In recent days and weeks, you will have seen a lot of talk about the UK breaking its temperature records and hitting 40C - a heat more commonly associated with north Africa than our green Valleys. Yet the projection of 40C, while still only one of a range of options for the weather in the coming days, is not just scaremongering.
Long-term weather forecasting is not a precise science, there are a number of different computer models which project where temperatures and weather conditions may head in the coming weeks and months. All of them have to incorporate so many variables and uncertainties that none alone is ever fully accurate.
When it first popped up on the computer screens of weather forecasters, the model projecting that in mid July temperatures might head as high as 43C looked like it was at outlier among many more milder forecasts. This is especially true given the fact that currently, the highest officially recorded temperature in the UK is 38.7°C, recorded at Cambridge Botanic Garden on 25 July 2019. Find out where and when it's going to be hottest in Wales this week here.
But as we have got closer to mid July, with Wales and the rest of the UK baking, more weather models are starting to show extreme heat on its way. While it still remains one of a range of possibilities for the coming days, the idea that temperatures may top 40C for the first time in the UK no longer seems so far fetched. In fact, the idea that it’s something wrong with the model causing the 40°C+ forecasts is now in the bin. The latest outputs from other models are suggesting something similar too.
In its latest forecast, the BBC said the heat wave in Europe was "now becoming more established" with temperatures already pushing above 40°C in Spain and Portugal. There are signals that that heat could push a "significant way north" towards Britain, it said.
Simon Lee, a weather expert who holds PhD and MMet degrees from the Department of Meteorology at the University of Reading has explained with welcome clarity exactly why the idea of 40°C in the UK is now a distinct possibility. It might be "seemingly unthinkable" for a country without widespread residential air conditioning but with anthropogenic global warming, 40°C is becoming more likely, Dr Lee said in a blog post.
Hitting 40°C could be seen as a big jump from 38.7°C. But it's not impossible: last summer, in June 2021, the Pacific Northwest heatwave obliterated local temperature records in Canada by as much as 4.6°C to reach a "mind-bending" record of 49.6°C in British Columbia. Dr Lee said: "To eclipse a prior record by such a margin shocked meteorologists and climate scientists alike, and showed what was possible in today’s world when all the favourable factors come together."
So what are those factors, and what are the chances of it happening in the UK?
Forecasters use a range of models to predict weather, including the NCEP GEFS model, a medium-range global forecast system for the UK. Eleven days ago, that model produced one output "unlike anything anyone had ever seen before". It showed 40°C and a huge area of southeast England eclipsing 39°C. Parts of east Wales were predicted to reach 29°C.
Dr Lee said no-one panicked at the time. He said: "That was just a single member of one ensemble forecast system. Ensembles are designed to capture a range of possible outcomes, and it’s common to see the odd member do something wild — especially beyond 10 days. At long lead-times, errors in the model can, given the right circumstances, occasionally lead to rapid error growth and produce unphysical outcomes."
So while the appearance of 40°C in that single ensemble member suggested it was a possible evolution over the next two weeks, it was also extremely unlikely. Like many, Dr Lee expected this would be the only time such an extreme temperature would appear in the forecast.
But it wasn't a one off and as the model produced more results, more of them were showing 40°C and even more broadly, more members started to show >35°C. Then came a "historic, unbelievable chart" with widespread temperatures of at least 40°C predicted for England on July 16.
As the models continue to run every day constantly refining and fine-tuning the forecast, the GFS/GEFS "continues to throw intermittent support to record-breaking heat for the UK somewhere around 15-17 July". Parts of south east Wales could see temperatures in the thirties while in the north east of Wales, the model indicated 33°C is possible.
And as mid-July gets closer, more and more models are showing extreme temperatures for much of the UK and a "big shift" from what might normally be expected.
As always though, each forecasting model is just one tool in the box to predict weather, which by its very nature is inherently difficult to predict. Dr Lee urged caution when interpreting the results and said: "Truly extreme heat in the UK is not yet the most likely outcome, even if the risk is likely the highest it has ever been. The median temperature prediction still sits just below 30°C. One forecast member is down in the low 20s — near the model average."
To reach 40°C in the UK requires a very specific set of circumstances to occur in order to pump the warmest air from Africa, through Spain and France, and then into the UK. This involves the development of a low pressure system west of Iberia and its subsequent northward drift during next week, with warm air advection on its eastern flank.
It's by no mean a given - in fact it's only a small possibility - but regardless of whether 40°C or even a new UK maximum temperature record is set this month, we have entered "uncharted territory in forecast-world", said Dr Lee. He added: "Eventually, this will translate to the real world, and with the global climate continuing to heat up, we can expect to see more situations where 40°C appears in forecasts for the UK in the coming years and decades."
He's not alone in his fears. Professor Nigel Arnell, from the department of meteorology at the University of Reading, said: "The current heatwave has certainly made the headlines. It’s not the first this year, and probably won’t be the last. And we’ll get more next year, and the year after, and so on. Our analysis1 shows that we are highly likely to get more and longer heatwaves in the future due to climate change, for the numbers of heat-health alerts, heat-stress days – days when it is too hot to work – and damaging heat extremes to increase."
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