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Dan's summer forecast 2021 - a classic?

Meteorological summer is here, and attention is increasingly drawn to what the weather is doing; in a time of limited foreign travel, the weather the UK experiences this summer will be of particular interest. In this blog post I will explore how this season could be shaping up, some summer thoughts are also given to wider European region in introductory section.



This summer offers at the very least to be a reasonable summer, summers such as 2007 which happened to be a particularly poor summer, could be well flagged, in contrast to this summer we are transitioning to neutral ENSO, instead of La Nina (cold ENSO) which these conditions developed in autumn 2020 and have now faded in spring 2021.


La Nina is associated with cooler equatorial Pacific waters this tends to affect global jet streams they meander more and there more extremes, for NW Europe where UK is positioned, this often means we sit on cool/unsettled side of jet stream with Atlantic blocking.


This is what characterised 2007-2012 summers La Nina was highly influential on their unsavoury note. Ideally El Nino (warm ENSO) is best teleconnection for long, warm and settled periods across Europe given we have not long moved from La Nina, it is probable we are still seeing feedbacks between the cooler oceans and atmosphere. And, therefore it would be unwise to assume this summer will be a long hot summer, the interaction of much cooler Atlantic air may trigger intense downpours and violent thunderstorms.


This summer I'm expecting the most anomalous heat to be located across southern Europe with blistering heat surging from North Africa with temperatures exceeding 40C in Spain, Portugal, Italy and possibly the south of France. It is possible this extreme heat might nudge further north and affect more of Northern Europe including UK, however, while I'm expecting Europe to have a warm summer. The slight mobility in North Atlantic region might just be enough to ensure hottest conditions are kept further east. Furthermore, ensuring hot weather is not as prolonged unlikely to be as fortunate across European continent.




QBO (Quasi-Biennial Oscillation)


Is essentially the term we use to describe 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 neutral, May's QBO number comes out as +0.31 therefore one would expect June's update to show a negative value. It is possible by winter it could be in strong easterly phase so that is of interest to winter junkies but none of that now. The transition to easterly phase of QBO this summer is largely good news, on negative side of things the positive is that extensive Arctic blocking which featured significantly and resulted in UK's coldest April since 1989 and coldest May since 1996 has subsided. The danger with eQBO this can help solidify these blocking regimes at these high latitudes, so it is reassuring the Arctic weather patterns has changed. Nonetheless, this may change and reemerge this would be more likely during La Nina summers and as previously stated it has faded.



North Atlantic Sea Surface Temperatures (SSTs)


SSTs in proximity of UK have warmed albeit they are only a bit above average +0.4C around Ireland they are cooler than average, approximately by -0.5C. In May 2018 SSTs were very elevated within UK coastal waters and this helped influence the extent of high temperatures. We similarly had a cold blob in North Atlantic but this one is further east 'aimed' at British Isles. Where waters are warmer this increases risk of high pressure formation, and therefore these cooler waters just positioned to our west might be less good news. Particularly for those further west which could be at more risk of Atlantic incursions. Indeed, this is a reason why I'm anticipating this to be a thundery summer where we see significant clashes of air masses.


Waters in tropical Atlantic are cooler than normal this is main development zone for tropical cyclones which can develop into hurricanes, we don't see them but they do tend to recurve up eastern seaboard and hitch a ride on jet stream bringing disturbed weather to UK. I am personally not expecting an active hurricane season which runs from August to October, therefore. late summer could be less threatened by the remnants of these hurricanes.



ENSO


I will not go into much detail here as I have discussed 'simply' before most models in IRI/CPC plume predict a continuation of ENSO neutral conditions throughout the summer. This indicates there's unlikely to be a major shake up in summer pattern, while indeed we could at times see cooler and wetter conditions this will not persist for long. The neutralisation of La Nina during spring has been very helpful in that respect.



Lastly looking at seasonal models the ECMWF model SEA55 has changed quite considerably from May update, instead of ridging (high pressure) being focused in North Atlantic this is common in La Nina regimes. In June update we see main ridging extend across all of Europe which has potential to bring very warm and dry conditions across region. The mean flow is coming from Azores so an unpleasant summer of high humidity and rising temperatures could be on cards. There are hints of Atlantic troughing (low pressure) with Greenland having a cooler summer, this may mean NW airstreams could be rather cool. Overall best conditions expected further south and east as is the norm and the greater risk of wetter and cooler conditions further north and west.



Summer Forecast Breakdown



Temperatures


Quite bullish on a warm summer potentially very warm for SE England, for areas further north and west the summer might be closer to average with less desirable weather while areas further south and east might escape. I consider a cool summer very unlikely <10%.


70% chance of a warmer than average summer

20% chance of an average summer

10% chance of cooler than average summer


Precipitation


This is very challenging to ascertain firstly a washout summer is not expected (sorry Jack Reeves), overall a drier summer is more likely than a wet summer intense and short lasting downpours, however, have the ability of really changing things but it must be emphasised of the large local variation. From a statistical pov a wet summer might not necessarily feel like it has been wet. Driest conditions are expected in SE with wettest conditions in far NW where the weather will take a more changeable flow through much of summer. I'm anticipating the driest month could well be June and this might be shared across country.


June


The month has began well with high pressure in total control, there is no indication that a major change will occur in next 1-2 weeks, in fact the modelling is growing increasingly confident of hot conditions developing later this week, which I do anticipate first 30C will be met this weekend. The second half of June it is probable things will be shaken up, there could well be limited success from westerlies, and high pressure and warmth rules all the way through. I am expecting June to be driest month of summer, I also think temps will be well above average potentially by more than +2C which is quite significant. I do not think we will see hottest weather of summer but very warm weather will be regularly occurring.


July


Confidence truly plummets, I can see early in month featuring some less desirable weather and nowhere would escape from cooler and wetter conditions. Overall, expecting July to be less abnormally warm compared to June, nonetheless, I think we will see hottest weather of summer and temperatures may exceed 35C. Continental Europe could be subject to very fierce and persistent heat and this might bring great suffering to vulnerable groups. I anticipate this summer month will be the most thundery with potential for severe storms.


August


We're sort of guessing at this point there has been a strange regularity to August's failing to impress in 21st century the 1990s were a particularly good decade (in more ways than one), we have not managed similar consistent highs since. Will this August offer something spectacular? Probably not, but, compared to last summer Nina conditions began developing in August this allowed the 2nd half of month to really deteriorated following on from intense heat earlier in month. Broadly expecting August to be a good summer month more traditional, rainfall closer to normal, June likely dry and July could vary extremely from dry to very wet.



So there we go that is my summer forecast for 2021, it offers to be an exciting season in my opinion to keep up with latest developments follow my tweets @TheSnowDreamer



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