Computer models are showing the possibility of extreme heat affecting parts of the UK during the middle of July. A number of simulations are forecasting temperatures to reach or exceed 35°C and there is a chance of a new UK all time record being set.
Most of the data is presently suggesting the hottest day is likely to be on the 17th or 18th, but there is still much uncertainty about how things will develop. Temperatures in parts of southern Europe have been at record breaking levels and some of that heat could be heading northwards. However, if it is to reach the UK then a number of things have to align and small adjustments would produce a very different outcome.
The highest temperature recorded in the UK was 38.7°C at Cambridge University Botanic Garden on 25th July 2019. It beat the previous record of 38.5°C, set at Faversham, Kent, in August 2003.
The chart below shows pressure patterns and air temperatures at approximately 1500m above sea level on 25th July 2019. The combination of low pressure to the west of the UK and high pressure to the east sucked up the extreme heat from southern Europe. It is similar to the pattern which could develop at times through the middle third of July this year.
Air mass temperatures on 25th July 2019
Air mass temperatures were close to 23°C in south eastern Britain. In the west values were lower as the plume of heat began to be pushed away eastwards.
A new record is possible and some computer model runs show the current one being smashed. The chart below is from the GFS 06z operational run on 9th July 2022. It shows forecast maximum temperatures on 17th July 2022. Values in much of central and eastern England are above 40°C with a maximum of 43°C.
It is not impossible that due to the relatively coarse resolution of the model grid temperatures could locally be 2C higher. That would be a truly astonishing outcome with the existing record being beaten by 4C or more.
However, the GFS 06z run was an extreme solution. Ensemble computer models are used to help determine how likely different outcomes are. Essentially, they are the same computer model run over and over again with the starting conditions tweaked slightly to help account for uncertainty about the starting state of the atmosphere. Although huge amounts of data are fed into them their starting point can only ever be an approximation of real world conditions.
The chart below also shows forecast maximum temperatures on 17th July. It was generated from one of the GEFS ensemble runs and it is a world away from the GFS outcome. Maximum temperatures of 25°C are being forecast and they are quite localised too. Therefore, a massive difference of 18°C from the GFS 06z chart above.
The run used to generate the chart above was probably the coolest run in the ensemble on July 17th. Therefore, it isn't representative of the most likely outcome. Nonetheless, it is a possibility and can't be entirely discounted.
With such big differences being served up by the computers what is the most likely outcome? One way of considering that is to use the forecast mean which is generated by averaging out the forecasts from all of the runs in the ensemble. The chart below is what is shows. Maximum temperatures are 30°C, so still in the hot category, but not the extreme one.
Unfortunately, there is another caveat to take into account. The ensemble mean is not necessarily representative of the most likely outcome. To take a very simple example, assume there are only two runs in the ensemble with one forecasting temperatures of 10C and the other 30°C. The mean of 20°C is something which neither of the runs suggested. However, due to the number of runs in the ensemble being much greater than two (effectively 32 in the GEFS) the mean often does provide useful guidance.
Perhaps a better approach is to look for is clusters of runs. In this instance it could be the number which are showing temperatures exceeding 30°C on July 17th. The data table below showing maximum forecast temperatures from all of the runs in the ensemble for the Greater London area provides this information.
London is a good location to check because it often the hottest part (or close to it) of the UK. The data table shows that 52% of the runs are forecasting temperatures of above 30°C on July 17th. That's a very high percentage at this range and 8 of them have temperatures exceeding 35°C. That equates to a 25% chance and bear in mind that maximum values locally often reach 2°C higher for the reasons discussed above. Therefore, the chance of extreme heat is significant, but far from certain.
Note: On 12th July 81% of runs are forecasting temperatures of over 30C, but most of them are in the low to mid 30°Cs. Therefore, although it is very likely to be hot, the chance of record breaking heat is low.
Computer models are pointing towards a reasonable chance of extreme heat affecting the UK for a time during the middle part of July with the 17th and 18th being the favoured dates. The raw data from the ensemble models suggests a 25% chance, although with the tendency to undershoot temperatures the actual likelihood is perhaps closer to 35% to 40%. A new UK temperature record is very much possible, but it isn't yet the most likely outcome.
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