Is disruptive snow on the way?

Temperatures are expected to remain below the average in the next few days and next week it looks set to turn even colder as a plunge of Arctic air sweeps southwards. There is uncertainty about the forecast details, but confidence in the general pattern is now high.

High pressure will become centred to the northwest, with very cold Arctic air on its eastern flank moving southwards towards the UK. By the end of the week the cold air is forecast to be covering most of the UK, but there is uncertainty about whether it will have reached the south and in particular the south west.

Snow showers in the north initially?

Snow showers are likely to push into the north, with significant accumulations possible in places, even at low levels. Nighttime frosts also become widespread. However, much of the uncertainty concerns areas of low pressure pushing in from the west or southwest. As weather fronts associated with them bump into the cold air mass, outbreaks of rain increasingly fall as sleet or snow down to low levels.

In this type of set-up, the area of snow is generally quite small, with milder air and rain never far away. Therefore, if a weather front tracks 100 miles further north than computer models predict, places which would have seen snow have rain. Conversely, some locations which would have remained dry and cold, finish up with disruptive snow.

Computer models at the range under discussion generate forecasts which cover the entire planet. Therefore, a tiny difference on the global scale can mean a big one for relatively small areas such as the UK.

Disruptive snow in the southern half of the UK?

The postage stamp chart below shows forecast accumulations of snow on Wednesday, 17th January. It is being used for illustrative purposes only, and the details will change as the time period approaches. Snow may well affect parts of the UK before and after this date. However, the pattern it shows is reasonably well supported at the time of publication.

In general terms, the risk of snow appears to be the lowest in areas to the south of London. That does not mean they won’t see snow or even disruptive amounts. It does suggest that the likelihood of those scenarios is lower. Past experience has shown that the M4 corridor often marks the boundary between snow to the north and rain to the south.

The graphs below help to assess the risks of accumulating snow, but are subject to change. Each line shows the snow depth forecast in cms from one run in the computer model. More lines means more runs are forecasting snow and taller ones mean deeper falls. The ones used are for London and Inverness, the locations chosen help to show the different likelihoods of snow in the north and south.

On the London chart immediately below a number of runs are forecasting accumulating snow from 16th January onwards. In fact, a few are showing significant accumulations.

However, the graph below for Inverness in northern Scotland shows a much greater likelihood of snow, as is usually the case. The majority of runs in the model are showing accumulations of snow, starting around January 14th and continuing until the end of the forecast period which is January 25th. Therefore, confidence in Inverness having significant accumulations of snow is high.

Will milder air return quickly?

Computer models often struggle to forecast the transition back to a milder and more typical Atlantic-based weather pattern. There are indications of it turning, at least temporarily, milder in the south at times next week. However, the bigger picture is an increased chance of cold conditions until around 23rd January. By then, the milder air may be making more significant progress northwards and eastwards across the UK.

Nonetheless, a number of the background signals are pointing towards an increased chance of further cold spells during the rest of the winter. Therefore, it wouldn’t be a surprise if snow returns in February and even March.

Wintry hazards likely but details uncertain

In summary, snow and ice appear likely to affect parts of the UK through the middle third of January. 

1) Initially snow is most likely in the north in the form of showers which could turn heavy and prolonged at times

2) At times next week areas of low pressure probably move in from the Atlantic and increase the risk of longer outbreaks of rain, sleet and snow in the southern half of the UK

3) The track that areas of low pressure will take will determine which areas see longer and potentially disruptive outbreaks of snow

4) If low pressure areas track further north they will introduce milder air into southern and possibly central regions, leading to snow turning back to rain

5) If they track further south (across France), much of southern and central Britain could remain dry and cold

6) If a very cold Arctic flow becomes embedded over all of the UK and low pressure areas remain to the south, there is a possibility of small scale weather systems moving southwards which could increase the risk of snow showers in southern areas   

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