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The first half of the meteorological winter has been mostly mild. December was a very mild month and the Central England Temperature (CET) for the first 10 days of January is 4.6C, which is 1C above the 30 year average. Despite that things are set to change in the coming days and there are reasons to think the second half of the winter will be significantly colder.
A Sudden Stratospheric Warming (SSW) event has been taking place recently. The term refers to an observed temperature rise of several tens of Celsius over the period of a few days. The current event is generally classified as a major one because the westerly winds in the stratosphere have reversed and become easterly.
The polar vortex - an upper level area of low pressure - over the North Pole has split and the easterly winds are beginning to move down through the atmosphere. The effect will be a disruption of the westerly flow at lower levels around the northern hemisphere in the coming weeks. In turn it means blocks of cold Arctic air are likely to be displaced to mid latitude locations such as Europe which could mean much colder weather developing in the UK.
However, despite the increased chance of cold weather it is by no means certain and some SSW events leave the UK under a mild pattern.
The postage stamp chart shows GEFS snow forecasts for Monday 21st January. On the plots pink shading indicates falling, but not necessarily accumulating snow. Each stamp represents the output from one run and they vary considerably, but the important point to note is quite a few of them show a risk of snow in parts of the UK.
In general terms the chance of snow is higher in the north. In large part that is due to parameters being quite marginal. Unlike the Beast From The East spells last winter, it looks as though a more changeable pattern will develop with deep cold not really becoming established before disturbances in the flow bring the risk of precipitation. That situation could lead to some parts of the UK seeing a lot of snow whilst other areas have rain instead.
It is too early to be confident about the weather details for the rest of January other than saying the risk of cold and snow is expected to increase significantly in all areas.
The westerly flow around the northern hemisphere is set to remain disrupted through February. Therefore the chance of cold periods continuing to affect the UK looks to be notably higher than the climatoligical average. In other words further spells of wintry weather and snow are possible in all regions. Nonetheless it is uncertain and a milder outcome is not discounted.
The first half of winter has been mild and most of the UK has not seen any snow. A change to colder weather is expected during the middle part of January and it may set the theme for February. The risk of ice and disruptive snow increases, especially but not only in the north.
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