By Brian Gaze
The meteorological spring started on March 1st but in the coming days temperatures will be dropping and wintry conditions returning. The expected weather patterns are similar to those in December which brought the cold spell dubbed the "Troll from Trondheim". High pressure over Greenland and low pressure over Scandinavia leads to a plunge of very cold Arctic air moving down across the UK.
ECM air mass temperature forecast chart
The days are longer now than in December so maximum temperatures will be higher, particularly on days when sunny spells develop. However, the nights are still long enough for severe frosts and, in areas where showers become prolonged or longer outbreaks of sleet and snow develop, the days could be particularly cold. Also, values will be lower where there is snow cover.
The chart below shows forecast maximum temperatures on Tuesday 7th March. It is based on data from the high resolution UKV computer model which is operated by the UK Met Office. In much of the UK temperatures are a few degrees above freezing point, but in places they are at or below 0C (32F), particularly in the north. The mildest part of the country is the south west.
The chart is only used to illustrate possible values; and significant day to day and regional variability is expected due to the reasons outlined above. In the southern half of the UK daytime temperatures could easily reach 7C in long sunny spells.
Frosts will be widespread on some nights. The lowest temperatures are forecast to be in the north, although even further south severe frosts are possible, especially if there is snow cover.
The UKV forecast chart below shows values dipping to -12C in the Scottish Highlands and -6C in central England on the morning of Wednesday 8th March.
The lowest temperature recorded in the UK in March is -22.8C at Logie Coldstone, Aberdeenshire, 14th March 1958. Despite the expectations of some low values, the record is unlikely to be challenged during this cold spell.
Temperatures will be low enough for snow but how much is on the way? Initially significant snowfall is likely to be focused on northern Scotland and, more generally, north eastern Britain. Disruptive amounts in these areas are probable and the UK Met Office has issued warnings.
In other parts of the UK things are less clear. However, the main risk in southern and central regions is likely to come from areas of low pressure moving up from the southwest. As they bump into the cold air over the UK they bring the potential for heavy and disruptive snowfall.
At this stage it isn't possible to say which parts of the UK are at risk, but often in the past it has been areas north of the M4 corridor with rain to its south. 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.
Nonetheless, there is the potential for disruptive snow in the southern half of the UK, but for it to happen a number of ingredients are needed. Timing could also be a factor, with snow being more likely to accumulate if it falls during the nights rather than days. Also, bear in mind that once areas of low pressure move away, the precipitation stops and the sun reappears, then snow will start melting rapidly.
The length of the cold spell is still uncertain, but it is be expected to last longer in the north than the south. The chart below is for London and 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 temperature at approximately 1500m above sea level. The horizontal axis shows the date range which is from March 4th to 19th. If the lines are mostly close to each other, there is a high degree of confidence in a particular outcome. If they diverge, confidence is low because a range of possible solutions is being suggested.
Between March 4th and 8th a vast majority of runs remain below the thick black line which shows the 30 year average. Therefore, confidence in it being colder than the norm is high. Things then change markedly with a cluster of runs showing a continuation of the cold theme for several days and another one bringing in much milder air. For locations further north more runs keep it cold for longer and in Scotland the ensemble mean (averaging out all of the individual runs) remains below the 30 year norm until the end of the period.
The differences between locations is largely down to the northwards progression of milder air pushing in from the southwest. It suggests the risk of snow will increasingly become confined to the northern half of the UK with rain becoming more likely in the south as time progresses.
A spell of cold and wintry weather is on the way for all of the UK. Sharp or severe frosts become widespread and daytime temperatures will be below the March average, but values will vary significantly due to factors such as amounts of sunshine and the chance of snow.
Disruptive snow is forecast in northern Scotland and northeastern Britain early in the cold spell. The risk could become more widespread as areas of low pressure start to push in from the southwest. Even the south may see disruptive snow, but conditions are quite marginal and there remains a lot of doubt about the track the low pressure areas will take. If they head towards northern France, the precipitation associated with them will stay to the south of Britain. On the other hand, if they go further north across central parts of the UK, milder air will push into the south and precipitation will fall as rain.
Computer models are suggesting the chance of it turning milder will start increasing from about 9th March in the south. However, there is uncertainty about the northwards progression of the milder air and the cold conditions could hang around in the north for quite a long time.
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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