Could snow be on the way?

Signs of colder weather later this month

Temperatures in early November have been well above the Central England Temperature (CET) norm. Up to the 7th the provisional value is 10.2°C which is 3.8°C above the 1961-90 norm.

The Weather Outlook's own CET tracker which uses slightly different data has the value a little higher, at 10.3°C. 

TWO tracker 10.3°C up to 7th November

• 1961-1990: 3.90°C
• 1971-2000: 3.30°C
• 1981-2010: 3.00°C
• 1991-2020: 2.90°C
• 1659-2020: 4.20°C

The values show the anomaly compared to different 30 year series, as well as the entire 1659-2020 CET catalogue.

Despite the large positive anomalies, the days haven't been particularly mild, with cloud cover suppressing  temperatures. It is the nights where values have been well above the average. Once again the main reason is cloud cover: at night it acts as a blanket, trapping heat and preventing it radiating out.

The air mass over the UK has been a warm one. Temperatures at about 1500m above sea level have been between 5°C and 10°C above the norm. Therefore, regardless of temperatures at the surface, any precipitation falling would be rain and not snow. For that to change a much colder air mass needs to be in place and there are some indications of that happening.

The chart below shows the forecast temperatures at approximately 1500m on 18th November from one run of the ECMWF model. A plunge of very cold Arctic air is sweeping down over the UK, leading to conditions which could produce snow.

The coldest air is in the north, with values around -10°C in Scotland and even in the south they are around -5°C. Showers would likely have a wintry flavour even in parts of the south and in the north snow could be expected to fall to low levels.

However, the chart above is for 10 days ahead and is from one computer model run. There are a number of computer models, each updating several times daily. Some of them, the ensemble models, contain many individual runs. When looking this far into the future small differences early on are quickly amplified, resulting in very big ones later. That means it is important to look for consistencies from the different model runs when assessing the probability of a given outcome.

How likely is a spell of cold and snowy weather?

At the moment it is far from certain, but can a percentage chance be given? The data table below helps do this. It shows the daily maximum temperatures which are being forecast for Newcastle between 9th to 23rd November. Initially the columns are mostly light green, which is used to show computer model runs forecasting between 6°C and 10°C.

A change takes place from the 17th when dark green starts appearing. It is used to show runs forecasting maximums of between 1°C and 5°C, which is cold for the time of the year.

The amount of runs in this category reaches 32%, so approximately a 1 in 3 chance of a cold spell developing between 17th to 22nd November. As the cold air is expected to be coming from the north (rather than the east) the chance will be higher in northern Scotland and lower in southern England.

Making sense of it all

The second half of November can bring wintry weather even to the south. The lowest November values in the UK were in 1919 when Braemar in Aberdeenshire recorded -23.3C and -22.8C on the 14th and 15th. There's no sign of anything like that this month, but there are indications of it turning colder, at least for a time.

Even if temperatures drop sharply there is no guarantee of snow. Usually in the UK it is only possible to be reasonably confident about the risk a few days ahead. Showers which can develop in an Arctic air stream often only affect northern parts of the UK and coastal counties. In southern and central regions more organised features are generally required to bring snow, for example weather fronts pushing up from the south west which bump into the cold air mass, or more organised disturbances in the flow.

Does it signal a cold winter ahead?

No. There have been examples of wintry spells in November which were followed by cold, mild and average winters. At the moment seasonal computer models are strongly favouring a milder than average winter. However, their skill level (or accuracy) for the UK remains low, so a cold and snowy winter isn't out of the question.

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