Similar Thinking, Another Perspective: Winter 2016-17 Forecast From Stuart Markham (chorleyweather.com)

Written by on November 30, 2016 in Rest of Europe, United Kingdom & Ireland, Winter 2016/17 with 0 Comments

UK Winter Weather Forecast 2016/17

Stuart Markham

Previous winters since 2012/13 have tended to be mild and somewhat unsettled as the Atlantic flow has been the dominant force during these winters. Certain variables where noticeably going to be a factor in those winters, take last year for example we had a very strong El Nino and a strong westerly phase of the QBO, both of which combined to bring a rampant polar vortex that sustained the westerly flow through much of last winter.

This winter we find ourselves with a very different set of starting points, which we will go into below, hopefully not being too technical in the process and making the forecast both enjoyable and understandable. Before we crack on we must add that long range forecasting isn’t an exact science so specifics beyond 7 days are simply not possible to determine. This forecast or a better word maybe ‘prediction’ just looks at current and historical trends, current data such as sea surface temperatures and many other variables that we mix together and see what we feel is the best path going forward, working alongside our own historical research. Long range forecasts can and will go wrong and should be used as guidance only.

Enso:

So we might as well start with the ‘goings on’ across the equatorial Pacific or ENSO region as we like to call it. Last year we saw a super El Nino with sea surface temperature values in excess of 2.3 above average in a tri-monthly period, with only one other year 1997, being on a par. Normally after a strong El Nino we see La Nina develop but that hasn’t looked the case for much of the year. That being said we are now perhaps in very weak La Nina territory and are probably going to remain weak through the winter period. Our research suggests that weak La Nina’s tend show an averaged view when it comes to temperatures across the UK but with a hint of colder about them, so certainly a good sign for cold fans.

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Solar:

Solar activity is declining towards solar minimum but we are not in solar minimum yet. This is more likely to come late next year or into 2018. That being said we have had more spotless days this year than in any other year going back to 2010 and activity remains very low. Historically however winters that occur as we head towards solar minimum tend to see positive NAO’s (North Atlantic Oscillation) more often than negative and this may be worth bearing in mind when we do the summary further below. Positive NAO’s are generally indicators of an active westerly flow across the N Atlantic and as such, the UK usually sees plenty of low pressure systems sweeping in as a result. However, for now the NAO looks like trending negative in the main or at the worst, more towards neutral. This will fluctuate at times though so periods of a positive NAO are certainly not ruled out

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N Atlantic SST (Sea Surface Temperatures).

During the last few winters we have seen very cold temperatures across this region, which perhaps was another reason for the strong westerly flow during these winters. The ‘cold blob’ as we called it was creating quite a large temperature contrast in this region, which could or probably did help the jet stream strengthen. This year there is no worries in that respect as temperatures look much warmer this time around and there does not seem to be a threat here. If anything the current SST may be more helpful to a blocking pattern but the connection is very thin.

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Polar Vortex:

Such a hot topic considering this feature is all about very cold conditions. Last winter saw perhaps the strongest vortex we’ve known since the vortex became part of our forecasts and maybe one of the strongest to be witnessed in quite some time. As mentioned this brought about an extremely dominant Atlantic driven flow. When the vortex is strong the winds surrounding the vortex become strong and of course westerly, this then enhances the westerlies across the N Atlantic. This year we have seen perhaps one of the weakest polar vortices for around the last 30-40 years. As a result the Atlantic flow has been very quiet so far this Autumn and indications are that there is no significant strengthening in the pipeline. That said don’t rule this out because the vortex has an amazing power of recovery. Current forecasts up to 240 hours show a warming, which will further displace the vortex across towards Siberia but the forecast thereafter becomes very unclear. The next few factors below may help with this however.

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Snow Advance:

October snow advance across Siberia has been a talking point for a good few years now. It is thought that large snow cover in the region, particularly below 60 degrees, helps strengthen the Siberian high, which then becomes influential later in winter in breaking the polar vortex down. However, last year saw a good snow advance which did lead to a strong Siberian high, however because the polar vortex was so strong, the high never got a look in. This year has seen the highest recorded snow cover since 1976 and with the vortex already weak, perhaps this time the Siberian high will be a factor. However there is one joker in the pack, which we’ll explain next.

