Insights & News

Big Changes Predicted for Europe in Summer 2021

18th May 2021

A detailed, analogues-driven look at the summer prospects for Europe, which shows signs of being on the warm side in the north and unusually rainy in the south.

Believe it or not, it’s that time of year already: the long summer season looms. To the hospitality sector, this is an important time of peak tourism for much of Europe. For retail, the season is usually less distinct… but this year is not usual.

Numerous countries are in a critical recovery stage after the easing or lifting of lockdowns, as non-essential customer-facing businesses open their doors for the first time in months.

This summer, more than ever, fine weather is not just desired, but needed. Sadly, the weather hasn’t been kindly inclined in recent weeks…

Spring’s Hardly Sprung

April was among the coldest on record, but it was also exceptionally dry for most places along a vast swathe spanning the UK to Turkey. Not too troubling for hospitality and retail. Unfortunately, for countries from the UK to Belarus, May has brought a much wetter run of weather, with milder nights but unseasonably cool days.

This is not good weather for those sectors. Another change is needed, sooner rather than later. Will summer 2021 deliver?

To assess the likelihood, I have identified a selection of ‘analogue years’ for guidance. These are ‘most relevant’ historical years chosen based on the behaviour of large-scale phenomena that drive our day-to-day weather patterns.

For each month, I’ve studied how the analogues compare to a long-term average (LTA), using two methods (see image annotations). Please note that like all long-range weather forecast methods, there is no guarantee that the predictions will prove accurate. However, the MetSwift analogues have proven skill, when compared to using a long-term average i.e. ‘how it’s tended to be lately’.

June: Hopeful for Many but Unsettling for the Mediterranean

For this opening month, the tercile probability (see below) suggests that the monthly mean sea-level pressure (MSLP) is likely to be low across some central parts of Europe. That corresponds to an increased frequency of wet, sometimes windy weather.

More notable, though, is a large area with low monthly MSLP favoured fully to the west of Europe.

Map showing the probability, based on the MetSwift analogue years, that the monthly mean sea-level pressure for June 2021 will be within the highest 33% of historical years 1970-present (yellow to red shading) or lowest 33% (blue to purple shading).

Map showing the probability, based on the MetSwift analogue years, that the monthly mean sea-level pressure for June 2021 will be within the highest 33% of historical years 1970-present (yellow to red shading) or lowest 33% (blue to purple shading).

In this area, large low pressure systems can become near-stationary for many days at a time. When this happens, hot air tends to advance northward across the western half of Europe. Signs are, this could happen more than usual in June 2021. It’s a theme reminiscent of mid-late July 2019 and the 1st half of August 2021. Both periods featuring intense, at times record-breaking heatwaves.

This signal is further supported when looking at the temperature and rainfall tendency:

The layout is warmer than average across much of western and central Europe, and wetter than average for much of the Mediterranean. This fits a ‘regime’ of high pressure being repeatedly displaced eastward from the Azores region by Atlantic low pressure systems. Each time, hot weather builds in intensity and northward extent, before a ‘breakdown’ brings a round of wet weather, usually with thunderstorms hitting some areas.

This outcome would be highly beneficial to hospitality and retail (where not locked down) in the north and west of Europe, spurring the public to splash out on seasonal goods and services (who’s up for some ice cream!).

At the other end of Europe, it’s a different story. When high pressure traverses the central belt of Europe, conditions tend to be unstable over the Mediterranean. That means frequent showery, at times thundery spells, and sometimes longer spells of rain. At least temperatures look respectable for the time of year!

In Scandinavia and eastern Europe, an drier but cooler than average June is suggested. That should mean enough fine weather to give hospitality and retail a helping hand.

July: Southern Sogginess Spreads Northward

The tercile probabilities for this month bear a striking similarity to those for June. Again, there’s the signal for low pressure systems to hang out to the west of Europe more often than usual.

Map showing the probability, based on the MetSwift analogue years, that the monthly mean sea-level pressure for July 2021 will be within the highest 33% of historical years 1970-present (yellow to red shading) or lowest 33% (blue to purple shading).

Map showing the probability, based on the MetSwift analogue years, that the monthly mean sea-level pressure for July 2021 will be within the highest 33% of historical years 1970-present (yellow to red shading) or lowest 33% (blue to purple shading).

Based on that, it appears July 2021 has potential to challenge high temperature records too.

