Winter 2020-21: A Dark Horse?

Well here we are, again, meteorological winter is here, and increasing numbers of you, and me, look for nirvana of winter weather to break tedium of monotonous nature of winter.

We move into this winter with very low expectation but is that unreasonable? Given the law of averages, and how shockingly poor it has been post March 2013. We head into this winter rather different on recent winters we have passed solar minimum, and have entered a new solar cycle, it is our first La Nina winter since 2017-18 which featured 'The Beast From The East', and the strength of La Nina is strongest since 2010-11. There are growing hints the stratospheric polar vortex (SPV) might be less of a bully this winter, in contrast to last winter which engulfed any opportunity to break out the rut.

Fading light. Canvey Island seafront late February 2018 during 'The Beast From The East' @olearydanny

Many argue a mild November has poor implications for winter, but this is unclear, this November has experienced very mild weather. We have seen a strongly positive Arctic Oscillation (AO) a low pressure dominative environment in Arctic. While across Europe we have seen persistent higher pressure centred right at heart of region. Refer to below 500mb height anomaly. The positive AO has meant the cold from high latitudes hasn't had chance to bleed into mid latitudes - two of strongest positive November AO months were 1978 and 2013, these winters on opposite ends of the spectrum.

Interesting Central England Temperature (CET) for November 2020 came out as 8.5C identical to November 1978. The winter of discontent 1978-79, Britain suffered socio-political crisis exacerbated by the coldest winter for 16 years. In some ways the country is at a similar crossroads, and in the midst of a pandemic the like we've never seen. A cold winter is not typically welcomed by Brits but perhaps even more so this year.

So, what can we expect this winter? I will explore the seasonal models, sea surface temperatures and various other teleconnections like ENSO to construct this winter forecast, seasonal forecasting is very challenging, it is a rather new science but there are methods in the madness, (scroll to the end to skip the jargon).

Pinky sky following recent snowfall in December 2017 near Goring in South Oxfordshire @HelenAdams85

ENSO

The El Nino-Southern Oscillation (ENSO) is one of Earth's most influential ocean-atmospheric climate drivers ENSO describes variations in ocean temperatures in Equatorial Pacific.

During a cold ENSO event referred to as 'La Nina' which happens every 2 to 7 years, we see colder waters across east-central Equatorial Pacific.

The trade winds which blow east to west become even stronger no one quite understands why, in El Nino it is vice versa they're weaker and less oceanic upwelling causes a warming.

This winter will be a La Nina winter and of moderate strength it has likely peaked and expected to weaken as we move into spring. During La Nina winters we in Europe and USA see more colder winters than compared to warm counterpart, however, when strong this greatly increases the risk of a mild, very strong Atlantic zonal flow to the winter. December 2010 was a huge oddity in that sense but again highlights Mother Nature will sometimes do as she wishes. What we have also observed is East Based (EB) La Nina winters in which the core of cold water is in eastern Pacific east of 160 W is more favourable to colder winters in Europe, and during Central Based (CB) the core of cold waters are west of 160W this has a greater tendency to the traditional La Nina, with encouraging undisturbed zonal/unsettled flow.

This winter we have a more EB La Nina than CB La Nina although it is no classic, the strength of La Nina expected and core of cold the analogues tends to favour a colder winter overall but not by much over a warmer winter.

@longrangesnow analogue for this winter with an east based moderate La Nina reveals an atypical picture evidence of northern blocking in Kara Sea. Interestingly, current sea ice coverage in region is only 50% of 2010s average, there's an idea more open waters causes more latent heat flux to escape into atmosphere. And, subsequently more high pressure formation perhaps explaining why Arctic is warming much faster than elsewhere.

Notably this Kara Sea blocking also has association to preconditioning for Sudden Stratospheric Warmings (SSW) of which 2/3 result in cold conditions in Northern Europe, across Northern Hemisphere the mid latitudes tends to see more cold or very cold outbreaks.

