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Winter returns with Arctic plunge

Paul Hudson | 17:24 UK time, Wednesday, 15 December 2010

After a brief respite from the exceptional cold that originated across Northern Russia and which led to numerous records broken across Yorkshire and the UK, winter looks set to return with a vengeance over the next few days.

This time the source of the air is the Arctic, and will cover the whole of the UK by the end of tomorrow. A cold front will bring the change to much colder air on Thursday morning, with frontal rain likely to turn to snow for a short time as it clears.

Following will be a number of snow showers - in Yorkshire and Lincolnshire mainly in coastal areas. Towards the weekend though the forecast becomes much more tricky as an area of low pressure becomes slow moving across the UK.

Forecasting snow is one of the most difficult tasks for forecasters. The previous cold spell that started at the end of November and lasted into early December was a little easier to forecast than normal - for two reasons.

Firstly, the air was so cold that all precipitation was always going to fall as snow. Normally across the UK temperatures are at levels that make forecasting when rain turns to snow and vice versa very tricky.

A fraction of a degree change in temperature can make the difference between rain and snow. A change in the intensity of rainfall can do the same, as heavier rain causes the air to cool down by a process called evaporative cooling - sometimes causing an abrupt switch from rain to snow, and back again.

Secondly, earlier in the month we had four days of an unstable northeast wind, with showers of snow running in across similar areas each day from the North Sea.

This time, the air will again be cold enough for all showers to fall as snow. The difficulty will be getting the precise wind direction right as a slow moving area of low pressure moves across the UK. Who gets the most snow will be critically dependent on where the low ends up and what track it takes.

If the low tracks close to Yorkshire and Lincolnshire, then the result can be seen below, from the midday run of the American model - snow in most parts on saturday, possibly heavy.



This, at the moment at least, seems to be the day which has the highest risk of disruption. But there is a lot of uncertainty about the precise track of this area of low pressure.

Take the European model as an example. The computer is run around 50 times, at each stage the starting conditions such as air pressure are changed very slightly. The blue squares seen on the diagram below show where each of the computer model runs places the centre of the area of low pressure at midday on Saturday.



As you can see, there are many different solutions, each position giving a different distribution of snow.

What we can say with high confidence is that a potent mix of deep cold air, a warm sea and the inherent instability of an area of low pressure will generate some heavy disruptive snowfall across parts of the UK from Thursday onwards.

By the end of the weekend it's likely that most areas will have seen at least some snow. But pinning down which areas get the heaviest and most disruptive snowfall, and at what time, will be difficult, more than 24 hours ahead. It will be a case of staying tuned to the very latest forecast.

How long the cold spell will last is very much open to question, with an increasing number of forecast models beginning to hint at milder air moving northwards by the middle of next week - but the outlook by this time is very uncertain.

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