BBC monthly outlook
Summary
Changeable for the rest of May, but fine by June
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Saturday 18 May – Sunday 26 May
A mixed bag for mid-May
Saturday will be rather wet and cloudy across Scotland, with outbreaks of rain pushing in from the east. This will keep it rather cool and dull, although England and Wales will see sunny spells and scattered showers develop, some of which will be heavy with the odd rumble of thunder possible. These will slowly ease overnight, leaving clear periods and a few mist or fog patches, although Scotland will continue to see some patchy rain. For many areas, Sunday will be the better day of the weekend, with more widespread sunny spells and temperatures a little higher than on Saturday. The sunshine will trigger some slow moving hit-and-miss showers through the afternoon, although many areas will also remain dry.
Next week will continue where the weekend left off, with light winds, sunny periods and some fair-weather cloud for many on Monday, but a few showers are also likely, some of which could again be heavy. More general rain may edge across Scotland from the east on Monday night and Tuesday, while the rest of the country will continue to see bright or sunny spells with just a few showers. Confidence is unusually low for the middle of next week, although it looks most likely that high pressure will build across the UK from the south or south-west. This will bring a lot of dry and calm weather, and it could become rather warm in the south, with any breezier and wetter weather limited to Scotland and Northern Ireland. However, there is around a 40% risk that low pressure becomes more widespread across the country from the west, bringing wetter and breezier weather for all. Either way, by next weekend, we should see a return to more widespread wet and breezy conditions nationwide.
Monday 27 May – Sunday 2 June
High pressure often to the south-west
Towards the end of May, it looks like high pressure will most often be centred to the west or south-west of the British Isles. This will allow occasional bands of rain and fronts to move south-east across the country, as areas of low pressure track east between Iceland and northern Scotland. All areas are likely to see some rain at times, although the driest and calmest weather is likely in southern and western Britain, where high pressure will most frequently ridge up from the south-west. This could also promote some rather warm weather to develop for a few days across southern Britain - although the warmth will likely be short-lived, as further fronts and Atlantic winds from the west or north-west bring cooler and wetter spells.
There are some indications that high pressure will begin to extend its influence north across Britain by early June, which should allow more widespread dry and calm conditions to develop (and prevent low-pressure areas bringing more widespread rain and winds). However, some uncertainty on the extent of low pressure across the country creeps into the forecast by this stage. This is most likely a consequence of intense thunderstorm activity which is expected across the central USA over the next few days. These thunderstorms can often generate large perturbations in the jet stream, the fast-flowing ribbon of air about 6- miles up in the atmosphere. These can often travel thousands of miles across the Atlantic towards Europe, and can be rather tricky for the computer models to accurately predict.
Monday 3 June – Sunday 16 June
Strong signals for intensifying high pressure
As high pressure continues to build across the UK during early June, there should be a lot of dry and calm weather to start the month, and temperatures should rise above normal for the time of year as warmer southerly or south-westerly airflows develop. In fact, southern parts of the country could see a couple of very warm days. It does look like the first half of June will continue this rather settled theme, with low pressure areas passing well to the north of the British Isles, and high pressure slowly shifting north towards northern Europe and Scandinavia. Many areas will be drier than normal; dry conditions may become widespread across Scotland and Northern Ireland, which are expected to remain closest to the high pressure centre.
It will be warmer than normal for all areas, although at present, it seems like heatwave conditions may struggle to develop. An area of lower pressure looks to develop to the south-west of Britain as we advance further into June. This could allow one or two heavy showers or thunderstorms to break out across southern areas at times, but these won't provide any meaningful rain for the gardens and fields which will no doubt be parched dry by this stage. Indeed, the main uncertainty is with regards to how far north this low pressure ends up; there is around a 30% chance that the low develops closer to southern Britain, bringing breezier and wetter weather to the south.
Further ahead
Will high pressure break to give a much-needed spell of rain to the UK, or will the expected fine start to summer continue?
https://www.bbc.co.uk/weather/outlook