Also analogue captures not the conventional west to east movement of Atlantic lows, the Azores high is nowhere to be seen a winter killer in Europe. This suggests a very wet winter for Iberian peninsula with the jet stream on a NW/SE axis, funnily we're seeing this now, with UK on colder more unsettled side of jet stream, I do not believe this suggests the winter will be very unsettled for us tending towards average rainfall, with wettest conditions in west the wetter weather perhaps going to Southern Europe which is anomalous. There's not a great indication for a particularly cold winter, frequent polar maritime remember 2014-15? Probably not if you're a southerner, it was an average winter only 0.2C warmer than 2017-18.

Overall this likely would result in a slightly colder and drier winter overall across British Isles and much of continental Europe.

Quite a bleak signal for SE Europe with ridge although inversion cold could be seen, there is also potential from this analogue of cold easterlies. Particularly in Eastern Europe, I get the impression at times this could creep our way, and bring very cold conditions, and with Atlantic trying to get in this spells potential for disruptive snowfalls particularly in eastern half of country. This winter is rather likely to offer much more wintry interest than preceding two.

Stratospheric polar vortex

Moving on the stratospheric polar vortex (SPV) a very vital element of consideration in our winter prospects essentially it 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 atmopshere 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.

The stratospheric zonal wind at 60N at 10hPa (mid level of stratosphere) is currently significantly stronger than average GFS analysis reveals 44.4m/s (03/12/20) the average for date is 30m/s. Immediately this may seem alarming for this winter but there is more than meets the eye by through looking at Northern Annular Mode Index (NAM) which is very useful as it shows a cross-sectional profile of the atmosphere.

Positive NAM values essentially indicate to positive AO when you see these positive numbers from top to bottom, it means the troposphere is coupled with stratosphere, very hostile for cold weather patterns. And, very briefly this was evident at the end of November. However, looking forward an increasing disconnect looks to open up between strong stratospheric vortex which is intent on making our lives a misery and troposphere. Which is very good news indeed there are also signs of a considerable weakening in the upper levels of stratosphere, particularly from mid December onwards. In this instance we do not want to see a disconnect between higher and lower atmosphere, seeing negative values from top to bottom would be very good in identifying a weaker polar vortex, and one which can impact on weather patterns lower down in troposphere. The good news there is evidence of this going forward.

A sudden stratospheric warming (SSW) is what we often look for in winter it refers to a rapid warming in stratosphere of up to 50C in 10-15km above the Earth's surface which can have a fundamental change on our prevailing weather. Typically, the polar jet navigates from west to east very much around the periphery of polar vortex, however, this ordinary westerly/zonal flow can be disrupted by large disturbances which propagate upwards from lower down in atmosphere in troposphere. This usually involves anticyclonic wave breaking which happens around large areas of high pressure, we are seeing such wave breaking now, and it is tied to weakening of SPV but a more significant warming isn't expected presently in near future.

Sometimes these disturbances are sufficient to weaken and even reverse the westerly flow very high in stratosphere to easterly. These easterly winds eventually propagate down it can take several weeks which can greatly increase the chance of northern blocking and cold weather patterns, however, it is worth noting not all SSW events manage to propagate to troposphere. With SSW approximately 2 in 3 result in extended cold weather in Northern Europe which is significant given we are a small island nation a little less likely.

February 2018 was our last successful example and involved a split in polar vortex which Europe spectacularly benefitted from, splits tend to be much more 'rewarding' than displacements in Europe. Often significant area of bitterly cold air is dislodged. We see splits from wave 2 attacks which could involve a large anticyclone in Scandinavia and North Pacific, think of a balloon being pinched on both sides, it is more likely to burst.

The SSW in early February 2018 and split notice two main vortices completely separated

The far majority of SSW events in 21st century including 2009, 2013 and 2018 have worked greatly in our favour January 2019 one occasion it didn't work out. I believe given we appear to be going against climatology this December, I strongly think a SSW is more likely than not this winter. More importantly 50% of La Nina winters with wQBO which I will go onto soon have featured SSW. I'm 60-70% confident of a significant disruption, possibly as we pass into the New Year. In this instance I think the earliest we will feel its impact will be in second half of January and into February, it is also typical when vortex is seriously perturbed for the westerlies to be flushed down into troposphere in which is why, I anticipate January to potentially have some of most unsettled weather.

