August bank holiday weather

A very British affair?

By Brian Gaze

The summer has been very mixed. June was the hottest on record, but July was notably wetter, with the UK situated on the northern side of the jet stream for much of the time. August has been more typically moderate, although the Central England Temperature (CET) is provisionally running close to 1°C above the norm. What sort of weather can we expect for the August bank holiday weekend?

Typical British summer weather?

Computer models are suggesting that pressure across the UK will be fluctuating around the late August average. That suggests a risk of showers or longer spells of rain, but not a washout. The day to day weather is likely to vary and regional variations are expected.

The chart below shows the pressure forecasts from the GEFS ensemble model for York. The columns represent all of the forecasts for a 6 hour time period, with the first one being for the start of Friday, 25th August and the last one for the end of Monday, 28th August.

The green shading is used to show the percentage of runs forecasting pressure values below the average. The yellow shading shows a range of outcomes, but many of them are forecasting pressure to be at least a little higher than the 30 year average.

The key points to note are:

1) Most of the runs show below average pressure on Friday, 25th August

2) During the second half of Saturday and through Sunday the balance tips towards pressure being a little higher than average

3) By Bank Holiday Monday the trend is slightly downwards

Therefore, at this stage there is a slight bias towards drier conditions on Saturday and Sunday, with showers or longer spells more likely on Friday and Bank Holiday Monday.

Where will high pressure be centred?

Most computer models are showing high pressure centred to the west or southwest of the UK during the bank holiday period. The pressure chart below is from the ECM model and is valid for the start of Sunday 27th August. 

It suggests that dry conditions are more likely in the south and west. However, the extent of the influence of the high pressure area on the UK's weather is still uncertain. Completely dry conditions are very unlikely, and all regions could see showers or longer spells of rain at times during the bank holiday period. 

What is the risk of rain?

By combining the forecasts from a number of different computer model runs the Will it rain calculator gives the percentage chance of rain on a given day for a given location. At the time of publication the chance of rain for the bank holiday period is: 

London 30% to 48% (Latest London update)

Manchester 70% to 79% (Latest Manchester update)

Newcastle 55% to 76% (Latest Newcastle update)

Cornwall 42% to 70% (Latest Cornwall update)

Lake District 67% to 7% (Latest Lake District update)

Glasgow 45% to 67% (Latest Glasgow update)

Barbecue and beer garden weather?

The possibility of showers or longer spells of rain at times means that it will be important to keep an eye on the short range weather forecasts if you are planning to sit outside in a beer garden or have a barbecue at home. However, even if it is dry, will it be warm? 

The chart below is in a similar format to the pressure forecasts table, but this one is for London and it shows daily maximum temperatures. Most of the runs are projecting forecast maximums between 16°C and 20°C, while a significant number indicate temperatures between 21°C and 25°C. Therefore, very warm or hot weather is not looking likely. Temperatures are expected to fluctuate around the average, perhaps a little below it during showers and cloudy periods, but slightly above it during longer, sunnier periods.

In conclusion, things are looking quite mixed. At this stage the details are still subject to change and there is uncertainty about the extent of the rain risk. The most likely outcome is that the weather will be characterised by fairly average late summer conditions, which would be in line with much of August so far.

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