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A cold start to the winter is likely

Published 25th November, 15:38

The weather has turned colder across the UK in recent days and there have been wintry showers in parts of the north. Are the colder conditions set to last and could the risk of snow become more widespread?

Cold start to the meteorological winter

The meteorological winter starts on December 1st. Currently, computer models suggest that it is likely to begin on a cold note.

The data table below, based on GEFS data, displays 850hPa temperatures (approximately 1500m above sea level) in the London areas, helping to assess the probability of various scenarios. The purple shading shows the percentage of runs in the model which are forecasting 850hPa temperatures to be below -4C.

Between 28th November and 6th December the number of runs falling into this category on each day varies from 39% to 74%. It suggests predominantly cold weather, although milder interludes are possible.

However, this update and other recent ones points to an increasing likelihood of milder conditions becoming established during the second week of December. In fact, some mild or even very mild scenarios are shown by the green, yellow and orange shading. With only two weeks until Christmas, I would expect some handwringing and disappointment if the cold patterns breaks and is replaced by a mild southwesterly flow which then locks in!

Is widespread snow likely?

Forecasting snow in the UK is tricky even at short range. Therefore, when talking about the prospects for the next two weeks it really is about considering whether the ingredients are likely to be in place. Basically, will the air be cold enough and will there be precipitation? Other factors such as evaporative cooling, diurnal variations and dew points will only come into play at the margins. 

At this stage computer models are showing areas of low pressure influencing the UK's weather at times. Also, as I discussed, the air mass looks like being quite cold. On balance conditions look quite marginal, so some of the other considerations I mentioned may come into play. I think it is relatively safe to say the northern hills will see snow and potentially significant amounts. However, the risk of lying snow in lowland parts of the south is a lot lower.

Will milder conditions then dominate?

If milder conditions return during the second week of December what happens next? Will an Atlantic flow become established, with high pressure sitting to the south or southwest of the UK for much of the meteorological winter? This has been the prevailing pattern in many recent winters, and discounting it is always risky!

The key to answering the question will be to watch how the medium range ensembles begin to shape prospects for the second half of December. One thing to bear in mind is that the UK's coldest winters tend to be ones where the snowy and frosty periods are focused on January, February and even March. Front loaded winters usually peter out by the end of December, and even 1981-82 was more or less done and dusted by mid-January. Last year was also a case in point, with the winter as a whole being slightly milder than average despite a very cold period in December. 

There is always "an exception that proves the rule", but I wouldn't be relying on this year being it. With that said, the El Niño climate pattern correlates with an increased chance of cold spell later in the winter.