Solar

We are heading into this winter entering a new solar cycle, 25, the beginning of a 11 year solar characterised by both ends with low activity and the maximum midway in cycle, solar maximum which is expected in 2025. We are in ascending phase from solar minimum in contrast to cycle 24 the wake up has been more abrupt than perhaps expected. Nonetheless, there's evidence in previous years of uptick following solar minimum this having strongest relationship on colder Northern Europe winters not in the thick of solar minimum. Indeed as we had seen in 2008, and colder winters which followed in those years after 2009-2013. Current solar flux interestingly is very similar to late 2010. Overall, solar activity is expected to be low or moderate this winter and following solar minimum this winter will be well tested.

03/12/20 - the emergence of sun spots in the Southern Hemisphere of Sun's surface

Arctic Sea Ice

Sea ice extent is currently second lowest 1979-present after 2016 this may indicate northerly flows may not be as cold, this is perhaps why seasonal models are showing some of most anomalous warmth in Europe across Scandinavia this winter. However, sea surface temperatures (SST) have lowered in Norwegian Sea from a very elevated state. On paper this bodes badly but there is early research which is attributing to a warmer Arctic with increased northern blocking, and cold weather patterns in mid latitudes.

Sea surface temperatures

SSTs in November took a large downwards plunge in North Atlantic cooler waters increase propensity for low pressure, and therefore the "warming" seen as we have moved into December is a positive development for -NAO and blocking patterns in this region increasing risk of cold northerly winds in Europe. During La Nina winters we tend to see increased Atlantic blocking, indeed for much of Autumn we did see this, October was cooler than average across Western Europe and France had their first below average month since May 2019. November, however, has been atypical for La Nina, there is evidence now of disconnect with La Nina state ending which particularly increases risk of colder weather earlier in winter.

Many winters in last decade which mild conditions have dominated have witnessed large cold anomalies in North Atlantic, to me this suggests the Atlantic flow will be weaker than normal this winter, and therefore such an unsettled winter like 2019-20 winter is extremely unlikely.

QBO

Sorry if you're getting increasingly restless at this stage but the end is nearly in sight. The QBO (Quasi-Biennial Oscillation) refers to the variation in winds we see in equatorial regions high in stratosphere. Roughly every 14 months the winds change direction from westerly to easterly or vice versa. It sort of has a butterfly effect around globe when the QBO is easterly there is a greater chance of a weak jet stream, SSW and therefore colder winters, when in westerly phase the total opposite.

This winter the QBO is behaving oddly it is sort of in a neutral position the lower atmosphere has weak easterlies which is most important, this is positive in that there are not westerlies to enhance the zonal westerly flow. Which may be a contributive factor why this December is expected to have little impetus in Atlantic and therefore a greater chance of colder conditions in Europe. We are also beginning to see descent of easterlies in upper atmosphere between 10hPa and 20hPa this is the beginning of an easterly cycle, which we are quite confident winter 2021-22 will feature eQBO, it is possible this could descend by spring 2021 which would increase the chance of an extended winter not welcomed by me. It is difficult to make sense of QBO and its future this winter, but overall I'm not expecting QBO to be hostile the westerlies in mid atmosphere are trying to descend, but thus far are being hold back.

Seasonal models

The GloSea5 model the best long range model in business, I believe in last 10 years it has got 8 out of 10 right in our part of the world, it forecast winters like 2009 and 2010. Unfortunately, the December update is still not out, in this time there's been increasing hints of disruption to SPV which could break normal Nina coupled state especially further into winter. There is clear evidence of a La Nina signature the North Pacific ridge, western Canada and Alaska seeing a very cold and snowy winter, very mild in the eastern half of USA. Europe seeing a blowtorch winter +NAO/+AO dominated, very anticyclonic.

ECMWF seasonal model MSLP probability shows evidence the Azores high will be retracted to our southwest possible ridging in North Atlantic interesting to note there's no strong signal either way for anomalous low or high pressure in Greenland. In recent years many seasonal models have taken to Greenland with significant low geopotential heights, +NAO identifier, very hostile profile for cold weather patterns in Europe. This doesn't scream a cold winter but nor a mild one, the polar jet is displaced bit further south, I can see potential for northern areas of UK seeing lots of snow potential from this, infrequently in south. Winter rainfall likely to be near normal the Atlantic isn't particularly energised.

