Well here we go again, winter 2021/22 is here, and many of you are eager to know how this winter may shape up to be.
The forecast headlines are:
Overall, temperatures are forecast to be colder than average expecting the coldest winter in UK since 2012/13
January expected to be the coldest month, however, coldest weather could happen later in winter, after mid month, possibly SSW induced and extend into February, potential for notably low temperatures with significant wintry hazards
50-60% likely to be a wetter than average Winter drier in the east
December forecast to be the wettest month
First, I will examine the main global climate drivers of winter 2021/22 of which when are in different amplitudes/phases can give a broad idea of how winter will turn out.
ENSO (El Nino Southern Oscillation)
This autumn we have seen La Nina conditions redevelop this is associated with cooler than normal sea surface temperatures (SSTs) in central and eastern Pacific. Nina conditions are expected to last through entirety of winter 2021/22. This is second winter in row with La Nina conditions we saw the ENSO go neutral in spring and this lasted through summer.
During La Nina, the easterly trade winds are stronger this means significant upwelling of deeper and cooler subsurface waters, to ocean surface, therefore, we see central and eastern tropical Pacific Ocean become colder than normal.
What we see is the strength of La Nina/El Nino affects the position and strength of jet stream the atmospheric changes in North Pacific in turn influences the weather in North America, and the altered jet stream continues onwards across North Atlantic and Europe in like a connected chain. A ripple thousands of miles away in North Pacific can have a large impact on the type of weather we see. During La Nina we often see an Aleutian ridge a big area of high pressure off western North America in North Pacific, this often causes North Pacific jet stream to bend sharply S/SE across North America. Often La Nina winters are colder and snowier in Canada.
The wavy jet continues as it exits east coast of US across North Atlantic and to Europe, and notably this meridional theme is more frequent with weak La Nina conditions, and coldest anomalies are in eastern Pacific we often see Atlantic ridging a blocking area of high pressure to west of British Isles. The mobile westerlies are gone and this encourages colder northerly or easterly winds. Interestingly we begin December 2021 with weak La Nina conditions and also the coldest anomalies are increasingly focused more in east. This means chance of cold snaps in early winter are rather high in Europe.
A strong La Nina favours a strong jet stream this is associated with +NAO (North Atlantic Oscillation), winters which are dominated by this are often very mild and unsettled winters. Indeed in last decade we have seen many of them. It is advisable to never take a broad brush approach with forecasting every winter is unique, indeed in 2010/11 winter which UK experienced its coldest December since 1890 the La Nina was strong although coldest anomalies were in eastern Pacific, which could have been crucial.
The 2010/11 winter although burnt out very quickly, which likely was due to strong nature of La Nina, but again highlights regardless of strength of La Nina we tend to see increased chance of cold weather patterns earlier in winter. This winter we could see further strengthening but likely no more than moderate. As such the second half of winter could have a higher chance of cold spells than the traditional La Nina which coldest anomalies are fairly evenly distributed west to east, and the further we head into winter the unsettled westerlies gather in strength.
In summary I find ENSO encouraging and to be highly influential this winter but uncertainty lays is how much strengthening of La Nina we see over winter, La Nina becoming strong while unlikely would increase probability of a mild and wet late winter.
The Quasi Biennial Oscillation (QBO)
This sounds complicated doesn't it? Essentially in a very simple way it refers to winds blowing above equator every 14 months or so, these winds can completely change direction from westerly to easterly or vice versa. This winter we will be in easterly phase of QBO and again the last time it was during 2017/18 winter. Notice the links which are being made with that winter? Which was also a La Nina winter. During easterly phase of QBO we tend to see increased risk of disruption to polar vortex, which slows down jet stream increasing risk of cold weather outbreaks. We are not only seeing easterly but a strong one, while sample size is not huge since 1950s we have never had a winter which was mild with deeply descended easterlies. Thanks Marco! Overall, this year QBO is particularly favourable to a cold winter and to a higher chance of polar vortex disruption, and I anticipate will be highly influential.
Stratosphere
Now onto the stratospheric polar vortex (SPV) which also feeds into previously discussed QBO, very important in determining winter prospects, this is is a huge area of very cold, low pressure which forms in autumn with return of polar night, it spins anticlockwise at North Pole in December-January it reaches peak strength.
The stratospheric circulation is higher in atmosphere above the troposphere, the troposphere is where the weather happens, what we see with stronger stratosphere polar vortex (SPV) is like paint dripping down a wall the enhanced westerlies high in atmosphere propagate down into troposphere and we see a strong tropospheric polar vortex (TPV). This, therefore, accelerates the speed of the jet stream a strong jet stream is likely to bring lots of low pressure, wet and windy weather at our latitude.
So knowing the state of polar vortex is very useful tool unfortunately predictability is rather low beyond week 2, hence, occasionally seasonal models can go epically wrong, disruption to these upper westerlies can totally radically change weather patterns across Northern Hemisphere to be more conducive to wintery outbreaks. This was exhibited in February 2021 and perhaps more famously in February 2018 which were a result of Sudden Stratospheric Warning (SSW) and featured a split - these serve like a sharp pin popping a balloon, significant cold is unleashed from higher latitudes but not everywhere can see this extreme cold.
