A cold end to November with a risk of snow?

Computer models are suggesting the possibility of a plunge of cold Arctic air moving down across the UK through the second half of next week. If this occurs, the risk of snow would increase, but how likely is it?

A change in the weather is expected next week. High pressure will move in from the west, bringing drier conditions for a time, but it is less certain how things develop beyond that. Some computer models show the high pressure building northwards in the mid-Atlantic, opening the gate for a plunge of cold Arctic air. However, other models show it quickly toppling eastward, which would either quickly cut off the Arctic flow or even prevent it from reaching the UK.

Arctic blast on the way?

The Global Forecast System (GFS) forecast chart below is valid for Friday, November 24th. It shows high pressure building northwards in the Atlantic and cold Arctic air sweeping southwards across western Europe. The UK is close to the edge of this plunge, so the weather conditions would be highly dependent on the exact positioning and orientation of the high pressure system.

The model run goes on to show high pressure remaining to the west and north of the UK for quite a long time, suggesting a relatively lengthy period of below average temperatures. Naturally, the longer the cold air sticks around, the greater is the risk of snow. Initially the greatest chance of snow would be in the north and east of Britain.

How likely is an Arctic blast?

An individual computer model run should not be looked at in isolation. In fact, there are literally hundreds of different ones to consider each day. Most of them are contributions to ensemble models, which help to assess the probability of different scenarios.

The Global Ensemble Forecast System (GEFS) chart below is one of the models used to gauge the likelihood of different outcomes. It is for London, with the top half showing forecast temperatures at about 1500m above sea level. The key takeout is that from 25th November the average of all the runs, shown by the thick purple line, falls below the 30 year average. It remains there until the end of the forecast period which is 3rd December.

How great is the risk of snow?

Even though the signal for below average temperatures is quite strong, it is still early for snow. In the UK, the coldest periods typically come in January and February or even March.

At the very bottom of the plot above is the snow row, which gives the number of runs forecasting snow to fall on a given day. The value can go up to 33, but on this update it does not exceed 4, which means the chance of snow falling in the London area peaks at approximately 12%. However, comparable charts for locations further north and east are showing a much greater chance: for example, for Newcastle the chance is 42%.

The chart below for Monday, 27th November is from one of the colder computer model runs. It shows sleet and snow in much of central and eastern England. 

The chart illustrates a possible outcome, but it is one of the less likely ones at the present time, for the reasons already discussed.

Ensemble snow depth graph

The snow row shows the number of runs in the ensemble model which are forecasting snow to fall, but it could just be a few flakes mixed into a heavier burst of rain. It does not necessarily mean snow will settle on the ground. To look at that possibility, the GEFS snow depth charts are useful. 

The one below is for Berkhamsted, Hertfordshire. The number of runs showing accumulating snow is 3, which translates to 9%.

In other words, the chance of snow falling over low ground in the south during the next 16 days is low. The chance of it accumulating is even lower. Nonetheless, the chance is substantially higher in the north and particularly the north east.

To conclude, the important points are:

1) A cold snap or spell may develop during the second half of next week, but it is far from certain. The chance of an Arctic plunge affecting the north is significantly greater than in the south. After all, if it is to reach southern Britain, it will have to cross the north first.

2) Even if it turns colder, the chance of snow in the south remains low. Once more, the risk is higher in the north but it is far from certain.

Check the charts and forecasts

There is a huge range of forecasts and charts on our website to help you decide if snow is on the way. The quickest is to use the 16 day place or postcode forecasts in the header bar. The Will it snow option gives a probability based forecast which will be less volatile. If you're looking for a deep dive, visit our Model inventory which provides a huge range of charts and data.

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