JMA model I do not have experience with this model but it goes further than ECMWF the macro scale features are very alike. There's more evidence of ridging in North Atlantic at a higher latitude into Greenland. This would have the potential to give a quite cold winter with plenty of cold northerly or northwesterly winds winter rainfall likely below average.

In conclusion none of the seasonal models I've come across are really forecasting a really cold winter the ECMWF and JMA are broadly similar with potential to bring quite frequent colder than average conditions from a northerly quadrant.

JMA model has greatest potential to bring something more significant and more countrywide, rather than limited to northern locations. It should be noted caution with interpreting a 3 month smoothed average, it is possible one winter month might feature more blocking for example. The GloSea5 very little wiggle room - very poor. Generally a positive change on recent winters in that Greenland doesn't look to harbour the PV.

In village of Iwade, Kent an 'Arctic landscape' February 2018 @iOS_weathervue

December

The first month of winter offers to be a seasonal month, I see it to be a remarkable month in the absence of exceptionally mild temperatures we have been so accustomed to. There is a good chance this will be coldest December since 2010, but this rather says much about how poorly we have done since 2010. The North Atlantic Oscillation (NAO) is looking to have a negative tendency, and therefore the pressure gradient between Azores High and Icelandic Low is reduced hence colder weather patterns are more likely to feature in Europe and a barrage of Atlantic lows are unlikely. Heading towards mid month I think it could it turn milder with Atlantic air breaking in, there could be snow in this process, however, there's a chance we could remain colder and go even colder. Towards Christmas week we may see reemergence of Scandinavia blocking given developments in MJO, and colder weather possibly from east, perhaps more dry weather, the chance of a white Christmas this year is higher than usual. I think a significant cold spell is unlikely to feature this month, nonetheless many could see their first snowfall, even disruptive snowfall, more likely in north and east. Rainfall probably below average, more likely wetter in west and drier in the east.

January

This month is particularly tricky in that I'm somewhat making an assumption but based on developing trends that a major disruption to stratospheric polar vortex is going to happen, I do not expect for this winter that we will have to have a SSW for there to be a chance of a good cold spell, as we had seen in December 2019 it become very rampant and only got progressively stronger. Vortex become one of strongest on record in February 2020 not coincidental it was the wettest February on record in UK.

With December 2020, it is going in a differing direction as December progresses a weaker vortex is signalled, and therefore this is very different to last winter. I consider it unlikely that much of this month will be cold, it may even turn out milder than average, but not particularly so. Later in the month I can foresee the potential for coldest spell of winter, and I believe this would come from the east or northeast. Beforehand it could be very unsettled as the westerlies are flushed down from stratosphere into troposphere to make way for the easterlies which will descend. Potentially January 2021 will be the wettest month of the winter.

February

I feel this month is more likely to begin on a cold or very cold side following on from the fallout of SSW and perhaps ending on the cold side as well, it is infrequent we just have one long continuous cold spell in Febuary-March 2018, we had a baby beast but as it was so late in season it was a damp squib. My inclination is that February will be driest month of the winter.

A snow coated bicycle in East Sussex early February 2012 @pete7777

Winter 2020/21 summary

* Near average winter leaning on the colder side

* A drier feel but not particularly dry or wet

* 60-70% chance of a sudden stratospheric warming in January

* Not exclusively front loaded often associated with a typical La Nina winter

* The greatest potential of significant winter weather coming January into February

Overall, I fancy this to be a near average winter which on recent winters will feel cold. I'm leaning more on the colder side than the warmer side of average, there is potential for some particularly cold weather coming from east, and I consider this most likely to happen in second half of winter from mid January onwards. Precipitation wise I cannot see it being nearly as unsettled as last winter particularly in the case of February, the signals aren't strong either way wet or dry, so I expect average rainfall to be more likely.

There's a small chance <20% every winter month could be colder than average, we could see an appreciably colder than average winter the coldest as far back as 2009-10. Perhaps more likely to be closer to winter 2012-13 which was 0.4C below average in UK a good winter.

