The hottest areas of the UK - namely southern England and parts of Wales - will see the thermometer rise to 38 or 39C, and this becomes particularly uncomfortable when you factor in the humidity.
Humidity is a measure of how much moisture is in the air.
During hot weather our body controls our internal temperature by sweating and as the sweat is evaporated from our skin, it cools the air directly next to us, making us feel cooler.
If the air is more humid, the sweat from our skin cannot evaporate and cool us down as effectively as it would do if the air was dry.
Meteorologists can calculate therefore how the combination of air temperature and humidity feels – as a "feels like" temperature.
While the air temperature in the coming days might be 35C for example, it may actually "feel" more like it's 41C.
With temperatures nudging 40C, there have been some comparisons to the historic heatwave of July 2022 when the UK recorded 40C for the first time.
But during that heatwave the air was very dry. So, while headline temperatures were higher then, this June's heatwave will feel hotter and potentially have more health impacts as a result of the humidity.
Overnight temperatures will remain high this week and some locations will not get cooler than 20C. These are called "tropical nights".
It is likely we will see the warmest June night on record on Thursday night as minimum temperatures across Wales, the Midlands and the South East are not expected to fall below 21 to 23C.
Some places could be warmer than the current UK and England record of 22.7C set in 1979, and the Wales record of 20.3C - set just this week.
"Feels like" temperatures may be even more noteworthy during the overnight period, when it may feel like 27C even at the coolest part of the night.
This is a major distinction to previous heatwaves in the UK.
During the hot spell of July 2022, for example, much of England and Wales experienced tropical nights - though few places had more than two consecutively.
In this heatwave, some locations may see three or four tropical nights in a row which becomes far more impactful.
This means the body will not be able to cool down in between daytime spikes in heat.
Though this is particularly dangerous for vulnerable people, it can put healthy individuals at risk too, leading to poor sleep, cardiovascular stress and a greater risk for heat exhaustion in subsequent days.
As the planet continues to warm, heatwaves like this will only become more common.
"Unless we drastically cut down the emission of greenhouse gases, global warming is not going to stop. In fact, it will accelerate," Deoras said.
"A warmer atmosphere holds more moisture, so the combination of increased temperatures and humidity is what is likely to make humid heatwaves like this become more frequent and more intense."
