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Weather updates from site founder Brian Gaze that keep you in the know.

Colder weather in January

Published 30th December, 16:15

December has been a very mild month, with the Central England temperature provisionally 7.0C to the 29th, which is 2.5C above the 1961-90 average. However, there are growing signs of cold weather developing as we head through the first half of January.

Chance of cold weather increasing?

The chart below shows forecast 850hPa temperatures from the ECM ensemble model. It covers the period between now and 13th January. In the short term most of the individual runs show temperatures to be above the 30 year average, which is highlighted by the thick black line. By 7th January the thick purple line - the ensemble average - dips below the thick black line, and it remains there until the end of the run.

Many of the runs show temperatures dipping to around -10C. In the United Kingdom that is often used as a marker for very cold weather. Given 850hPa temperatures at those levels any precipitation falling is almost certain to be snow, even at low altitudes. Nonetheless, the period under discussion is over one week away which is a long time in terms of the UK's weather.

Cross model support

The GEFS ensemble model shows something very similar. On the data table each column shows how many of the individual runs on a given day full into each temperature category. Purple is used to 850hPa temperatures of -5C or lower, and the number in brackets is the percentage of runs which are dipping down to or below -10C.

In recent updates the amount of purple in the columns has been increasing, indicating growing confidence that it will turn colder. On this data table 65% of the runs show temperatures of -5C or lower on January 9th and 10th. Beyond that, the percentage begins to decrease, but at longer ranges the amount of scatter usually increases.

Therefore, the chance of a significant cold spell is increasing. Will it snow? At this range it is impossible to discuss the specifics of snow fall and which areas are most at risk, other than in very broad terms. Many of the runs are showing the cold air coming from the east or the northeast, if that is the case the highest risk of snow would probably be in the north and the east of Britain. Of course, other scenarios are possible, for example areas of low pressure could push up from the southwest. They would lead to the possibility of more prolonged periods of rain sleet and snow as weather fronts associated with them moved northeastwards.

Too early for the details

In the summary, it is too early to be talking about the details, but the chance of a significant cold spell as opposed to a relatively brief cold snap is quite high. Other outcomes remain very possible, for example high pressure could become centred close to the United Kingdom. That would bring a quiet spell of weather, with temperatures and the extent of frost depending on its exact positioning and orientation. Another outcome would be for high pressure to slip away southeastwards into continental Europe and quickly allow milder air to return from the west or the southwest. At the moment that is not the most likely development according to the computer models, but it has been a common pattern in recent years so it should not be discounted.