Heading over 40°C? Long-term trends need a pinch of salt

Heading over 40°C?
Long-term trends need a pinch of salt

Temperature map of England showing widespread extreme heat, with temperatures reaching 39°C near London, 37°C near Norwich, and over 30°C across much of the country, alongside a large warning symbol indicating dangerous conditions.

Another heatwave is likely, but hitting 40°C is less so.

Once again, weather models are being posted online showing widespread regions of the UK passing 40°C at the start of August, and once again these need some extra context.

While some reports are writing of the next severe heatwave, it is unlikely to be exactly what is currently stated. Beyond three to five days it gets incredibly difficult to pinpoint specific weather conditions. So, posts suggesting highs of 40 or 41 °C around August 1st should be approached with caution.

What is actually happening? These posts are using one extreme output of one weather model. Beyond the five day mark a forecast is no longer possible, only a trend of where conditions are roughly expected to reach.

The particular 40°C+ claim on August 1st is an extreme scenario which is unlikely. If several model runs point in this direction, it becomes more reliable. This is not the case currently.

At this point we know that we are likely to see another hot spell, very likely becoming the fourth heatwave of the season, but we do not yet know how hot it will be.

Forecast vs trend

Forecasts focus on specific conditions in the short-term, usually accurate up to five days. Trends use data and probabilities to determine a more general outlook for a longer timeframe without details.

Trends are updated regularly with new data, and can change many times before we get to the dates mentioned.

It is best to remember that the details will only become clear as we get closer to the date. Keep an eye on the 14 day weather card and the TemperatureRadar in the meantime.

Ryan Hathaway