Experts say a second 40C peak is a possibility in an extreme August heatwave 

The UK could be in store for another extreme heatwave within weeks as climate models that correctly predicted this month’s record temperatures suggest they could reach 40C again in August.

A long-term outlook by a major US forecasting system has shown temperatures could reach 40C in Greater London and parts of the East of England in early August.

Temperatures across the south of England as a whole could hit the high 30s, according to the forecasts.

However, experts stressed the 40C prediction has only been made in two of the 31 individual forecasts that make up the overall US National Centers for Environmental Prediction (NCEP) modelling.

The NCEP system is designed to cover many different variations, but was the first to predict record-breaking 40.3C temperatures seen in the UK earlier this week with one of its 31 forecasts.

It correctly predicted the temperature would exceed 40C – although the specific forecast, of 43C, did overshoot the mark.

While it is far from certain temperatures will hit 40C again in August, experts say the forecast should be taken seriously as both an indicator that we could see extreme heat next month, and as further evidence of a steep rise in underlying temperatures.

“This is the world we are in: having seen 40C in reality, we can also expect to see it appear as a possibility more frequently in ensemble forecasts,” Simon Lee, an atmospheric scientist at Columbia University in New York, and co-editor-in-chief of the Royal Meteorological Society’s Weather journal, said.

The forecast is from the GEFS, or Global Ensemble Forecast System, a weather model created by the National Centers for Environmental Prediction in the US.

It generates 31 separate forecasts – ensemble members – to address underlying uncertainties in the data, such as limited coverage of sensors or biases in instruments.

Dr Lee said: “What we’re seeing in some forecast model output has unsettling similarities to what we saw at the start of July – when 40C appeared in these forecasts for the first time.

“Back then, it was perhaps easier to discredit these scenarios as simply too extreme and too unlikely to warrant much further consideration – but then the extreme scenario became a reality on Tuesday 19 July. 

“That changed everything, because now we know from observations that temperatures above 40C are achievable in the UK in the current climate. Previously, this had only been demonstrated as a possibility in climate models.”