BBC monthly outlook
Saturday 5 May—Sunday 13 May
A dry and warm Bank Holiday but cooler by midweek
Western and northern Scotland will often be cloudy on Saturday with occasional light rain or drizzle and hill fog, while banks of low cloud and mist may affect western Britain at times. Elsewhere, it will be warm with light winds and lots of sunshine. Over the next few days, high pressure will dominate, so there will be a lot of dry weather but north and north-west Scotland will have a chance of rain on Sunday and perhaps again on Tuesday, with occasionally fresh winds. However, much of the country will be dry and warm with lots of sunshine and temperatures perhaps reaching 26 or 27C in the south-east on Monday and Tuesday.
After the high pressure and warm weather this weekend and early next week, a return to a west or south-westerly airflow from the Atlantic can be expected later in the week. Frontal systems, bringing bands of cloud and rain will attempt to invade the country from the north-west, perhaps bringing some remnant cloud and patchy rain into south-east England at times too. The wettest and windiest weather, as well as coolest temperatures, will be reserved for Scotland, with temperatures around or perhaps slightly above average elsewhere. By the weekend, it looks like high pressure will attempt to build across the southern half of the country from the south-west, attempting to bring drier conditions, although there is still the potential for some showers or patchy frontal rain to affect western Britain for a time. Temperatures will still remain around average.
Monday 14 May—Sunday 20 May
High pressure dominating - dry, settled and warm
During the beginning of the following week, high pressure is expected to ridge towards the UK from the south-west; there are then robust signals for high pressure to dominate through the middle of May, not just across the UK but across much of northern Europe and Scandinavia too. Such a 'blocking' high pressure area often becomes very stubborn to move anywhere very fast, and this certainly looks to be the case. There will be a lot of dry weather with light winds too, with Atlantic fronts kept at bay well to the north and west of the UK. The only meaningful rainfall may come from slow-moving thunderstorms edging towards the UK from the near Continent. These may produce locally torrential downpours across southern, and perhaps at times, central areas but many areas are expected to remain dry. Therefore, this does give a risk of some heavy, thundery showers for the Royal Wedding on the 19th although statistically speaking there is still a greater chance of it remaining dry. Some early morning mist may develop at times inland although this should soon dissipate once the Sun rises. Additionally, temperatures are expected to be above average for the entire country. The warmest spots will be in the south, where already warm air will waft in our direction from France, Germany and the Low Countries. There is a small chance of some low cloud and mist lapping at North Sea coasts of northern England and eastern Scotland, should winds veer a little more to the north-east, although this appears unlikely at present.
Monday 21 May—Sunday 3 June
High pressure still firmly in the driving seat.
The pattern of high pressure dominance across the UK and much of N Europe is expected to continue, with current indication for the centre of the high to drift east from Scandinavia towards the UK. Low pressure areas and associated rain bands are likely to remain well to the south of the UK. The greatest risk of thundery showers or thunderstorms would remain across the southern half of the UK, where high pressure is expected to have less of an influence and a rather summer-like pattern of convective precipitation may develop here. Somewhat breezier conditions may develop at times across southern Britain too, as an easterly airflow blows around the southern periphery of the high. Often fine and settled weather with light winds can be expected for much of northern Britain, while temperatures are likely to be above average for most of the country. How warm it will get depends on a fine balance between the wind direction and wind strength - any easterly wind flow will likely bring cooler temperatures, especially to eastern Britain, as the waters of the North Sea typically remain rather cold well into May. Model ensembles suggest a 25% chance of low pressure located close to Iceland becoming more influential, allowing a cooler and more 'progressive' westerly airflow from the Atlantic to move across the UK. This would bring wetter, windier and cloudier conditions for most. However, the signal from the models over the last few weeks has been remarkably consistent with high pressure dominating for much of May - and so it would be tough to be against them.
Next Update
What will June, the first month of Meteorological Summer have to offer on the weather front?
https://www.bbc.co.uk/weather/outlook