However, the tendency view tempers expectations. The analogues suggest that both the west and southeast of Europe will see more rainfall than usual.

That implies more unstable conditions there compared to June, leading to frequent outbreaks of showers or longer spells of rain. The cooling rain mitigates the warming effect of large lows to the west of Europe. While the outlook could be better, it’s still looking plenty warm overall, so it wouldn’t be the worst of outcomes for the hospitality and retail sectors.

Such a situation to the south suggests that imports of hot weather from there to Northern Europe will be more likely to breakdown in a thunderous manner. By which I mean, via outbreaks of thunderstorms, sometimes severe. Even so, rainfall looks close to normal for most, so it could be another good month for helping economic recovery efforts.

August: More of the Same or a Decline in the North?

At 3 month’s range, there’s inevitably a lot of uncertainty over the details, but even so, a stark shift is evident in the tercile probabilities.

Gone is the low pressure hanging about to the west of Europe. Instead, it’s suggested to be unusually prevalent across the western and central Mediterranean, as well as southern England.

Map showing the probability, based on the MetSwift analogue years, that the monthly mean sea-level pressure for August 2021 will be within the highest 33% of historical years 1970-present (yellow to red shading) or lowest 33% (blue to purple shading).

Map showing the probability, based on the MetSwift analogue years, that the monthly mean sea-level pressure for August 2021 will be within the highest 33% of historical years 1970-present (yellow to red shading) or lowest 33% (blue to purple shading).

We can also see that the monthly MSLP is indicated to be in the upper tercile across Greenland and toward Svalbard. That means more high pressure than usual in that region. When that happens, low pressure systems moving east from the Atlantic tend to pass close to or over the UK.

Usually, this means plenty of high pressure across the Mediterranean – yet we see a contrasting signal above. To me, this dissonance implies two distinct outcomes with similar probability. The first is cooler and wetter in north-western Europe compared to June-July, while warmer and drier in the south and southeast. The second is a continuation of the preceding months’ pattern, making for a poor southern summer by Mediterranean standards.

Studying the tendency view, the pattern is more ‘scattered’ compared to those for June and July. More evidence of a split in favoured outcomes.

Even so, we can see a coherent signal for eastern Europe: Predominantly drier and warmer than usual, suggesting an abundance of fine weather there.

Meanwhile, it appears summer may end early in Scandinavia – but confidence is lower here. It’s lower still for western Europe, where we’ll need to wait for the June issue of the MetSwift analogues for a clearer picture.

Overall: Wetter South, Warmer North

At face value, most of the Mediterranean and some neighbouring countries appear to be in for a soggy sort of summer, at least until August. But we should bear in mind that ‘normal’ rainfall amounts are low in this season, especially July and August. Even a ‘wet’ summer tends to feature some long spells of dry, sunny weather.

Further north, the analogues suggest the main story to be more in the way or very warm or hot spells of weather than usual.

It’s also worth noting the persistent below-normal rainfall in the eastern reaches of the continent. Appreciated by hospitality and retail, but not by farmers, considering the raised risk of drought development.

Do External Long-Range Forecast Models Agree?

Around the world, there are several institutions which routinely release predictions from their in-house long-range forecasting models. Unlike the MetSwift analogues, these are numerical prediction models that attempt to resolve the way forward using complex mathematics to simulate physical processes.

Handily, Atmospheric Scientist and University of Reading PhD student Simon Lee has performed some detailed analysis on the latest predictions from these models.

He uses a ‘regime’ based approach, which categorises weather patterns based on leading modes of variability. Casting an eye over those, the MetSwift analogues are predicting a +SNAO, +SEA pattern for June-July, then either that again or a -SNAO pattern with either +SEA or -SEA for August.

Of the models Simon assessed, DWD (German source), JMA (Japanese) and ECCC (Canadian) favour the same pattern for the summer season. Meanwhile, ECMWF (UK) and CMCC (Canadian) favour a variant that has much more high pressure across north-western Europe (2018 style).

Coherent Predictions

I see that as a strong vote for the analogues’ wetter south, warmer north pattern. Both +SNAO, +SEA and +SNAO, -SEA can support such an outcome. The main difference is an unusually dry north-western Europe in the latter case.

For that region, this is a welcome prognosis for those whose business benefits from such conditions. I just happen to reside there and after such a cool and then rainy spring, can’t wait for the change!

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo by Unknown Author is licensed under CC BY-SA-NC

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