To wrap it up this winter promises to be no different to any other, the inevitable feelings of euphoria following a frigid GFS model run only for it to be coldly snatched away. However, this time I do not expect an endless chase, the pointers are different this winter and present is unordinary which inspires hope. I hope this forecast provided to be of interest and of educational benefit I enjoyed typing it, I'll catch you later!

Dan

Autumnal times lie ahead, early winter clues?

Following weeks of unseasonable sunny and balmy weather you would by now have caught on a major change in our weather, reflecting the descent into crazy season. It was only last Tuesday, we had a very warm southerly flow, which enabled temperatures to reach as high as 31.3C in Kent the first 30C+ in September since 2016.

A major change occurred on Wednesday and it is expected these cooler and more unsettled conditions will continue for remainder of month and continue into early October.

The cause for this change and big fall in pressure, stems from what was Hurricane Teddy which has trundled through easternmost Canada and to Greenland its impact on the jet stream causing a wavy configuration. As such amplifying the ridge (dry and settled) poleward in mid Atlantic and sending a trough (wetter & cooler weather) downstream into NW Europe.

The weekend will be on cool side temperatures generally 10-14C warmest in south, however, following some recent rain, we will see high pressure, nudging in from west and hence the weekend will be largely dry with sunshine. On Saturday there's likely to be showers moving in across easternmost counties off North Sea... particularly Kent & Norfolk. Sunday will see it turn dry across all areas, with lighter winds making it feel less cool especially in east.

Winds will fall light especially further north and west meaning frost is likely to develop Saturday and Sunday AM. In contrast less light winds and cloud further east may keep frost at bay.

Saturday morning minimums

Next week: on Monday we will see a frontal band moving eastward with cloud and rain across all areas, however, it will be a decaying feature and therefore may give no rain for South East and East Anglia. Feeling milder with southwesterly winds instead of cool northerly winds. Tuesday sees a brief area of high pressure nose in south, meaning mostly dry and bright conditions particularly in south, temperatures near normal for time of year. After a break from more unsettled conditions midweek onwards we see the jet stream strengthen and this helps introduce a series of low pressure systems.

Wednesday likely remaining dry for many however low pressure is expected to dive in from northwest late in the day, there is the expectation this area of low pressure will linger around not necessarily meaning continuous rain, but very squally, showery conditions.

Model output deepens this low pressure through Thursday and Friday bringing very windy conditions and gales for exposed locations. Feeling cooler once more on north side of jet stream with cooler Polar Maritime airmass firmly established.

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What of winter? As you know we're still in September with still 9 weeks till December 1st, so much can chop and change, nonetheless, the atmosphere can give some longstanding clues in autumn with possibilities for early winter.

The autumn so far has certainly been interesting, and is progressing quite unusually. So far one overwhelming characteristic has been jet streams behaviour, it has been more meridional (wavy) than normal. The mid month European heatwave was caused by jet stream looping aggressively north of British Isles and up to Scandinavia, allowing very warm continental air to seep north from Spain and North Africa. Being south of jet stream means 'warm' and settled conditions. Now, for foreseeable future we look to sit on northern side of jet stream the cooler and wetter side. These wild fluctuations in jet stream, potentially has links to solar minimum conditions. Indeed there has been a lot of similarity with 2008 that year it was also declared. During that October, London had experienced earliest snow since 1934, I am not suggesting we are going to see the same, but there appears to be a greater risk of a more unseasonable, potent cold snaps in October and November and potentially into early winter.

That winter began a cluster of much colder winters between 2008-2013. The autumn of 2008 similarly featured a wavy jet stream, and Atlantic blocking which looks to dominate persistently in near term, this continued through winter and indeed this winter was coldest winter in UK since 2000-2001. The winter which followed this was 2009-10 which was coldest winter since 1978-79, and not long after in December 2010 we witnessed coldest December since 1890, there is literature which suggests the effects of solar minimum are most pronounced 2 years after. It is worth noting these winters came after a long barren spell through much of late 1990s and 2000s we are no stranger to lengthy snowless periods.

Ural blocking over NW Russia looks to feature extensively in outlook, there are studies which attributes the diminishing sea ice (this year we experienced the second lowest sea ice minimum extent after 2012 since 1979), to greater likelihood of blocking in these higher latitudes. The ice free areas release more heat to the atmosphere, and can affect the atmospheric circulation patterns. An interesting but very plausible theory, it would mean in this event the Arctic would experience more rapid warming and indeed it already is experiencing but the mid latitudes like Europe would be more prone to 'The Beast From The East', however, remaining uncommon but perhaps with increased frequency of extreme cold spells.

