BBC monthly outlook
Summary
Easter warmth easing this week. Changeable weather
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Wednesday 24 April—Sunday 28 April
Warmth fading, and turning more unsettled
After a glorious spell of largely dry, sunny and very warm weather over the Easter weekend, changes are afoot in the atmosphere now that will gradually introduce more unsettled weather back across the UK. Recent days have seen a vast area of high pressure over Scandinavia and central and eastern Europe influencing the UK's weather, feeding in warm air from the south-east. Low pressure has been gathering strength over Spain, Portugal, Biscay and north-west Africa and it is this feature that will introduce the weather changes over the UK this week. The main uncertainty in recent updates has been how quickly and how far north-east this feature would push up across the UK, but there is now much better agreement in the forecast.
During the middle of this week, there is a slight risk that rain showers may bring some Saharan dust deposits, especially over central, northern and eastern areas. You may see these on your car after a shower of rain. This is due to recent strong winds and thunderstorms over the northern Sahara, whipping up the fine Saharan sand and dust into the lower atmosphere, before being transported northwards by the aforementioned Spanish low pressure area. Temperatures falling back closer to average as we head through the second half of this week, as cooler air sweeps down from the far north Atlantic and Iceland. Daytime maximum temperatures will be some 10ºC lower than they were over Easter in many areas. A deepening low pressure area on Saturday looks set to spread cloud and rain eastwards, while winds over Wales and the southern half of England could become strong. Sunday should see more sunshine than Saturday, but there will still be scattered showers.
Monday 29 April—Sunday 5 May
Cool and unsettled for a time, drier later
Similar to April last year, despite a very warm and dry spell in the third week of the month, the weather likes to remind us that we are still in the middle of spring and we have not yet reached summer, with a noticeably cool and unsettled spell to round off the month. With high pressure intensifying close to Iceland, this will deflect the track of North Atlantic low pressure areas towards the UK.
Each one will bring extensive cloud cover and spells of rain, with the heaviest and most widespread of this falling over the southern half of the UK. Chilly north-easterly winds over northern areas, but this will also mean that sheltered western parts of Scotland could end up with plenty of dry weather and sunny spells, despite the lack of warmth. Snow seems unlikely, despite the cool and unsettled pattern, but we cannot completely exclude the threat of some wintry precipitation over the mountains in the north.
Gardeners should keep a very close eye on forecast updates, as cool, clear nights between the low pressure areas could threaten a late season frost, especially over northern parts. There is some uncertainty during the latter part of this week and confidence is fairly low as a result. The most likely outcome is that we see high pressure becoming a little more widespread closer to the UK, with the unsettled weather tending to fade as low pressure slips away to the east. With winds often from the north, the nights will still be chilly, even if the daytimes feel pleasant in the sunshine. There is a 25-30% chance that high pressure builds more strongly over the UK and then the near continent, rather like it did this time last year. This would bring a warmer scenario, with temperatures back up into the low 20Cs by daytime.
Monday 6 May—Sunday 19 May
A lack of sustained wet & windy weather
The first full week in May should see high pressure over Iceland nudge closer to the UK. This will serve to reduce the chilly north-easterly wind flow and introduce a slightly warmer weather pattern.
It is encouraging to see the latest longer range forecast models both predicting a similar story, which helps with forecast confidence. High pressure areas in late spring are often associated with plenty of sunshine, which allows daytime temperatures to rise quickly in the mornings, following cool and locally misty starts. However, as we saw last May, stubborn areas of coastal mist and fog can result in large differences in the weather over just a few miles.
Some beaches can smothered in cold, dank, foggy weather all day, while just down the coast, or a few miles inland it can be warm and sunny.
The main risk on the forecast is that the high pressure is less resilient and slips away to the south and east during the week, allowing Atlantic fronts to introduce a wetter and windier picture. By mid-May, confidence in the forecast becomes low. The most likely outcome is for further extensive high pressures areas over northern Europe, overhead the UK at times. The high pressure areas should be located further east than in early May, allowing the UK to tap into some much warmer southerly winds at times. There is about a 30% risk that high pressure could be located too far east to bring dry and settled weather to the UK, so wetter and breezier weather would be more likely.
Next Update
We saw some big week-to-week swings in weather type over the UK during April. May looks likely to see more changeable weather patterns. We'll attempt to add more detail for mid-May.
https://www.bbc.co.uk/weather/outlook