Insights & News

August Scorcher: Big West. Europe Heatwave Incoming

3rd August 2020

In what’s fast becoming an annual event, a spell of exceptionally hot weather is expected to affect a large part of Europe. Only last year, an extreme late June heatwave set numerous high temperature records in France. In the south, a staggering new national record of 45.9°C (114.6°F) was set.

One month later, a second intense heatwave set new national highest temperature records in the UK, Belgium, the Netherlands and Germany. All this, in a summer that was overall less hot than the one preceding it. 2018’s was in fact the hottest on record for Europe as a whole. The year before that, Spain set its still-standing national record of 47.3°C (117.1°F; in Montoro).

Even before this incoming heatwave, 2020 hasn’t been without its heated moments, either. During the final week of July, hot weather developed over Western Europe and then surged northward across England. The result was the third hottest temperature on record for the UK: 37.8°C at Heathrow (London).

That makes it the 2nd year in a row 100°F has been reached in the UK, after 38.7°C was observed 25th July 2019. Never had such a feat been reliably recorded before. As shown below, the UK is seeing increasingly short intervals between summers with a peak temperature of 35°C or more. As the 20th Century began, it was a once-per-decade phenomenon. Now, it’s happened in 5 of the past 6 years!

Chart showing the number of years between each occurrence of temperatures exceeding each of three thresholds. The dashed lines show the linear trends.

Chart showing the number of years between each occurrence of temperatures exceeding each of three thresholds. The dashed lines show the linear trends.

How Hot This Time?

In a word: Very.

We’re talking daytime maximums widely in the mid to high 30s °C across France, Spain and Portugal. Locally into the low 40s. Not just for a day either – at least three, probably more than that (perhaps well into the week of 10th Aug). The Netherlands and Belgium also look very hot, widely reaching the low to mid-30s.

There’s a good chance some of this heat spreads across southern England for at least a few days starting 6th or 7th. Here, temperatures aren’t likely to reach so high, but could still hit the low to mid-30s widely. Somewhere could conceivably reach into the 37s°C again as soon as Friday – only a week after it last happened. I honestly can’t believe I’m writing that!

Will More Records Fall?

In the mainland countries I’ve listed, temperatures will likely come close in some spots. But the weather forecast models suggest that national records are unlikely to be challenged (Portugal’s is a blistering 47.4°C (117.3°F), set 1st August 2003, The Netherlands’ 40.7°C and Belgium’s 40.2°C).

Having said that, it’s hard to anticipate this far in advance where localised ‘hot spots’ resulting from local interactions of the airflow with the land. It’s too uncertain exactly what those fine-detail airflows will be.

For example, the France record in 2019 was made possible by air descending the southern slopes of the Alps and becoming hotter (known as the foehn effect).

So, new national records can’t entirely be ruled out. At the very least, some more local (county / province) records are likely to fall. Heatwaves vary greatly in where the highest temps occur.

Regardless, this heatwave will bring the usual challenges of very hot weather to health and productivity. Thankfully, the provision of air conditioning is much better than it was even two decades ago, but there are still exceptions. Despite seeing not so high temperatures, southern England may see some of the strongest socioeconomic impacts, as air conditioning is much less common in households there. Not only that, but the traditional housing design is one that works to trap heat, not release it, or deflect it away.

What’s Behind This One?

The two charts below summarise the situation at hand, weather-wise. There will be an unusually strong area of high pressure (for the time of year) stretching east from the UK, right across Scandinavia. On the southern flank of this, a broad airflow from the east will occur. This gives that air a long time to respond to sun-heated surfaces each day before it reaches western Europe. It’s why the highest temperatures look to occur in the westernmost countries.

Charts showing an ensemble mean model forecast for mean sea level pressure (left) and surface temperature anomalies (right) for the period 6th-11th August.

Charts showing an ensemble mean model forecast for mean sea level pressure (left) and surface temperature anomalies (right) for the period 6th-11th August.

It’s a big deal to see a pattern like this at this time of year. It’s the point at which the long-term average reaches its warmest. This is when the balance between ocean and land heat build-up and day length is at its optimum for producing the highest surface temperatures.

That’s why we’re looking at an exceptional heatwave and discussing the potential for high temperature records.

…and What’s Behind That Pattern?

Until late in the month, July 2020’s European weather pattern was strongly driven by vigorous tropical thunderstorms over the western Indian Ocean. This is represented in the left-hand plot below by the Madden-Julian Oscillation (MJO) being in phases 1 and 2. Because of this, a measure known as global atmospheric angular momentum (GLAAM) was driven to very low values (see below-right). Historically, this has corresponded to cool, changeable July weather for western Europe – especially the UK. July 2020 was little different in that respect… until the last week of the month.

Charts showing observations of the Madden-Julian Oscillation (MJO; left) and Global Atmospheric Angular Momentum (right). In the MJO chart, anticlockwise movement of the observation points and line represent eastward movement of an area of strong(er than usual) tropical thunderstorms.

Charts showing observations of the Madden-Julian Oscillation (MJO; left) and Global Atmospheric Angular Momentum (right). In the MJO chart, anticlockwise movement of the observation points and line represent eastward movement of an area of strong(er than usual) tropical thunderstorms.

See how the MJO suddenly moved across (eastward propagation) to phase 3. This set in motion a large rise in GLAAM, in fact one of the biggest one-week climbs ever observed. The closest historical analogues for this show that such an event causes high pressure to extend north-eastward from the Azores islands, toward Scandinavia. Very much as is being forecast for later this week.

Hope for the Best but Prepare for the Worst

When an intense heatwave is forecast for your area, it’s a good idea to make special preparations. Ensure you will have plenty of drinking water available. See what arrangements can be made to help keep you cool during the hot weather. If you have air conditioning, be prepared for the possibility that it fails or there’s a power outage.

Take care, everyone.

James Peacock Msc
Head Meteorologist at MetSwift

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

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