There are several computer models that provide medium range weather forecasts. These generally come in two types: deterministic and ensemble. The former provide a single, specific forecast for future weather conditions based on initial conditions and physical equations. The latter generates multiple forecasts to assess the probability of different outcomes.
As a result, hundreds of runs (both deterministic and ensemble members) are analysed daily. In recent days the consistency between the various model runs has been notable, at least in certain aspects that are crucial for assessing the risk of snow on the big day.
The first key point is that the models have been showing high pressure building from the south. This suggests there won’t be much precipitation in the southern half of the UK. However, there is more uncertainty as you move northwards, particularly into Scotland.
Once this general pattern is accounted for, the question of temperatures in southern and central regions becomes slightly less significant because the likelihood of precipitation falling from the sky is low. However, the definition of a technical white Christmas at a given station is for a single snowflake to be reported within the 24 hours of 25th December. Therefore, it doesn’t take much precipitation and other factors must also be considered.
Another consistent factor in the computer model output has been the mild air mass covering most or even all of the UK. For example, in the London area the positive temperature anomaly is close to 10°C at the 850 hPa level, approximately 1,500 meters above sea level. However, this doesn’t necessarily mean it will be very mild at the surface. Even with warmth aloft it can become very cold at the surface, particularly if the air is dry and conditions are calm and clear.
Here’s the key point: even if it were to turn very cold and precipitation fell, it would not be snow. Instead, it would be rain or freezing rain due to the high temperatures aloft.
This situation is known as a temperature inversion, and a small number of computer model runs have shown one developing. However, as explained this would not result in snow, meaning Christmas would still be green.
The key factor is the northern extent of the high pressure. It’s possible that the high pressure could build far enough north to cover the entire UK with a mild air mass, but this isn’t certain. Several computer models suggest that colder polar maritime air may occasionally reach Scotland, particularly its northern half.
If that happens, showers could bring a wintry mix with snow in some areas, especially over higher ground. These showers would be scattered and some may fall as rain. Nonetheless, snow could be recorded at certain reporting stations which would be enough for a technical white Christmas at that location.
In many recent years the likelihood of a white Christmas (or at least a technical one) only became clear a few days in advance. However, for the reasons discussed here, confidence in a green Christmas for the majority of the UK has solidified earlier this year.
Of course, until the event occurs it’s impossible to be completely certain. A forecast by definition is “a statement of what is judged likely to happen in the future,” and it can be wrong. However, at this point there is a very strong body of evidence supporting the prediction that Christmas 2024 will be snow free for the vast majority of people in the United Kingdom.
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