The Ural blocking is also linked to disrupting the Polar Vortex particularly pertinent the link from November and increasing the chance of -NAO in December and January, hence a greater chance of colder weather patterns for NW Europe and more settled conditions.

It is linked as a robust driver to early winter stratospheric warming for NW Europe the polar vortex is a winter troll, a strong vortex encourages strong thermal gradients, therefore the intense cold remains bottled in Arctic & mid latitudes do not see this cold air. And, therefore we see see a strong baroclinic zone, where low pressure can rapidly form. When it is disrupted, the very cold air is much more likely to bleed out and increase cold risks markedly.

Guidance resolutely is going with a weaker polar vortex through much of October even adjusted CFS which is known for its weaker bias, is going for a weaker polar vortex into early winter, and therefore we may have the greatest shout of cold and snow early in the winter.

In conclusion I must say I'm feeling quite positive overall for early winter prospects, it is even possible November could deliver something wintry, which we haven't seen in 10 years. I see something a little different about this upcoming season, you may feel you hear that every winter, but we have decidedly different variables to work with this winter. I've chosen not to go in great length, there'd be no point now. In contrast to last two winters, snowless and terrible, we are set to have La Nina conditions again this is correlated to a front loaded winter. Although of course it's worth noting winter 2017-18 was also La Nina, the most intense cold of the winter did come in February and March that was brought by a Sudden Stratospheric Warming (SSW). However, the December was not without interest with multiple snow events but SE England largely missed out there. It illustrates things are never black and white with weather, which makes it very challenging, but there's no harm giving it a good go.

Summer Forecast 2020 - 'On a Tightrope'

Would you believe it the time has come? A new season brings its new challenges. In deciphering the longstanding oceanic/atmospheric teleconnections, one can gain an idea of how the summer may be framed. Specific details cannot be provided. Please be warned this forecast should not be treated like gold dust, however, I hope it provides for some helpful insight in how the summer may turn out in a general sense. Overall I am expecting a reasonable summer not excessively warm or dry, carry on reading to find out more....

A nice scene in Brighton seafront - ensure social distancing this summer please ;)

First we are going to look at North Atlantic, Sea Surface Temperatures (SSTs) being a maritime nation we are particularly influenced by waters surrounding us and Atlantic Ocean, which weaken the potency of hot and cold weather. SST patterns in Atlantic can impact the atmospheric conditions which Northern Europe sees, hence it is very important to monitor, given that water is a slow conductor of heat SSTs experience slow changes.

Figure 1. Observed north Atlantic SST anomalies in May 2018 and 1976, relative to 1960–2017 climatology in HadISST. ( Dunstone et al, 2019.)

Research has been done which strongly correlate a tripole pattern in Atlantic (warm/cold/warm) SST anomalies in May with drier summers in Northern Europe. With areas south of Greenland seeing persistent low heights where suppressed SSTs are, with areas of elevated SSTs supporting ridge development meaning settled.

As Figure 1 shows joint record warmest summers of 2018 and 1976 in UK experienced a very similar pattern. This May we are similarly seeing this tripole pattern (Figure 2) which may suggest this summer has a greater likelihood of being warmer and drier. However, there are noticeable differences the main cold anomalies extend further south, and waters running through Caribbean Sea to West Africa are warmer perhaps illustrating the background state is dissimilar to these years. Warmer SSTs in these regions increase chance of formation of tropical cyclones. The band of cooler waters being more expansive further south may suggest the jet stream is further south nearer to UK than 2018 and 1976 meaning not as settled.

While in my opinion this summer is unlikely to experience exceptionally long anticyclonic conditions akin to 2018 and 1976. Due to such a benign spring around British Isles and the lack of upwelling of cooler subsurface waters, SSTs, are generally 1-2C above climatology. The elevated SSTs within our coastal waters will influence magnitude of high temperatures seen this summer. This is positive for seeing high temperatures. Indeed as we have seen this spring frequent easterly winds but not the cold sort, as we experienced in 2013. A legacy of a mild winter and enhanced by a mild spring. Overall a good picture with SSTs but not perfect.

