Computer models exist that provide seasonal forecasts, but interest around the Christmas period is truly sparked when it comes into view of the medium range models. The deterministic European ECM and American GFS models provide forecasts updated several times each day, reaching 15 and 16 days ahead, respectively. Several other deterministic models forecast between 7 and 10 days ahead, so at the time of publication Christmas remains beyond their event horizon.
The models which usually garner a lot of attention are called deterministic. A deterministic weather computer model provides a single, specific forecast for future weather conditions based on initial conditions and physical equations. Their forecast output is quite simple to understand, but when looking ahead more than about five days they are subject to wild swings from update to update. This is because small changes early on are quickly magnified, leading to significant differences later.
The deterministic models have already shown a wide range of scenarios. As expected, they suggest that lying snow is most likely over high ground in northern Britain. However, a few of the early updates have indicated weather patterns that could lead to colder conditions even in southern counties. At the moment, the chance of very cold weather in the south is low, but it is too early to discount the possibility.
The Christmas Day chart above is from a deterministic model. It shows high pressure centred to the northeast of the UK and the potential for cold air to move in from the east. With low pressure approaching from the southwest it wouldn't take much adjustment for snow to be a risk.
The scenario is possible but unlikely. Why? Because it is in a small minority with most model runs showing a different pattern. To help establish the likelihood of different outcomes ensemble models are used which generate multiple forecasts. The most commonly occurring pattern is considered the most likely to happen. However, probability and uncertainty in weather forecasts adds a layer of complexity and doesn't make for snappy headlines in the media.
The important point is that deterministic models should not be relied on when looking more than about five days ahead. Even at shorter ranges there is a move towards using ensemble based systems to generate forecasts.
Is it possible yet to identify more likely scenarios than the one shown above? Analysis of ensemble runs suggests the two charts below both depict more likely scenarios.
The first shows high pressure building up from the south. In general, this type of setup suggests that dry conditions are most likely in the south, with a greater risk of rain in the north. The exact position of the high pressure would determine the risk of frost and daytime temperatures. However, a white Christmas would be very unlikely, apart from higher ground in Scotland.
The chart below shows low pressure centered over the North Sea. Colder air is sweeping southwards on its western side, moving down across the UK. In general, this type of pattern suggests a good chance of snow falling over high ground in the northern half of the UK. It could even produce snow at lower levels.
The risk of snow could extend southwards, but the devil is in the details. For example, how cold the air feed is around the low pressure system and how quickly it moves southwards. Nonetheless, if a chart like this were to appear a few days before the big day, it would be reasonable to assume that a number of locations might record at least a technical white Christmas.
Even at very short ranges there can be a significant element of uncertainty. Much depends on the weather patterns in place. For example, conditions for snow in the UK are often marginal. It’s not uncommon for factors such as precipitation intensity and wind strength to make the difference between snow and rain when conditions are borderline.
Despite this, the big picture usually starts to firm up about one week before Christmas Day. However, the details may not become clear until 48 hours or even 24 hours before 25th December. For instance, if colder air is expected to push southwards, its southern extent may remain uncertain. On many days of the year, a small shift in the details may not have a significant impact, but when discussing a technical white Christmas, it could be critical. A weather station only needs to report one flake of snow falling within the 24 hours of 25th December for a white Christmas to be declared.
For the latest view check out our White Christmas forecast which is being updated every day between now and 25th December.
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