BBC monthly outlook
Summary
Cooler weather giving way to a warm end of summer.
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Wednesday 11 August to – Sunday 15 August
Unsettled, slightly cooler finish to the week.
A low pressure system pushing into the UK from the west this week will keep things unsettled and feeling a little cooler than normal for mid-August. For northern areas, the low pressure centre will bring some bands of heavy rain, strong winds even up to gales on some Scottish coasts, and chances for heavy, thundery showers. Southern areas will tend to be breeze with a stalling cold front keeping it cloudier with patchy rain. Towards the end of the weekend clouds should begin to clear as the low heads east into Scandinavia.
Monday 16 August to – Sunday 22 August
Wet and cool to start but turning dry and warm.
As we make our way into the second half of August (and the final few weeks of meteorological summer), low pressure is likely to become increasingly less influential throughout the week as it shifts away into Scandinavia. High pressure is expected to build in from the southwest, and for the first half of the week this means we will likely see some cooler northerly winds with the wettest weather in the eastern half of the country. From midweek and through the second half of the week, high pressure is expected to be more influential and shift overhead, bringing some increasingly dry, sunny, and warmer days. Temperatures will gradually climb a bit above average for most areas and as we approach late August we can expect some decidedly more summer-like weather for most.
The timing of the arrival of the high is still a little bit uncertain, and our computer models have recently been in some major disagreements on the exact day. However, we are starting to see some encouraging signals for high pressure to arrive in the second half of the week, with cooler, wetter weather for the first half. There is a roughly 30% chance of high pressure arriving a bit earlier, perhaps even as early the start of the working week. This would mean dry, sunny, and warm weather would be present throughout the entire week instead. Confidence is still medium overall due to the inconsistent signals for the timing of high pressure arriving, but we have high confidence that it will arrive eventually and be in place by the weekend.
Monday 23 August to – Sunday 5 September
Warm and dry for a time. Cooler into September.
High pressure is anticipated to arrive late August, and once it moves in, we should enjoy some more summer-like weather for a time. The high will be an extension of the sub-tropical high that lingers near the Azores through summer, so it will bring in some warmer air and sunnier skies. As it sits overhead for a few days, temperatures should quickly climb above average. This final week of August brings the best chances for a lengthy stay of high pressure and with it a slight risk of a heat wave. It's still a bit difficult to pin down the heat chances at the moment, largely due to the persistent inconsistencies in the computer models for this time frame.
In light of the poor performance of the computer models, for September we are leaning more on our historical analogues for guidance. These are where we look at previous years which saw similar large-scale weather patterns around the globe to what we are observing now. For the spring and summer months, some unusually warm waters in the North Pacific and North Atlantic have been proving to be a useful tool, and they support more low pressure systems and a more active storm track for the first month of meteorological autumn.
Therefore, we are anticipating that high pressure later this month will not tend to linger, but rather shift away after around a week or so and build in the Atlantic. This would send low pressure into Scandinavia again, so while this isn't a particularly wet pattern for us, it is still cooler with rain chances increasing in the north and east. September is likely to see more cooler, wetter weather develop later on, but the timing of the high's departure is the biggest uncertainty.
Further ahead
High pressure looks likely in late August, so we will try and determine the risk for late summer heat waves or early autumn rainfall and coolness with better confidence.
https://www.bbc.co.uk/weather/outlook