A late blast of wintry weather?

Temperatures will be spring-like over the next few days, with 18°C possibly being reached in some locations. However, there are growing signs that much colder weather will return next week, suggesting that winter may be making a late comeback. Is there any sign of snow?

Arctic plunge on the way?

In recent days computer models have increasingly been forecasting a cold spell during the middle third of March. The details are still being firmed up, but in general terms high pressure is expected to become centred near Greenland. This will open the gate to a plunge of cold Arctic air which is set to head towards the United Kingdom and western Europe.

There is uncertainty about how far south the cold air will reach and the extent of any snow risk. At this time of year the increasingly strong sun means that lying snow melts quickly. That said, falling snow even in southern counties is more common now than in November and the first half of December. Additionally, sharp frosts remain a possibility in all areas.

A blocked weather pattern

High pressure is expected to build towards Greenland by Saturday, 8th March. In the following days a strong blocking pattern is set to establish itself, preventing disturbances in the Atlantic from progressing eastwards towards western Europe as they usually would.

That would allow a plunge of cold Arctic air to sweep southwards. Computer models are increasingly suggesting it could reach all parts of the UK, but there is still uncertainty. There remains a viable scenario in which the Arctic air mass does not reach the south, allowing low pressure to the southwest to remain more dominant. If that were to happen southern counties would be more likely to experience showery spells of rain, with winds coming from an easterly direction. Although it could feel quite raw in the wet conditions, frost and snow would be unlikely.

Arctic air is shown sweeping southwards

The animation above shows cold air reaching all areas during the early part of next week. The dark blue shading indicates where the coldest air is, and the numbers represent forecast temperatures at approximately 1,500m above sea level. At the surface it wouldn't be as cold but if clear periods develop, sharp nighttime frosts could be expected and showers may well turn wintry even in southern locations.

How widespread is the risk of snow?

At this stage it isn't possible to be specific about the likelihood of more widespread falling snow or the risk of accumulations. However, ensemble models can help assess the probability and the chart below provides an example.

It uses data from the ECMWF Ensemble and represents the precipitation type forecast for Berkhamsted, Hertfordshire between 5th and 19th March. The spikes in the upper part indicate forecast precipitation at a given time, while the numbers below show the percentage chance of different types.

On this chart rain, snow, wet snow, sleet and ice pellets are displayed. The chance of snow peaks at 18% on 12th March, so not a very high risk but it is significant enough to keep an eye on developments in the coming days. The chance is higher in locations further northwards and eastwards.

How long will the cold spell last for?

At the moment it looks as though colder than average conditions could persist until around 17th March in the south and perhaps a little longer in the north. However, when looking this far ahead there is always a high degree of uncertainty. In spring identifying trends can be even trickier, especially as the Stratospheric Polar Vortex weakens or, as is likely this month, a Sudden Stratospheric Warming (SSW) occurs. 

Despite the presence of a cold air mass and the potential for frost, daytime temperatures at this time of year can still rise quickly due to the rapidly strengthening sun. As a result a large diurnal (day-to-night) temperature range is possible, with values perhaps exceeding 20°C in a few locations.

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