Autumn Leaves

DISCOVER THESNOWDREAMER'S

WEATHER KALEIDOSCOPE

Your Go-To Weather Man

 
Search

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.

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.


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


1,208 viewsWrite a comment
  • Dan

Updated: Dec 3, 2019

Winter is here... so here I will go again and try to find some direction to how the winter will progress amongst all the chaos. I will not be looking at analogues for this forecast as I do not believe they are useful anymore in a time which arctic is seeing unprecedented change.


First looking at the drivers which impact weather patterns in UK and afar.


The stratospheric vortex is a vital element in winter forecasting for the UK and Europe but limited use the models have beyond 2 weeks. The vortex naturally strengthens and expands over pole through late Autumn as polar night returns. Typically it reaches its apex strength around late December, it is therefore no surprise Christmas often is mild with no snow boo! The stratospheric polar vortex (SPV) is currently weaker than average but not significantly so. Today (02/12/19) the zonal mean zonal wind at 10hPa from GFS analysis is 15.2 m/s it should be around 30 m/s, it is forecast to remain weaker and weaken further, in a time which it should be at its most rapidly intensifying phase. A growing envelope in GEFS seem keen for a reversal in zonal wind to E'ly and therefore a Sudden Stratospheric Warming (SSW) approaching mid month. I however do not think this is likely as often with GFS we are chasing rainbows, at earliest I see this happening in late December add a few weeks and cold surface impacts?, would be quite a bit into January. With SSW approximately 2 in 3 result in extended cold weather in Northern Europe with nuance the UK is a little island. February 2018 was last recent example which spectacularly benefited W-Europe. If we do see a major perturbation which I'm inclined to believe we will, it is likely we will not see possible positive effects of this until midwinter, January or even into February.


Sea Surface anomalies in the North Atlantic as of now favour a slightly south of normal storm track indeed this has been observed through Autumn. With a shocking lack of gales in Scotland the jet stream has been further south than normal on an unusual axis, depressions which usually ride jet stream have been instead passing through central swathe of England. Sheffield has experienced its wettest Autumn on record, flooding has never been far from news. The current SST anomolies are not particularly promising for a relent in wet weather in these places as we go through December. However, very warm waters around Greenland's southern coasts may help to support ridging in area, indeed which we have seen of late in a colder spell of weather with Greenland blocking. This more hospitable region for blocking in conjunction with a jet stream which has been already more south solar? May allow for significant snowfalls in December and beyond but this is much more likely for northern UK.




Snow cover & ice extent


Snow cover is very high across Northern Hemisphere this October had 5th largest snow cover extent on record. it is believed a strong and extensive snowpack across Eurasia allows for a strong Siberian High, fostering disruption to vortex with a linked connection for a negative Arctic Oscillation (AO). This promoting a southerly tracking jet stream and very cold air bottled up over the high latitudes to spill into mid latitudes a greater risk of cold weather in Europe and US. The Siberian High can become more influential for UK later in winter as robust N Atlantic westerlies die down, I do see the second half of winter featuring coldest weather *spoiler*. While onto sea ice the Atlantic side only really relevant for UK and Europe has seen significant very much above average progress in the last month, something we have not seen in last 10 years. This will mean northerlies are likely to be a little colder than normal this winter and a higher chance of snow falling and accumulating to lower levels and further S.

Solar forcing

There is a problem equating a "cold winter" to solar minimum as a direct relationship but it is not truly understood some brilliant winter spells have been seen near to solar maximum such as February 1991 which London did not see equivalent winter weather till 2009. Although dissimilarly this was a solar minimum winter, the observational note in last 10 years has shown a cluster of blocked and colder winters at or following a solar minimum 2008-2013. Some literature suggests there is two year lag, with an uptick in sunspots in 2013 followed a cluster of much milder winters, but again careful pinning it on just one variable. Overall I'd say this winter solar minimum, increases the chances of blocking and therefore a colder spell of wintry weather but where it positions itself is key. A "bendy" jet has been present this autumn not typical flat W-E may already be having an influence with Autumn 2019 being the first cooler than average average to 61-90 average since 2012.


Monthly forecast


December


The first month of December is likely to be generally chilly and wet while the jet stream may remain strong it is in my opinion likely to be deflected south. Current modelling paints a likely cold snap/spell around mid month with cold northerly winds, and rather unsettled with it and therefore the risk of snow particularly in the north, significant snowfall(s) a distinct possibility there. I do not think significant cold is likely to feature in this month - but it is not necessary for snow to fall even in SE England, it would just not lay on ground for long. Looking towards pacific forcing the next ENSO cycle expected later this month looks to bring the MJO back into equation, with colder phase 7-8-1, possible this may allow for blocking to develop. So a drier final third I feel but potentially cold or very cold, rather than chilly which could prove good for festive period, with severe frost potential any significant snow I feel this is limited more likely into 2020. The month generally as a whole looks quite cold and seasonable.


January


This is difficult month I see there being two possibilities the first rests on major perturbation to vortex and of course downwelling, at earliest I can see this truly manifesting on ground level is middle of January. With possible much colder and drier weather, and enhanced risk of disruptive snow just about anywhere. Before this point it could be wet start to January and around average if milder temperature wise, somewhat what I feel like a fair swathe of December will be like. The jet stream remaining predisposed to south, I can see a wonderful season for Scottish mountains. If we fail to see a major disruption to vortex I firmly believe January could be the most stormy month of the winter, with a turbocharged +NAO pattern, this is what I believe seasonal models have gone with. No stratospheric impact for winter 19/20 which may prove wrong in my opinion & if is true wintry weather could be very limited to northern hills. I am placing my bets with first scenario.


February


It probably will be the driest month of the winter I will not give much detail here but as above in regards to January, this month may be the coldest of winter. With easterlies perhaps bringing exceptional cold from Siberia, in this case not like 2018 possibly much earlier in month, with the QBO transitioning to E by late winter. This aids in the polar vortex not being menacing and on top with a possible major perturbation to vortex and climatology support, northern latitude blocking may prove to be extensive and resilient a very strong -NAO signature this month might have. In contrast to my last winter forecast which I thought would bring an early start to Spring I do not believe this is particularly likely in 2020.


.....


So that is my winter forecast this may be seen as a daring forecast but it is a forecast which I see as being plausible and rooted in science. However, much emphasis on it leaning towards a colder winter rests heavily on a SSW happening and imprinting favourably. If this does not occur then the winter may be rather uninspiring and wet, if opposite is true which I think will be so, expect the winter to become increasingly drier as we progress but also colder. A winter we may not have seen in a number of years, feet firmly on ground but I am looking forward to what may come. I do not for the life of me foresee it being a snowless 2013/2014 winter low solar also to add, expecting for this winter colder than average departures to north and east and more around average for south and west. Thanks for reading. :-)


2,187 viewsWrite a comment

©2018 by TheSnowDreamer - weather kaleidoscope. Proudly created with Wix.com