur, there is still likely to be cold weather in the UK beforehand.
February 2015
Really tricky to call. Low confidence but high pressure looks like becoming more of a dominant force, possibly after a bit of a colder start to the month. Incidentally, eastern USA looks really cold but as you can see, apart from far northern Europe and perhaps n Scotland, Portugal and west Spain, the vast majority of Europe sees temperatures ranging average to slightly above average. I will look again at February, depending how early January pans out, and may need to update the February forecast.
So its all to play for. There are the usual obstacles to UK cold weather that are often present like geographical position (tiny island on the edge of a huge ocean) etc and as always the UK needs everything to fall into place to benefit from cold and snowy weather. But that’s the UK for you, and arguably a reason why we are so obsessed with snow in the UK! I’m fairly confident that many of us will see at least some snow this winter and the vast majority will see below average temperatures at some point.
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This article originally appeared at LONDON & SE WEATHER blog.