Record breaking June heatwave

The period from 22nd to 26th June is expected to produce record-breaking heat in the UK, and more widely across western Europe. The existing June record in the UK is 35.6C, set in 1976. It is likely to be beaten, possibly by a large margin. What is the immediate cause of the heatwave and how will it break?

Heat pushing up from France

A slow-moving area of high pressure over western Europe has formed a heat dome. The air underneath it is descending and being compressed, causing it to warm. In the coming days, that exceptionally warm air will filter northwards across the UK.

Southern and central regions will see the highest temperatures, but the warmth is set to spread northwards, with temperatures well above average in Scotland and Northern Ireland too.

How hot will it be?

Every day there are many computer model simulations, each one providing forecast temperatures. Therefore, a range of outcomes is presented and, even at relatively short lead times, there can be significant variations. The emerging consensus is for temperatures to peak between 35°C and 40°C, with 38°C or 39°C the most likely outcome. Were that to happen, the existing June record set in 1976 would be beaten by a large margin.

It's worth noting that, at the very top end, some models have shown temperatures peaking at 42°C. This is unlikely, but not completely out of the question. If it happened, a new UK temperature record would be set, as the current one is 40.3°C, set on 19th July 2022.

High levels of humidity

It will feel particularly oppressive, even compared to the 2022 heatwave, which saw temperatures exceed 40°C for the first time in the UK. Dew points, which are a measure of humidity, will be extremely high.

In July 2022 they were in single figures, but on Wednesday and Thursday they are forecast to be as high as 22°C in places.

When will the heat peak?

Most models are showing the heatwave peaking on Wednesday or possibly Thursday. Confidence that this is correct is quite high, but it is likely to remain hot in eastern and perhaps central areas on Friday. Through the weekend, temperatures should continue to fall.

How will the heatwave break?

The catalyst for the heatwave to break will be an area of low pressure pushing in from the west. It will lead to cooler Atlantic air gradually returning eastwards. As pressure falls, the risk of showers and thunderstorms will increase.

The details of this are uncertain, and sometimes this type of breakdown isn't as clean and well-defined as earlier modelling can suggest. In particular, the cooler air can take time to return to the south east and East Anglia.   

Will there be more heatwaves this summer?

It is still only June, and statistically the hottest weather in the UK is more likely to occur in July or August. That said, it is unlikely that an even hotter heatwave will develop in the coming weeks.

However, further very warm or hot periods may be more likely than usual, as there is a reservoir of heat over the continent and sea surface temperatures around the UK are above the norm. The evolving El Niño phase in the Pacific is likely to have an impact on global weather patterns, but how that maps to western Europe isn't always well defined or straightforward.