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A major SSW is expected

Published 11th February, 11:22

A major sudden Stratospheric Warming (SSW) is expected in the next week. Wil it lead to a return of wintry weather? Is a Beast From The East style cold spell a possibility. If so when could we expect it to arrive?

Sudden Stratospheric Warming (SSW)

A major SSW occurs when the winds in the Stratospheric Polar Vortex (poleward of 50°) reverse. Cold air descends and temperatures in the stratosphere rise. An SSW is a warming of up to 60°C (the amount temperatures increase by, not the absolute value) in a few days.

Although a minor SSW (zonal winds slow but do not reverse) occurred earlier in the winter the one now modelled is more likely to have a significant impact on the UK's weather.

The GEFS chart below shows forecast zonal winds in the stratosphere during the next 16 days. It is calculated by averaging out the zonal component (west to east) of winds around the globe at 60 degree north. All of the runs in the model show zonal winds becoming negative which means reversing from westerly to easterly. They stay in this phase for several days and indicate a major SSW. Therefore, confidence in a major SSW taking place is very high.   

GEFS 60 deg north zonal wind forecasts

The next question is will it have an impact on the UK's weather or not. The likelihood is yes it will, but with that said the data suggests the probability is around 70% of the time, so it is not certain.

If it is assumed there will be an impact the next question is what will it be? Again the answer is nuanced because although the chance of cold weather in the UK is increased it is not certain. This SSW is expected to displace the Stratospheric Polar Vortex (SPV) unlike the 2018 episode which caused it to split and led to the Beast From The East cold spell.

Beast From The East in late February?

If the SSW is to have an impact on the UK's weather the ECM ENS and GEFS ensemble models are the places to look for signs. The days are rapidly lengthening now, but as 2018 illustrated it is still possible to record ice days (when temperatures remain below 0C) in southern Britain even in mid-March. Head northwards and the statistical chance increases.

However, for this to happen more things need to fall into place and the "margin for error" is smaller than when the days are shorter. In particular, a Beast From The East style cold spell would require 850hPa temperatures to drop below -10C.

If you want to keep an eye on how things are shaping up one of the easiest ways is to check the GEFS 850hPa temperature data tables. To do this go to the GEFS custom data tables, select 850hPa C and Location buttons then Show table. To filter use the Summarise by hour radio buttons.  

The things to look for are the amount of purple in the columns and the percentage number if brackets. Purple indicates 850hPa temperatures (those at approximately 1500m above sea level) below -4C and the figure in brackets is the percentage which dip to -10C or lower. If a major cold spell is on the way this number and the amount of purple in the columns will both increase a lot. 

At the time of writing the London data table shows the number of runs in the purple category peaking at 39% on 24/02 and the figure in brackets at 13% on 23/02. The data tables are fully updated every 6 hours so watching them through the coming week will be a great way of keeping an eye on how things are looking. At this stage it's much better to stick to what the data is showing rather than be swept away by headlines which may be focusing on the more extreme possibilities.

I've tried to remain completely objective so far in this post, but now to venture an opinion! At this stage I think a cold spell comparable to the 2018 Beast From The East is unlikely. By UK standards that was quite an extreme event and it resulted from an SSW which (as mentioned earlier) if I recall correctly caused the SPV to split. To see something as severe we probably need to roll a 1 on the dice, but for a cold spell a 1, 2, 3 may do the trick. Finally, even if a cold spell (I'm talking about 4 days or more, not just a brief and chilly northwesterly wind) develops it doesn't necessarily lead to widespread snow, as December 2022 proved beyond doubt.       

TL;DR

A major SSW is expected in the next week. Statistically it means the chance of cold weather in the UK will increase in late February and early March. However, it is possible the SSW won't have an impact on our weather and even if it does there are no guarantees it will tip things in the cold direction.

Finally, remember that the 2018 Beast From The East was very cold for the UK and the chance of a repeat as severe is relatively low despite the major SSW.    

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