BBC monthly outlook
Summary
A cooler, wet end of spring and start of summer.
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Wednesday 12 May – Sunday 16 May
Cool often showery week. Occasionally cloudy.
The rest of this week is expected to continue much as it started, with a high pressure system in Russia preventing low pressure over the UK from moving away to the east. This means the weather will tend to be unsettled and cooler than normal, but also not particularly windy.
Afternoon showers will be likely most days with some thunderstorms mixed in too. A few days, namely Thursday and potentially Saturday, may see some longer spells of rain and cloudier skies. Although temperatures are a bit below average, afternoons will still feel warm with the higher sun angles of May.
Monday 17 May – Sunday 23 May
Cooler, showery, and unsettled weather.
Next week, and the final full week of May, we expect the high pressure system in Russia to linger, keeping a broad area of low pressure overhead. These highs are often called "blocking highs" since they act as roadblocks and prevent weather fronts from moving east. Instead, low pressure will remain in place and keep things unsettled and showery for the UK. In addition, the jet stream, a ribbon of fast-moving air in the upper atmosphere that drives weather systems, will be displaced slightly to the south of us.
This is the main reason for the below average temperatures, as cooler air that is normally near Iceland is now over us instead. Around the middle of next week there is potential for a deeper low pressure system to send some weather fronts into the UK from the west. This would bring some widespread bands of rain and stronger winds for a few days. There is still some uncertainty on how far north the low will be. There's potential for some warmer air from the sub-tropical Atlantic to reach southern areas for a few days too, bringing temperatures briefly above average.
Confidence is medium for next week, mainly due to the low pressure system around midweek. We have high confidence it will be a cooler, wetter than normal week for most mixed with some sunny spells in the afternoons. The main risk for the forecast is that high pressure in Russia shifts too far west and reaches Poland or Germany. This would send warmer air into the UK from the south, and while it would still be wet it would also be much warmer with temperature a few degrees above average. There is perhaps a 30% chance of this developing.
Monday 24 May – Sunday 6 June
Staying rather cool and wet into early June.
For the end of May and early June, confidence is a bit lower, especially for temperatures. The computer model forecast skill is very poor at the moment due to the models struggling with high pressure in northern Europe. We expect the blocking high in Russia to tend to be a tenacious feature, but also gradually shift westwards with time. This will be a very slow movement and likely take weeks for the high to even reach eastern Europe. What this means for the UK is that low pressure troughs will tend to be nearby or overhead, keeping the weather unsettled and feeling cooler than normal for the start of meteorological summer.
We also expect to see occasional strong low pressure systems track in from the Atlantic. These will bring some wetter, windier spells with a chance for some brief warmth too. We are leaning more on our historical analogues for this range of the forecast. This is where we examine previous years with similar large-scale weather patterns to what we're seeing this year. 2001, 2012, and 1996 are proving to be useful years, with 2012 and 2001 being cool and wet while 1996 saw a brief heatwave in early June. 1996 highlights our main risk scenario, which is that high pressure shifts into Central Europe and send hot African air into the UK from the south. This could result in some well above average temperatures for several days. There is perhaps a 35% chance of this developing in early June.
Further ahead
We will consult some of the latest data to see if our risk of warmth in early June has increased and when we might break out of our cooler, wetter spring pattern.
https://www.bbc.co.uk/weather/outlook