QBO (Quasi Biennial Oscillation)

In brief these are stratospheric winds above the equator that run westerly then flip easterly, roughly interchanging every 12 to 16 months with the full cycle generally lasting 28/29 months. A westerly phase generally helps to maintain or even strengthen the polar vortex in winter and an easterly phase is known to help weaken the vortex. This year or this winter was supposed to be in an easterly phase but something rather unprecedented has occurred. The westerlies failed to flip and have now strengthened again, which has never been recorded ever. This has possibly been a cause of the very strong El Nino but ultimately the reason is still relatively unclear. This leaves our forecast in a bit of a conundrum as this could help strengthen the vortex, whereas most other signals suggest a weakening. That being said the vortex is weak and no strengthening is forecast beyond climatology so perhaps the QBO needs help in that respect from other sources, which are simply not there such as the strong El Nino like last year. It is our belief that the QBO alone cannot determine a winter by itself.

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Arctic Sea Ice:

Very low ice or the refreeze of sea ice since the summer has been extremely slow, in fact it’s the slowest ever recorded through Autumn which is a result of anomalous warmth across the Arctic region. However the region of interest which is also the regions with the lowest refreeze is the Barents-Kara seas. Very low ice cover in these regions are known precursors for a negative Arctic Oscillation through winter, as 2012/13 saw. A negative AO basically allows colder air to flood to lower latitudes such as Europe for example. This doesn’t guarantee cold to the UK but certainly helps.

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Long Range Models:

There are many to choose from these days and some perhaps are not so good but the most trusted sources suggest a front loaded winter. By that we mean colder weather is perhaps more likely through the first half of winter rather than the second, with the JMA, ECMWF, MET OFFICE for examples perhaps agreeing on this. The models certainly suggest a very different winter than last year and most effectively show a weakened westerly flow.

Historical Data:

We’ve researched back to 1950 with as much data and variables that we possibly could and no year stands out singularly and it’s very difficult to match single years anyway.  We like the combination of winters 64/65 & 66/67 as we feel this best represents our current winter variables. The resulting map shows a large amount of northern blocking, which is being touted by many sources for the coming winter. This pattern shows a fairly blocked pattern which will lead to a weakened vortex for sure and if there is sufficient cold across the continent, we may be able to tap into that cold. This however is far from straight forward.

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Summary:

A very different winter is expected this year compared to recent winters. Firstly the signal for a much drier pattern is strong, particularly through the early part of winter but the drier theme should follow through most of winter. This doesn’t mean we won’t see any low pressure systems flowing through, there’ll just be less of them with longer drier spells in between. The current state of the vortex would suggest a risk of cold weather developing through December, most likely during the second half, with a milder SW’ly regime perhaps developing beforehand. This could lead to wintry conditions developing through Christmas week, perhaps lasting through to the New Year but the timing of this is unclear as yet.

For the rest of winter there are many question marks. Computer models suggest a return to the normal westerly flow, although not severely so but this could attributed to the westerly QBO. However there are many cold winters that have had westerly QBO’s and it’s our opinion that the QBO would have played its hand already, which clearly hasn’t brought any effects on the Polar Vortex. Instead it is our belief that the strong snow advance coupled with the low sea ice refreeze will play a significant role this winter, especially as there is no other strong indication of any other variables perhaps interfering with this signal.

So with this in mind we think the rest of winter through January and February will remain no worse than average, with the potential for colder than average. By this we mean there is likely to be more colder periods of weather, especially relative to what we have seen in recent winters, however, as we think the Atlantic jet will be largely amplified this year (wave like) it won’t take much to be on the wrong side and experience mild conditions, although remaining in a generally dry/drier pattern. If however we get a sustained period of blocking which our historical data suggests, then a prolonged period of cold isn’t ruled out but it would be foolish to suggest such a pattern will develop, however, what we can say is the risk this year is far higher than in recent winters and we have a greater chance of seeing more seasonal and perhaps wintry conditions.

Here are our final temperature and precipitation thoughts.

Temperatures: Average to slightly below average overall.

Precipitation: Average to below average.

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