BBC monthly outlook
Summary
Often unsettled with regular swings in temperature
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Wednesday 4 December – Sunday 8 December
Turning milder, wetter and much windier
So far this week, we have had a large area of high pressure located across the south of the UK. This has brought dry, calm but also chilly conditions to southern areas, with some overnight frost and patchy fog, while milder westerly winds have spread slowly in across northern and central areas. On Wednesday, the southern and eastern half of the UK can be expected to stay dry, following another chilly start, with some patchy fog and frost. A weakening cold front will push a band of cloud and increasingly light and intermittent rain south-eastwards across Scotland, Northern Ireland and into north-western England. Breezy and showery weather will follow to western Scotland on Wednesday afternoon.
A big change is on the way for the rest of this week, as a powerful westerly jet stream (high above the far North Atlantic Ocean) forces a vigorous low pressure track and associated fronts into the UK and pushes away the high pressure. These fronts will arrive over northern areas on Thursday and linger through Thursday night. A conveyor of strong to gale force south-westerly winds and prolonged heavy rain over western Scotland and Cumbria could bring widely over 50mm of rain and locally between 75mm and 100mm in less than 30 hours.
These normally very wet areas have avoided the heaviest of the UK's autumn rains, but there could still be some flooding issues, as the river levels and stream rise rapidly. Further south, turning much milder and windier than recently, but the wettest weather will on Friday and Sunday, with Saturday offering a brief window of drier and brighter weather.
Monday 9 December – Sunday 15 December
Unsettled, but also colder with risk of some snow
An unsettled and windy week is expected, especially so across the south. Winds are likely to come from a westerly or north-westerly direction early in the week, keeping temperatures close to the seasonal average. However, there is increased evidence on the latest model guidance for the low pressure track to shift just to dip just to the south of the UK from midweek, allowing colder wind flows from the north and north-east to extend in from Scandinavia. Many areas are likely to have showers and some lengthier outbreaks of rain spreading from the west and south-west. Rainfall amounts are favoured to be above average for many southern and western areas, but less so over northern and north-west Scotland.
The most important aspect of the forecast involves the fact that some of the rain may start to fall as snow, as the cold air moves down over the UK. The extent and the timing of the snowfall threat is extremely uncertain, not least because temperatures will be very marginal regarding the rain / snow mix. While the higher parts of central and northern Britain will be most likely to see some snow, lower levels could see some at times, too. Later in the week, night frosts are likely to return, especially to northern half of the UK, but perhaps further south as well, especially where there has been some snowfall.
Monday 16 December – Sunday 29 December
Variable temperatures, but often wet and breezy
The second half of December is expected to be wet and windy at times with swings in temperature. Low pressure areas are likely to be often nearby or to the west and southwest of the UK, bringing wet conditions with above normal rainfall expected quite widely. The rainfall could lead to some further flood issues too, especially where river catchments are already saturated, such as across parts of Wales and the Midlands. It is likely to be breezy at times, too, with the strongest winds often over western and south-western areas.
By the festive week, confidence in the forecast remains quite low. However, there are still indications that the low pressure areas will often sink southwards near or over the UK and down into France, while high pressure builds over Scandinavia and Eastern Europe. Wettest over southern and western areas of the UK and quite dry over northern Scotland. It is very possible that some colder air could tuck into the UK on a few days and even bring some wintry showers. But the signal is for short-lived chilly spells, rather than a prolonged period of significant cold.
Further ahead
We will provide more detail on the threat of some snowfall over the UK later
https://www.bbc.co.uk/weather/outlook