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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


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