By Brian Gaze
Temperatures will be dropping during the coming days as a cold spell, which has been referred to as the "Troll from Trondheim", becomes established. The combination of high pressure over Greenland and an area of low pressure over Scandinavia (hence the name) will lead to cold Arctic air being pulled southwards across the UK.
There are indications that some very low temperatures may be recorded and that the cold period will be quite lengthy. There is a risk of snow too.
ECM air mass temperature forecast chart
Computer models are suggesting very low temperatures indeed. The combination of a cold air mass, very short days and slack areas of low pressure all play a role. Also, values will be even lower where there is snow cover.
The chart below shows forecast maximum temperatures on Thursday 8th December based on data from the high resolution UKV computer model which is operated by the UK Met Office. In much of the north temperatures are at or below 0C (32F) and even in most of the south they are only rising a few degrees above it. The mildest part of the country is the south west which often sees the highest temperatures during the winter months.
It's a very cold picture and although computer models are showing some variations, the general view is consistent.
There is also a likelihood of very cold nights. Frosts, which were almost absent in much of southern Britain during the meteorological autumn, will become widespread and sharp. Temperatures could widely dip to -5C or lower on some nights and in places where there is snow cover exceptionally low values are possible.
The forecast chart below is generated using data from the Global model operated by the UK Met Office. It shows values dipping to -17C in the Scottish Highlands on Thursday night and some computer model runs have suggested that -20C may be reached.
The lowest temperature recorded in the UK is -27.2C. It has been reached twice in Scotland, firstly on 10th January 1982 at Breamar and then on 30th December 1995 at Altnaharra. Although the record is unlikely to be challenged, the temperatures which some computer models are forecasting are unusually low for the UK.
Temperatures will certainly be low enough for snow but can we expect to see any? In Scotland significant snow is forecast on Wednesday with accumulations of up to 10cm possible in the northern half.
Elsewhere there isn't a clear signal and it looks as though there will be a lot of dry weather. However, there is a chance of a weakening band of sleet and snow pushing southwards through Thursday. It has the potential to bring a covering even to parts of central and southern Britain, but amounts aren't likely to be great and there is uncertainty about the details.
The chart below generated from the GFS model shows forecast precipitation at 6pm on Thursday 8th December. The white hatched areas indicate the possibility of snow.
In the longer term snow showers are most likely in northern and eastern coastal counties, although there is a chance of more organised bands pushing further inland. However, computer models often only pick up small disturbances a couple of days ahead so it is not possible to be more specific.
Another possibility which becomes more likely from around 12th December is that areas of low pressure and their weather fronts start to push up from the southwest. They bring the potential for milder conditions but also more widespread snow along the boundary zone between cold and mild air masses.
In this type of scenario the area where snow falls is very small when considered on a global scale. That means small adjustments in its position can have a massive impact on the weather we experience and computer models often only firm up on the details shortly before the event.
By UK standards the cold period looks set to be quite lengthy. The chart below is generated using data from the ECM ensemble model. It is run many times with the starting conditions fed into it, adjusted slightly to help account for uncertainty in the present state of the atmosphere.
Each line on the chart represents the forecast air temperatures at approximately 1500m above sea level. The horizontal axis shows the date range which is from December 5th to 19th. Between December 5th and 13th virtually all of the runs forecast air mass temperatures to be significantly below the thick black line which represents the 30 year average. Therefore, confidence in it staying cold until then is high.
Beyond December 13th the spread of solutions increases and it becomes very wide. An increasing number of the runs climb above the thick black line and show mild conditions returning. Nonetheless, a number remain in the cold category. That means forecast confidence is much lower by then, but the chance of it turning milder rises considerably. The chart is for London and comparable ones for locations further north suggest the cold conditions could hold on for a while longer, although they follow a similar trend.
Cold or very cold weather becomes established in all of the UK this week. Sharp or severe night frosts develop and even during the days temperatures struggle to rise much above freezing point.
There is a risk of snow, especially in Scotland, but possibly more widely on Thursday. In the longer term snow showers are most likely in northern and eastern coastal counties, but there is a chance of them forming into more organised bands and pushing inland at times. More widespread snow could occur if milder air moves up from the southwest and the boundary zone become slow moving over the UK.
At the moment computer models are suggesting the chance of it turning milder will increase from about 13th December, but there is a lot of uncertainty about the timing of a transition. The north is likely to hold on to the cold conditions for the longest.
If you would like to check the chance of some of the white stuff falling in your locality check Will it snow for the latest view. The percentage chances are generated using forecasts from multiple computer models to help improve accuracy.
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