The latest weather buzz

Weather updates from site founder Brian Gaze that keep you in the know.

Is an early March cold spell likely

Published 22nd February, 08:52

The combination of the recent Sudden Stratospheric Warming (SSW) and the imminent onset of another one will increase the risk of wintry weather in the UK during the coming weeks. In the short term it is expected to be chilly at times, but could a Beast From The East style cold spell develop during the first third of March?

Is very cold weather heading our way?

Computer models are showing the possibility of a significant cold spell affecting the UK, or at least parts of it, during the first third of March. However, there is a lot of uncertainty when looking this far ahead and particularly so when the normal zonal (west to east flow) is being disrupted.

Although it is late in the season snow is still quite common at this time of the year even in southern Britain. Nonetheless, the lengthening days and strengthening sun mean that unless the air is exceptionally cold, as it was during the Beast From The East period in 2018, any lying snow will quickly melt. That is particularly the case in the southern half of the country. 

Model agreement on cold air moving southwards

The GFS model chart below is for March 4th. It shows 850hPa temperatures, those at about 1500m above sea level, and surface pressure. The deep blues and purples are used to show where the air is very cold.

GFS 850hPa temperature forecast chart

This particular model run moves an area of low pressure up from the southwest and the Arctic air sinks southwards mostly to the west of the UK. Therefore, it is basically a close miss as far as cold weather is concerned. The scenario is very plausible and it highlights how small changes in the synoptic pattern across the North Atlantic region can have a big impact on the weather we experience.

The Canadian global model chart below illustrates the previous point well. It also has very cold Arctic air plunging southwards, but this time the core of it is over western Europe. Therefore, the UK turns cold and the potential for snow increases, especially in the east. 

Finally, the ECM global model chart below shows the cold plunge being a little less vigorous and centred further east. As a result the air over the UK is chilly, especially in the north, but it isn't anything of note at this point.

At this time of the year 850hPa temperature values often need to be below -10C for daytime temperatures at the surface to be kept very low. For example, March 2018 started with 850hPa temperatures close to -15C in much of the UK and that was enough to scrape ice days in the south.

On the ECM chart 850hPa temperatures are only a little below -5C in north eastern Britain and in the south they are close to 0C, so it is a very different scenario.

How likely is a major cold spell?

Although the computer model charts above show a similar broad scale pattern the devil is in the detail as far as the UK's weather is concerned. Therefore, to asses the likelihood of different scenarios ensemble model data is used. An ensemble model is one which is run many times with the starting conditions adjusted slightly to account for uncertainty.

The data table below is generated using data from the GEFS ensemble model and it shows the 850hPa temperatures all of the individual runs are forecasting for London with each column representing one day.

The key points to note are:

1) There is a strong signal for rather cold air between 24/02 and 27/02, but the number of runs showing very cold air (-10C or lower) is only 3% so that is a very unlikely scenario

2) During the first third of March the number of runs showing rather cold air peaks at 48% and the number showing very cold peaks at 10%. Therefore, a very cold spell remains a low probability, at least in this part of the UK

Computer models are generally showing cold air moving approaching from the north rather than east. Therefore, the chance of it reaching northern parts of the UK is higher. For example, the comparable data table for Inverness has 19% of runs bringing in very cold air.

Summary

In the next week it will be quite chilly at times. However, the big questions are focused on how the weather will develop during the first third of March. Although an increasing number of computer models are showing an Arctic plunge, the chance of it hitting the UK directly currently appears to be quite low.

Latest charts