BBC monthly outlook
Summary
Potentially stormy and wet for Christmas period
Wednesday 5 December—Sunday 9 December
Wet and breezy but often mild in the south.
The rest of this week looks to be unsettled with plenty of rain and cloud at times, and some breezy conditions mainly in the west. Wednesday will have a wet start for many, with a warm front bringing rain to most of England and Wales. Through the afternoon this rain will spread north into northern England and Scotland, while the cold front moves into the south-west. Rain will begin to clear from the west in the evening and overnight, with areas of mist or fog possible. It will be a mild night for most.
Thursday will see another frontal system moving into the UK from the west, bringing another round of widespread rain for most. Winds will be stronger with this front, especially for western and southern coasts. Overnight Thursday into Friday will see yet another frontal system move across the UK, bringing more wet and windy conditions for most. By Friday morning things are looking mostly dry for the south and east, with showers in the west. A low over Scotland will bring plenty of rain throughout the day, with occasional heavy bursts in the north.
The weekend continues the unsettled trend, with Saturday looking like the drier and brighter of the two days, as another frontal system moves through Sunday bringing rain for most and strong wind gusts in the west.
Monday 10 December—Sunday 16 December
Becoming cooler but drier, with showers midweek.
In the second full week of December, high pressure pushing north into Scandinavia will bring some cooler easterly winds for the UK with occasional showers in the east. Generally, a lot of cloud is likely at times while some of the showers in the east could turn wintry over high terrain. The best of any sunshine is likely to be in the west and northwest, particularly in Scotland. Temperatures are expected to be mostly below the seasonal average, but with a recovery to near-average temperatures possible over the weekend.
Indications are for wettest conditions to be over eastern areas of the UK midweek, with scattered showers drifting in on the chilly easterly breeze. The moors and other hilly spots will likely see wintry showers at times. It is also likely to be rather breezy across eastern and central areas, with some biting, chilly winds along the east coast. There is a slight risk of some of these wintry showers reaching low-lying areas in the east, particularly on Thursday. There is still a slight risk of a generally mild, wet and breezy pattern if the high pressure shifts further east into Russia, or fails to build strongly into Scandinavia. This would lead to more mild and unsettled weather with storms moving in from the west.
Monday 17 December—Sunday 30 December
Unsettled weather returns bringing mild air.
Indications are that the second half of the month will be characterised by wet, windy and mild conditions with a strong jet stream moving in from the Atlantic at times. The jet stream is a ribbon of strong winds in the upper atmosphere and helps to drive in weather systems that bring wet and windy conditions to the UK. Again, western areas of the UK are expected to be the wettest. There are indications that rainfall could be well-above normal for December, bringing a risk of flooding. Strong winds appear likely at times, perhaps most likely towards the end of the month.
However, the positioning of the jet stream will dictate where the wettest conditions will be. There are chances that the jet stream lifts northwards for a time before Christmas to bring a brief drier period of weather to southern areas. After Christmas, there appears to be some risk that the jet stream drifts further south, bringing a risk of colder conditions into northern parts of the UK. Anyone hoping for a white Christmas could end up being disappointed as milder air from the Atlantic will keep temperatures around or just above average. However, it is still far too early to say for certain what weather we will have on 25th December. Even in some of the mild Decembers of the past few decades, short-lived cold spells did crop up and even produced short spells of snow. A green Christmas is still not guaranteed ... although a white Christmas is looking increasingly less likely.
Next Update
We will have an updated look at what this waving jet stream may have in store for the holiday celebrations.
https://www.bbc.co.uk/weather/outlook