Figure 2. Global Sea Surface Temperatures (SSTs) courtesy NOAA

Moving onto QBO (Quasi-Biennial Oscillation) which essentially is stratospheric winds around the equator. There are two phases either westerly or easterly in a 9-18 month cycle. What we tend to see is during strong positive/westerly, we see increased westerly flow across Atlantic. This summer we are entering the summer weakly negative/easterly (Figure 4). This means activeness of Atlantic flow may be reduced, hence, increased chance of warmer and drier conditions. Alternatively this may mean persistent Atlantic blocking with the UK east of high subject to cooler and showery northerly winds.

Furthermore, QBO given its expected weak state may not be particularly influential. However, while overall I would say it is in a good state, if blocking develops in unfavourable location it may be harder to budge. And, therefore this would bring cooler and more unsettled weather if blocking is positioned to our north or west.

Figure 4. QBO progression

We will now look at ENSO which is SSTs of a region in tropical Pacific which influence global weather patterns you may have heard of La Nina and El Nino (Figure 5).

Figure 5. What is La Nina and El Nino? NOAA

In the last few weeks SSTs and upper ocean heat content in equatorial Pacific have experienced a dramatic cooling (Figure 6 ). An unexpected early progression towards La Nina. The May ensemble guidance remains out of kilter with what we are observing now. A SST anomaly of more than -0.5C was experienced in 3.4 Pacific region in last week threshold for La Nina all regions have since seen a warming in recent days however note this is more a blip.

Figure 6. SST anomoly 3.4 region relative to 1981-2010 average tropicaltidbits.com

This summer we are expecting SSTs to be below average across central pacific indeed they are already. A SST anomaly of -0.5C in 3.4 region of Pacific is indicative of La Nina regime it often then takes a few weeks for ocean to couple to atmosphere. For much of summer NOAA are expecting neutral ENSO conditions, therefore it is unlikely to have much impact on global weather patterns. However, most guidance is lurching towards Nina by late summer and autumn correlating with peak hurricane season running through August to October in Atlantic. With La Niña we see reduced wind shear in tropical Atlantic essentially wind shear tears tropical storms apart. Often we see these recurve our way across North Atlantic hence an active hurricane season will increase chance of unsettled weather but most notably from August. Late in summer we are most likely to see negative effects. In my opinion La Nina will develop faster than expected which will have implications for later in the summer.

Figure 7. Model predictions of ENSO from May 2020

Now looking at solar we see sun go through cycles every 11 years we start a new cycle in these cycles we either experience a solar minimum or maximum. It is theorised low solar activity causes a more meandering jet stream, we are currently at solar minimum and the lowest part of this cycle. Hence we tend to see more extreme weather and stuck weather patterns. High pressure, blocking patterns, persisting over long periods and well this has certainly been evident this spring, but, if you are on the wrong side of the jet stream we may see persistent unsettled weather. This is interesting when you consider the last year we have experienced both unusually long, wet and dry periods. Overall, I would say solar background gives a mixed signal for summer 2020.

Lastly we are going to look at seasonal models for June/July/August first starting with CFS typically this model tends to go OTT with warm anomalies, so it is quite interesting to see close to average temperatures for summer 2020. You can also see significant warm anomalies stretching from Greenland Sea to Kara Sea this is indicative of high latitude blocking. Therefore, this may illustrate cooler and wetter weather downstream in mid latitudes as you can see parts of Southern Europe see below average temperatures.

Precipitation there is no strong signal for either a dry or wet summer there is slightly drier departures to our northeast, again this is indicative of blocking in this area this high pressure establishing further north is not likely to bring a hot summer. However, with potential Scandinavian blocking there may be plenty of easterly winds which we have seen through much of this spring, so overall a pleasant and near normal summer forecast from CFS.

Moving onto UK Met Office seasonal model GLOSEA in my opinion this is the best seasonal model out there, it had very good performance for last winter. This model is going for very average temperatures in NW Europe this is likely indicative of more cooler maritime airflows. Interesting to note Greenland is much warmer this may illustrate GLOSEA thinks there is going to be Greenland blocking which tends to bring rather cool and showery weather to UK.

