BBC monthly outlook
Summary
Mild & wet for a time. Colder at Christmas!
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Wednesday 16 December – Sunday 20 December
Windy and very wet, especially in the west. Mild.
Wednesday 16th will see a small but rather intense area of low pressure moving north across Ireland. This feature will already be producing widespread strong to near gale force south-easterly winds across the western half of the UK by dawn on Wednesday, combined with persistent and heavy rain. The rain will push rapidly north across all of Scotland by midday. But its eastwards progress across England will be much slower. East Anglia and the far south-east of England will stay dry until after dark. Most of the UK will have extensive cloud cover.
Thursday will be a much brighter day for most of the UK, with widespread sunny spells. These will be most prolonged of these over central and eastern England, where it will be a predominantly dry day. Scattered showers will affect southern and western coastal counties of the UK. These will be heaviest close to the Kent and Sussex coast and also over Wales, north-west England, Northern Ireland and western Scotland. Still windy in the north, but less so than on Wednesday.
Friday and the weekend will be influenced by a new large low pressure over the north-east Atlantic, moving slowly eastwards towards the UK. Extensive cloud cover on Friday, with intermittent rain and drizzle and hill fog over western hills and mountains. A broad band of heavy rain moving slowly east across the country on Friday night, with strong south-westerly winds. Brighter and showery conditions over the weekend, but staying windy, with brisk west or south-westerly winds.
Monday 21 December – Sunday 27 December
A mild & wet start. Turning colder by Christmas.
The first half of this week will continue in the same vein as the previous week, with additional low pressure areas continuing to track north-eastwards over the UK. Because we have relatively high pressure to the north of the UK, the north Atlantic low pressure track is displaced a bit further south than where it would normally be in December. This means that Wales and south-west England will continue to bear the brunt of the strongest winds and heaviest rainfall, rather than western Scotland and Cumbria.
Milder than average conditions in the early part of the week, due to a combination of south-westerly winds, extensive cloud cover, rain and often windy weather. This will prevent a frost forming at night. However, there is a slight risk that Scotland and even northern England could get into some chillier air on Tuesday 22nd and Wednesday 23rd , if the low pressure areas fail to push up quite as far over the UK. Some forecast computer models support this risk. This would mean the chance of a slight frost at night here, and maybe some sleet and snow over the mountains of Wales and northern UK.
By Christmas Eve and Christmas Day, confidence is growing for at least a couple of days of colder and brighter weather, as winds change to a north-westerly direction and a ridge of high pressure then moves in from the west. Scattered wintry showers in this north-westerly flow may fall as snow, raising the possibility of a white Christmas in parts of the UK! Scotland and the hillier parts of northern England and Wales stand the best chance. Stay tuned to the forecast over the next few days. Some sunshine by day, but a frost is very likely at night for many places.
Additional fronts moving in at the end of week will bring some rain. A slight chance of this falling as snow for a time as the fronts move into colder air over the UK.
Monday 28 December – Sunday 10 January
Changeable, with cold & mild spells alternating
The latest long range forecast models are having a hard time finding some consistency and predictability during this period, with a lack of any robust signals for either sustained cold weather or sustained mild, wet and windy weather.
What seems fairly clear, though, is that the north Atlantic low pressure track should remain active and keep fronts moving towards the UK at fairly regular intervals. Transient high pressure ridges are predicted to shift over the UK from time to time, giving us a few days break from the rain, but a prolonged dry spell doesn't seem likely.
At the same time, there is also a signal for a large area of high pressure to remain over eastern Europe, Scandinavia and north-west Russia. This feature will help to steer low pressure areas to the south of the UK at times, allowing for less rain and wind over Scotland, but wet and windy weather over south-western UK at times.
Similar weather patterns over Europe in the recent past have resulted in a highly changeable weather regime over the UK. Even more so than usual. It's very likely that there will be another cold spell during this time, with the risk of some short-lived sleet and snow and overnight frost. This variable weather pattern will certainly keep long range forecasters on their toes!
Further ahead
Any updates to the festive season weather forecast will be covered, but we will start to look in more detail at the key trends for the first half of January 2021.
https://www.bbc.co.uk/weather/outlook