BBC monthly outlook
Monday 4 December—Sunday 10 December
Mild at first, then wet and windy, then very cold!
The working week will get off to a relatively settled and chilly start, with high pressure centred to the southwest of the British Isles. Any mist and fog patches will slowly lift through the morning, leading to a mostly dry day with variable amounts of cloud and a relatively mild feel. Western districts may received the odd spot of drizzle.
Tuesday will be largely similar to Monday, with the exception that westerly winds will strengthen across central and northern parts of the UK as the high pressure system recedes into the near continent. By Wednesday, winds will back around to the southwest and strengthen significantly, making for a mild and windy day for much of the UK with fairly widespread gales developing, particularly around coasts. Western areas will be increasingly affected by rain though, as an Atlantic low pressure system edges in from the west. Indeed, the rain will slowly become quite heavy across parts of western Scotland and northwest England.
Overnight into Thursday, the low pressure system and its attendant fronts and rain bands will sweep southeast. A very cold and strong northwesterly wind will follow. Showers will turn increasingly wintry, with snow on the higher ground of the north. Many inland areas will stay mostly dry and sunny, but despite the sunshine, conditions will feel bitterly cold. Expect a very cold day on Friday with a cold northerly wind and further wintry showers affecting many parts that will turn increasingly to snow at lower levels. Coastal districts will most likely experience showers, but even some central parts of the UK could be see wintry showers too.
Through Saturday, forecast confidence is relatively low, but the most likely scenario is that the cold northerly winds will start to abate, with the possibility of conditions turning slightly less cold and more settled by Sunday.
Monday 11 December—Sunday 17 December
Not much sign of any lengthy milder interludes!
As we head into the middle of December, the majority of computer forecasting models suggest that weather patterns across the UK will tend to change very slowly. The favoured forecast theme for this period is that the jet stream will develop long, often narrow north/south meandering kinks that will only very slowly traverse the North Atlantic towards our shores. Such an eventuality will mean that the UK will often be in cold air of polar origin, as the UK will mainly lie on the cold side of the jet stream. Occasionally though, Atlantic low pressure systems will likely cross the UK from northwest to southeast, bringing milder air across mainly southern parts of the country.
This means that it is likely that most parts of the UK will experience fairly lengthy spells of colder weather. Frosty nights with patchy fog, followed by chilly days that combine sunny spells and showers (often of a wintry nature in the north) will be the main theme. Any low pressure systems that occasionally move across the Atlantic will likely introduce slightly milder interludes, mainly for southern parts of the country. Milder interludes will be accompanied by rain though, with northern edges of any rain areas introducing the threat of snow for some central and northern parts of the UK at times.
A weather phenomena known as the Madden-Julian Oscillation is also currently lending confidence to our UK forecast for this period. Across the tropics, a pulse of convective cloud (that manifests as thunderstorms) tends to move from west to east across the globe once every 30 to 60 days. Research has revealed a link between the position of this convective activity, and weather conditions across UK several days into the future. The current phase of the Madden-Julian Oscillation lends good support to the idea that the UK will be experience mainly slow changes in weather conditions through much of the month. Indeed, other meteorological phenomena occurring in other parts of the atmosphere also lend support to the latest UK forecast.
Monday 18 December—Sunday 31 December
Still on the cold side!
By the end of the calendar year, there isn't a particularly high level of consensus in the details of the UK weather forecast. That said, some data suggests that pressure will tend to be lower towards the south and west, with high pressure dominating towards the north and east. Such a synoptic pattern will lead to a continuation in colder than average conditions. Frosty nights, with cold days would be the most frequent and widespread occurrence, with the chance of wintry showers. Any wetter, milder interludes will likely be most prevalent further south, but with the jet stream forecast to continue to behave in a slowly meandering pattern, any such milder interludes will likely be fairly short-lived.
Of course, we're reaching the stage of the year when the weather question on the minds of many relates to the prospect of a white Christmas. At time of writing, it is simply just to early to give a definitive answer! As already discussed, the current favoured forecast solution for this period does lean towards conditions often being slightly colder than average, so we cannot rule out the possibility that somewhere in the UK (most likely further north) might see some snow at some point during the festive period. Regardless, it is very important to state that confidence in any forecast regarding this matter at this particular point in time is too low to lend any credibility to any definitive statements. Keep in touch with the BBC Weather Centre to stay abreast of the latest thinking!
Next week
New year, new weather. But what form are January's weather patterns likely to take? Find out next week ...
http://www.bbc.co.uk/weather/2635167#outlook