Precipitation it is going for a slightly wetter than average summer, continental Europe drier this suggests we will see less in way of warm continental flows. Also, interesting to note the North Atlantic has dry anomalies this likely suggests blocking in mid Atlantic. This would rule out a washout but there may be cooler and showery conditions for UK. One can infer this is going for increased winds coming from a northerly quadrant. Similarly to CFS a quite pleasant summer with near normal temperatures and rainfall.

Lastly we are going to look at model from Meteo France we could be here all day with these seasonal models but I believe what I have shared has highest skill. This is going for generally a warm summer in contrast to other two seasonal models, particularly towards SE England a very warm summer possible here. A mixed summer for Scandinavia suggestive the cooler northerly winds do not make much progress south in contrast to GLOSEA and CFS.

With precipitation this model is going for average precipitation echoing all other seasonal models which suggest a drought this summer is not likely.

Overall, I am quite struck by how much the seasonal models want to go for an average summer in terms of temperature and precipitation this is actually quite unusual to see. Typically in warming world we live in they are usually bullish on warmer summer but it is not evident here, remember the models follow the signals not the other way around. As such the earlier discussed teleconnections fit very well with these seasonal forecasts, which inspires some level of confidence with my forecast.

Summer Forecast Breakdown

Temperatures

Generally a warm summer but not hot, close to average, the higher confidence of a warmer summer drops further NW one goes in which I'm not favouring a warm summer.

40% chance of warmer than average summer

40% chance of an average summer

20% chance of a cooler than average summer

Precipitation

A rather variable summer in terms of rainfall across UK. I have higher confidence Ireland/NI Scotland, Northern England, parts of SW England and Wales seeing a wetter than average summer closer to Atlantic influence. Generally for south and east of England near normal rainfall much coming from thundery downpours, potentially drier for some in East Anglia. A small possibility <20% but not insignificant we all experience a wetter than average summer, if La Nina strengthens and arrives faster July and August could be very poor.

50% chance of near average summer rainfall

30% chance of wetter than average summer

20% chance of below average precipitation

June

We will see start month as we were through much of the spring, sunny and warm for all. However, this will be cut very short. A significant change in our weather taking place midweek for Scotland and NI this will begin on Tuesday. A burst of amplification (high pressure) in mid Atlantic, ridging poleward, meanwhile pressure around UK falling sharply, in the second half of next week we will see introduction of much cooler northerly winds, and blustery of that. All parts of country are likely to see some useful rain. I do expect an eventual recovery but I see the first half of month being more changeable and cooler, there is expectation atmospheric angular momentum (AAM) will rebound +ve when this happens, we tend to see high pressure less on retreat from our shores hence a more likely return of a warmer and drier theme sometime in the second half of the month. Overall a near average month, nothing great not like April and May but I'm not seeing much signal for it to be that wet either.

July

Confidence is of course lower, however, the general idea is that this will be the warmest month of the summer, most likely to feature hottest weather. While, I do not think we will see a prolonged period of hot temperatures, I would not rule out 35-38C temperatures possibly even higher in these times we live in, but this being more likely to be confined towards London and Home Counties. I also especially see this being the most thundery month of the summer recent summers have been very poor owing to a large CAP from persistent high pressure. But I do think it is likely through July there will be periodic breakdowns on occasion the more cooler and unsettled weather may not reach SE, however these sharp thermal contrasts are likely to allow for the breakout of intense downpours and thunderstorms. Overall, I see this month being drier for S+E and possibly wetter for N+W areas, however due to localised nature of downpours some may see much more.

August

Confidence is of course much lower, however with how La Nina is expected to develop by late summer NOAA are giving 60% chance of an above average Atlantic Hurricane season. This is the month most likely to go sour in my opinion. I do not think it will be a complete write off particularly earlier in month, however I do not see this August breaking the curse. Cooler and generally unsettled weather prevailing, the south and east will hold it off for longest.

So that wraps up my summer 2020 forecast something for everyone I would say, I hope the forecast made for an enjoyable read. To keep up with most recent developments keep following my latest tweets on @TheSnowDreamer.

Ta, Dan