Autumn heatwave on the way

Hottest day of the year is possible

By Brian Gaze

The meteorological autumn has started, but the weather will be taking on a very summery theme in the coming days. In fact, heatwave conditions are forecast in parts of the UK. Thresholds vary for different locations, but absolute values are expected to be the highest in the south where 30C is likely to be exceeded.

The very warm or hot period may be quite lengthy and there is a chance it will produce the UK's hottest day of the year. The cause of the heatwave is the jet stream migrating northwards, in part due to the impact of Hurricane Franklin, and high pressure building on its south side close to the UK. This combination causes exceptionally warm air to be pulled up from north Africa and southern Europe. The upper air temperatures which are being forecast could well be record-breaking for the United Kingdom in September.

How hot could it be?

Most computer model runs are indicating maximum temperatures reaching the upper 20s to low 30s Celsius in the southern half of Britain. In the north values are lower, which is what would typically be expected. 

However, the number of runs forecasting temperatures peaking above 30C has been increasing in recent days. What is more uncertain is whether the hottest day of the year will be recorded. The target is 32.2C which was reached on 10th and 25th June.

The chart above shows forecast maximum temperatures on Wednesday 6th September. Values of 32C are shown in a number of locations in southern and eastern England. Local variations aren't well picked up by a global model such as this one, so the chance of some locations pipping the 32.2C is high, if the general pattern is being correctly forecast.

How long will the heat last?

Medium range forecast confidence at this time of the year is lower than normal because of the impact of ex-hurricanes and tropical storms on weather patterns in the North Atlantic. Their interaction with the jet stream is difficult for computer models to handle and relatively small changes upstream can have a huge impact on the weather we experience in the UK.

Nonetheless, the ensemble models which are run many times to help account for uncertainty are suggesting temperatures will remain well above average for the first third of September. The chart below shows forecast air temperatures at about 1500m above sea level and rain amounts. Each line represents one individual model forecast. 

Most of the runs stay well above the 30 year average until 10th September, but beyond that a big spread develops. It suggests increasing uncertainty about possible outcomes. This means that cooler and more changeable conditions become more likely. 

Heat probabilities for different UK cities

The Will it be hot calculator combines forecasts from many computer model runs to give the percentage chance of temperatures reaching or exceeding a number of thresholds.

The table below shows the percentage chance of 25C and 30C being exceeded in a number of UK locations between Wednesday 6th - Saturday 9th September. As is expected, the chances for locations in the south of the UK are a lot higher than for those in the north. That is particularly true when considering the prospects for over 30C.  

Percentage chance of temperature thresholds being reached between 6th to 9th September

*Despite the low percentage values for the thresholds (over 25C and over 30C) shown in the table, the chance of temperatures reaching between 21C and 25C in Glasgow and Belfast is high.

The forecast is updated every 6 hours by using new computer model data. Follow the links above for the most up to date information or select any UK location Will it be hot.

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