The Met Office and BBC Weather on how and when disrupted polar vortex could affect the UK’s weather

The Met Office is expecting a sudden stratospheric warming to occur this week, with the UK to potentially feel the effects of the rapid warming towards the end of November.

The forecaster says it could be the earliest major sudden stratospheric warming on record by around three weeks.

It is still uncertain what the sudden stratospheric warming could mean for our weather, but the Met Office says a cooler, drier period might come a bit earlier.

Sudden stratospheric warming explained

Sudden stratospheric warming (SSW) is when rapid warming occurs high up in the stratosphere.

A rapid warming (up to about 50­°C in a few days) takes place between 10 km and 50 km above the earth’s surface. We don’t feel the ‘warming’ due to it being so high up but we can start to see knock-on effects on the jet stream a few weeks later, which in turn effects our weather lower down.

The stratospheric sudden warming doesn’t happen every year and it doesn’t always affect our weather when it does.

What polar vortex means and how a sudden stratospheric occurs

In winter, strong westerly winds circle around the pole high up in the stratosphere - the stratospheric polar vortex - and it circulates around cold air high over the Arctic.

In some years, the winds in the polar vortex temporarily weaken, or even reverse to flow from east to west. The cold air then descends very rapidly in the polar vortex and this causes the temperature in the stratosphere to rise very rapidly.

How a sudden stratospheric warming affects our weather

The stratospheric sudden warming can sometimes cause the jet stream to ‘snake’ more, which tends to create a large area of blocking high pressure.

Typically this will form over the North Atlantic and Scandinavia, meaning northern Europe, including the UK is likely to get a long spell of dry, cold weather. Southern Europe will tend to be more mild, wet and windy. On the boundary of these areas, cold easterly winds develop and in some cases the drop in temperatures leads to snow, which is what happened in early 2018.

How and when disrupted polar vortex could affect the UK’s weather

Met Office

The Met Office says model forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) are predicting the winds to slow down or even reverse in the early stages of November.

Alex Burkill, Met Office Presenter, said: “This looks like it could be a fairly major SSW and it could be the earliest major SSW on record by around three weeks.

“As a result, it's somewhat new territory for us. We don't exactly know how it's going to play out, what the high pressures will do, how they will be influenced by other factors. Likely that the El Niño is driving the earlier onset of an SSW, but it will also be playing a role in different ways.

“I just spoke about the fact that with this SSW occurring it means an increased likelihood of a blocking pattern as we go deeper into November and December. But that's quite a contrast to what we were saying is most likely with an El Niño event for the UK, where previous El Niño events they often lead to milder, wetter, windier starts to winter and then something cooler and drier for the latter stages of winter.

“But now it looks like that cooler, drier period might come a bit earlier. And that's probably partly due to this El Niño being exceptionally strong.”

BBC Weather

BBC Weather says there is a chance of a cooler and drier development in the north during the last week of November due to disruption to the polar vortex throughout early November.

The forecaster writes: “The last week of November is more problematic, with competing atmospheric configurations indicated. A similar pattern may continue for most areas, but there is a chance of a cooler and drier development, at least in the north.

“Current happenings in the stratosphere might eventually play a role as the polar vortex is getting disrupted throughout early November, which is unusually or even uniquely early. If any effects were to filter down to the surface, it would take at least a couple of weeks, so that could be towards the end of November. It's nothing more than an outside risk at the moment but if this were to manifest then chiller weather could arrive.”