This winter we begin with a stronger than average SPV, this is not positive for cold weather patterns in mid latitudes, if this is to continue a wetter and milder winter would be favoured with a ++ Arctic Oscillation (AO) trapping cold in North Pole, however, what we've seen so far these strong westerlies have failed to really descend and influence our weather patterns. You will have observed how dry November has been.
In recent days we've seen a sign of a disturbance in the force with Dr Cohen well on the case, the signalling of Ural Blocking was also seen last winter this helped result in SSW in January 2021, and weeks later in February we experienced some very cold easterlies albeit not as dramatic as 3 years earlier.
The signalling for this atmospheric "road block" earlier in mid December is interesting, as this could weaken the polar vortex and increase risk of a significant cold spell in January the coldest month of winter. The strong easterly phase of QBO this winter in tandem with La Nina also enhances risk of disruption to polar vortex and cold weather patterns in Europe, therefore, I believe the probability of an "attack" is quite high this winter.
Solar Cycle
This is a one which the science does not agree upon, nonetheless I am an observer and advocate. I noticed the increased northern blocking patterns and wavy jet stream after 2008 solar minimum, and again following late 2019 solar minimum our weather patterns have become more 'unstable' oscillating between v wet and dry. We are no longer in solar minimum but it is believed the ascending part is where we tend to see lag effects, interestingly this December will be 2 years since solar minimum. December 2010 was also 2 years following 2008 solar minimum and activity is very similar, it might be a tenuous link but an interesting one.
Northern Hemisphere snow cover and Arctic ice
Snow coverage across NE Europe is more extensive than this time last year, the Scandinavia region is also significantly colder. Sweden in recent days has witnessed its lowest November temperature since 1980. This thickening snowpack and rapid fall in temperatures means northeasterly winds are likely to be unusually severely cold for so early in winter.
Current Eurasian snow cover
Eurasian snow cover 26th November 2010
The Eurasian snow cover we see it closely resemble late November 2010, also we observe uncanny similarity with Arctic sea ice, in fact as of 28/11/2021 we have over 400,000 square km more ice. This is reflected by a colder Arctic of which I have mused over this autumn. Last winter we greatly struggled with having sufficiently cold air, the patterns were favourable with good air flows but they did not live up to expectations. Winter 2021/22 likely for this to be less problematic, and thus could make a major difference in marginal snow situations.
North Atlantic Sea Surface Temperatures
The North Atlantic at present is warm, however, lately with chilly northwesterly winds this has caused SSTs to drop closer to normal NW of British Isles, meaning Polar Maritime northwesterly winds will not be particularly modified. The subtropical Atlantic is particularly warm this has contributed to some of exceptionally mild conditions this autumn, any southwesterly flows this winter are likely to be very mild across Europe.
In winters 2009/10 and 2010/11 the Atlantic SST anomalies were quite favourable we had a tripole pattern, this is when you have cold waters sandwiched between warmer waters to the north and south. This winter we do not have this, but I theorise the widespread above normal SSTs could be ironically beneficial, temperature gradients fuel the jet stream and associated wet and stormy weather. The North Sea is also comfortably above average this may make easterly winds less cold, however, if we see bitterly cold Siberian winds there would be vigorous convection, frequent heavy snow showers in east/southeast. Overall with SSTs there are good and bad aspects I don't anticipate Atlantic SSTs to be particularly influential.
Seasonal models
In last 15 years particularly we have seen great advancements in long range seasonal modelling most of models UKMO, ECMWF and Meteo-France are going for a milder than average winter +NAO. It is worth noting a three month mean can mask an awful amount of variation for instance one cold winter month against two much milder months the cold anomaly is completely lost, being a country which does not typically experience prolonged cold/snow they must be used carefully.
UKMO's Glosea depicts a mild winter with dreaded Euro high, the jet stream is kept to north of UK allowing for a dry winter with an unusual lack of winter storms.
French seasonal model is likewise going for an above average winter, but increased mobility of Atlantic westerlies a wet winter in this case, which also might lessen the mild feel.
The European model by far most compelling for a cold winter signal, average temperatures are favoured overall, there is also evidence of a high latitude blocking signal Scandinavia is shown warmer than average which possibly suggests an anticyclone. This encourages cold easterly flows into Europe. What is more stark is strength of the warmer signal between Iceland and Svalbard, I believe this suggests a strong northern blocking signal, blocking here historically has resulted in some rather extreme and protracted cold winter conditions.
Typically I give a monthly breakdown but I do not think this is sensible, no one on this planet could give you a 100% roadmap of how this winter will evolve particularly this one.
In summary, as earlier mentioned the headlines are;
Overall, temperatures are forecast to be colder than average expecting the coldest winter in UK since 2012/13
January expected to be the coldest month, however, coldest weather could happen later in winter, after mid month, possibly SSW induced and extend into February, potential for notably low temperatures with significant wintry hazards
50-60% likely to be a wetter than average Winter drier in the east
December forecast to be the wettest month
This is the first winter forecast I have gone for a colder than average winter, I am not super bullish it will be cold, so I am taking a calculated risk the benefits I suppose of not having clients, but this winter will likely go one of two ways. Despair or jubilation. Very wet and Atlantic driven or the blocking wins out and cold if very cold weather dominates. I hope that wasn't too wordy, I look forward to sharing more with you @TheSnowDreamer
P.s. If you are appreciative and have a little to spare, contributions to keep site up and running are greatly appreciated, many thanks
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