BBC monthly outlook
Mostly unsettled with a warm, dry spell later
Saturday 3 July to – Sunday 11 July
Unsettled and cool week with a few dry spells
Low pressure is set to move into the British Isles this weekend, bringing some heavy rain in places and chances for thunderstorms. A secondary low pressure system is expected to move in from the southwest early next week, making for a stormy few days for southern areas. Unsettled weather should persist throughout next week, although as the weather systems weaken later there may be a few sunny and dry spells mixed in too. Temperatures will tend to be near or a touch below average, with the coolest days expected around midweek.
Monday 12 July to – Sunday 18 July
More unsettled, cooler, and wetter weather
Through the middle of July, we expect that the most likely scenario is for a large-scale low pressure trough to persist over northern Europe, including the British Isles. This means that we should see a continuation of the cooler, wetter pattern as low pressure systems are often nearby or overhead. One complicating factor for the longer range forecasts this month is that the jet stream, a ribbon of fast-moving air in the upper atmosphere that helps drive weather fronts, is rather weak at the moment. This is normal for summertime, but it also means that low pressure systems that tend to move through and head east will instead stall and sit in place, lasting much longer. However, it also means these lows will tend to be weaker, so it shouldn't be particularly windy.
Our best chances for warmer weather will be if the winds shift out of the southwest or south. There's little support for southerly winds through mid-July, but south-westerly winds are possible if high pressure builds into Germany. The main risk to the forecast is that high pressure in the Atlantic may build stronger into northern Europe than we expect. This would be a drier pattern for southern and eastern areas although wetter weather would still be possible in the west and north. Temperatures would also be a little warmer, rising slightly above average especially for southern areas. Our shorter-range computer models are showing good support for this, but they have been tending to overestimate the extent of high pressure in Europe, so caution is needed. There is perhaps a 30% chance of this pattern developing instead.
Monday 19 July to – Sunday 1 August
Decent chance of a warm, dry spell later
For the second half of July the large-scale pattern may see a bit of a shake-up thanks to some tropical rainfall anomalies around Indonesia and Papua New Guinea. Historically, this supports high pressure in the Atlantic to shift eastwards across northern Europe and head into Russia in the following weeks. In recent months this has been a useful forecasting aid and brought a couple of warm spells, such as the one we saw during the first half of June.
We think the best chances for a warmer and drier spell will be in late-July as high pressure shifts overhead from the southwest before continuing eastward into Russia. This period will likely last around a week or so and see some prolonged dry, sunny, and warm weather for the whole UK. This would be a more south-westerly wind carrying sub-tropical Atlantic air, which is warmer than normal but not overly hot. There is a chance that we could see a hot southerly wind for a few days as high pressure shifts into Germany, but confidence is low on that.
Towards the end of the month and into early August as the high moves further east, low pressure will likely return from the northwest. This will bring some fresher Atlantic air and more unsettled weather back to the UK. The cooler weather will likely be the norm for this summer, with only temporary deviations to warmer, drier spells. Confidence is low for the second half of July due to the potential for the tropical rainfall anomalies failing to develop, or to fail to link to the European weather pattern. In this case low pressure will linger throughout the month with only brief dry spells as weak highs move through. There is a 35% chance of this developing instead.
Any more signs of warm and dry weather in the forecast? How could tropical rainfall patterns influence the weather pattern through late July?