Arctic blast starts this weekend

By Sunday, 14th January, much colder air will be pushing southwards across the UK. The driver for the cold spell is high pressure building towards Greenland, causing cold Arctic air on its eastern flank to plunge southwards.

The latest guidance from computer models is suggesting the Arctic plunge will be a sharp one, with deep cold making it down across all parts of the UK. Even in the south, air temperatures at about 1500m will be close to -10C, which is often used as a marker for very cold conditions in the UK.

Although the risk of widespread snow is looking lower than a couple of days ago, it remains very uncertain. During the middle of next week two areas of low pressure come into play, but it is also possible that small scale disturbances could pop-up at relatively short notice and lead to snow showers becoming more widespread at times. Predicting these is very difficult, so the best advice is to keep up to date with the short range forecasts and weather warnings.

Snow risk initially in Scotland

Initially, the risk of disruptive snow will be focused on Scotland where significant falls are possible on Sunday and Monday. By Tuesday snow showers may become more persistent and prolonged in parts of the north as an area of low pressure moves in. Details are very uncertain, but there is the potential for significant accumulations of snow in places. However, there will be a sector of less cold air mixed in, so snow could turn back to sleet or rain, particularly on the western side of the system.

A second low pressure system may threaten southern Britain with snow during the middle of the week. However, computer models are showing a range of solutions, with some taking it across France, leaving southern counties drier and bitterly cold.

The chart above shows forecast snow depths at 18:00 on Wednesday 17th January. Each "stamp" represents the forecast from one run in a computer ensemble model. It is used only to illustrative different possibilities. Most of the runs show accumulations of snow in Scotland, but it is a more uncertain picture in southern and central regions. 

Severe frosts are expected

Another hazard will be very low temperatures, with severe frosts developing at times. In areas with snow cover, notably low values may be recorded. In the Scottish glens -20C is not out of the question and in northern England temperatures may dip close to -15C. Ice days are also possible in some locations, defined as those where the maximum air temperature remains below freezing point. Frozen pipes, which is one of the biggest reasons for boiler failures, are likely to become a problem in places.

How long will the wintry spell last?

The wintry weather is likely to lead to disruption next week. However, it may not last for long, with most computer models showing a transition back to milder conditions from around 20th January. As weather fronts move in from the Atlantic and bump into the cold air over the UK there could be some transient but disruptive snow, especially in the north. 

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