• Dan

Sudden Stratospheric Warming declared (05/01/21) - what does this mean for our winter weather?

Yesterday, witnessed a major Sudden Stratospheric Warming (SSW) it is an event in which westerly wind 60N at 10hPa roughly 30km high in atmosphere, above Arctic, reverses to an easterly wind indicated by negative value, the easterlies will continue to strengthen.

Not all SSW events give the same outcomes, but far majority reshuffle weather patterns across Northern Hemisphere, sometimes the easterlies do not trickle to lower atmosphere and there is no impact as was evident in January 2019.

Some feature displacements and others splits, sometimes both, this Met Office video offers great visualisation and insight.

This reversal to easterly wind in upper atmosphere is an unnatural occurrence in winter time, particularly approaching mid winter, when stratospheric westerlies are at their strongest. This is why first half of winter tends to be more unsettled and mild, westerly driven, the strong stratospheric westerlies bleed into troposphere the part of the lower atmosphere, where weather happens, as we witnessed last winter. The extremely strong stratospheric westerlies were the direct result of February 2020 being the wettest February on record in UK.

Typical winter killer patterns with strong stratospheric westerlies/vortex, westerlies driven across North Atlantic and Europe is very mild with coldest air bottled up near Greenland.

There is greater climatology support for SSW as winter starts to fade out in Northern Hemisphere, why they're more common in February, as we had seen in 2018. The reversal of westerly wind is accompanied with a dramatic warming of up to 50C this warming fundamentally changes the structure and strength of stratospheric polar vortex, what we see is it goes from a concentric shape, compact keeping very cold air in Arctic to one that becomes more mouldable, in that parts can break away far south into mid latitudes.

Typical pressure patterns when stratospheric westerlies/vortex are weak often allows for high latitude blocking into Greenland ect, and the jet stream is much weaker and goes further south. We therefore often see the displacement of very cold Arctic much further south than usual. Approx 66% of SSW result in extended cold conditions in Northern Europe, so there's a greater likelihood in bringing cold, in fact the relationship is greatest in Europe. Hence why these events often excite, as we saw in February 2018 with the 'Beast from the East' spectacularly showed what a SSW can usher in.

But what is happening with this SSW? There are promising signs from ECM model that we're going to experience a relatively quick coupling towards day 10 - usually it takes 14-21 days to see an impact at earliest, worth noting we didn't see coupling happen in January 2019 SSW, the easterlies did not descend to lower atmosphere in which it can influence our weather patterns, so the early indicators with this SSW are I believe much more promising.

As you can see from the blues the easterly winds the day 9 chart from ECM shows the easterlies feeding all the way down indicative the impacts of this SSW will be seen.

In contrast to other winters we're not critically relying on a SSW the westerlies in lower atmosphere are unusually weak, and this has nothing to do with a SSW. As you will know the last 2 weeks have been on the colder side, a large swathe of country has seen snowfall but alas the lack of cold air, has meant significant cold has been absent.

And, we have seen more in the way of rain and sleet in SE England unfortunate given patterns but hold in there. A displacement is expected from this SSW they tend to not be as favourable as splits, but this one could position itself to be rather favourable to Europe. There is some hints in modelling we could see a split mid month in February 2018 we had one.

The modelling is suggesting a transfer of the polar vortex to Russia and a part to N/E Europe what this means there's going to be extensive deep cold relatively close to UK something of which so far this winter has been missing.

ECM 12z of yesterday very much displayed an air mass slamming SW into Europe evocative to the Beast From The East and that raised a few eyebrows.

I do think realistically this is a bit too rushed, nonetheless in the final third of January, there is a risk of exceptionally cold conditions to come from east the Met Office in their extended outlook have in fact mentioned these very cold conditions in Scandinavia could cross the North Sea. This transpiring would result in heavy, disruptive snowfalls across eastern half of England. I am not guaranteeing it, we are trusting on blocking close to UK to help direct it our way, but there is a heightened chance.

The position we find ourselves has the greatest interest since February 2018.

Even in event later January was to fail to deliver, given how powerful this SSW is looking it is likely the remainder of the winter through February, and even into early Spring will have a higher chance of cold and wintry conditions. So a lot to watch closely right now. Buckle in.


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