5th August 2023
By Brian Gaze
Summer 2023 has been a season of extraordinary contrasts. June was confirmed as the hottest on record by the UK Met Office, while July, taken across the UK as a whole, was the sixth wettest on record. The Central England Temperature (CET) was 16.1C, compared to 17.0C for June.
August began where July left off, with an unusually deep area of low pressure for the time of year becoming the UK’s first named storm since February. Storm Antoni brought another period of wet and very windy conditions to much of the country.
Will the topsy-turvy weather continue through the rest of August, or will summer return?
The unsettled weather in July and early August was caused by the position of the jet stream, which took a more southerly track than normal for this time of year, steering a conveyor belt of low pressure systems across the UK. However, there are signs of change in the coming weeks. The jet stream forecast for August 13th shows it to be weaker and more fragmented than recently, and further north, placing southern and central regions on its warmer southern side.
The weakening jet stream suggests a greater chance of drier and warmer periods developing.
As the jet stream migrates northwards, high pressure pressure centred to the southwest of the UK over the Azores is able to start building northeastwards. Therefore, it begins to have more influence on the UK's weather, leading to more settled periods, especially in southern and central regions. Just how strong is the signal for high pressure?
The data table below summarizes the daily pressure forecasts for York from all of the individual runs in the GEFS computer model, reaching out to August 20th. Most runs forecast pressure to be between 1011mB and 1025mB, with relatively small minorities going higher and lower.
Similar data table for locations further south, such as London, suggest slightly higher pressure being the likely outcome.
Nonetheless, although forecasts indicate that pressure will be higher than recently, they don’t suggest high pressure becoming completely dominant across the UK. Areas of low pressure pushing in from the Atlantic could still have an influence, particularly in the north and west.
The jet stream and pressure forecasts point towards more settled periods developing at times this month. However, showers or longer spells of rain remain likely, particularly in the north. The data table below shows the rain forecasts from all of the runs in the GEFS model between 12:00 - 18:00 GMT for the next 16 days. It gives an indication of the likelihood of rain and the possible amounts.
The risk of rain on a given day increases again after August 10th, but the data suggests it is unlikely to become as wet as it was in much of July. It could be summarised as being more typical for a British summer.
Another consideration is the distribution of rain. The chart below shows the percentage chance of 5mm or more of rain falling on August 15th. The greens and oranges over northern and western parts show a chance between 40% and 60%, while white and light blues covering much of the south and east put it at between 0% and 25%.
The same basic pattern is indicated on the following days: significant amounts of rain are more likely in the north and west, with drier conditions in the south and east.
At this time of year, drier and sunnier conditions are very likely to lead to higher temperatures during the days. The graph below gives some indication of what values can be expected in the south, but locations further north are likely to be cooler.
Of course, this graph is showing forecasts and there is no certainty that they will be correct. If they are accurate, maximum daytime temperatures in the mid to upper 20Cs look probable between August 9th and 12th, which would be a big contrast with recent times. Temperatures then dip but start to climb again from mid-month, indicating another pulse of summer warmth.
So after a June heatwave and July washout, it looks like summer will return at times during August. The chance of dry and warm periods is higher than it has been in recent weeks but a completely settled pattern is not likely. Showers or longer spells of rain remain possible, with wet periods being more frequent in the north.
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