Increasing possibility of a Sudden Stratospheric Warming in the New Year which may bring colder weather

There is growing support for a possible Sudden Stratospheric Warming (SSW) in early January, which could “load the dice” in favour of cold and wintry weather in northern Europe including the UK in the New Year.

A Sudden Stratospheric Warming (SSW) occurs when the usual westerly flow over the arctic is disrupted by large-scale atmosphere waves (called Rossby waves) in the troposphere which get pushed higher into the atmosphere. These waves can “break” (like waves in the ocean) on top of the polar vortex and weaken it. If waves are strong enough, the winds of the polar vortex can weaken so much that they can reverse from being westerly to easterly. This leads to cold air descending and warming rapidly, with a sudden jump in temperature.

Although a Sudden Stratospheric Warming refers to a rapid warming (in extreme cases as much as 50°C in just a couple of days) in the polar stratosphere, between 10 km and 50 km up, a major SSW is often defined as a wintertime event in which the zonal (westerly) mean wind at 60N and 10 hPa reaches zero and turns easterly.

Most models predict that the polar vortex will remain slightly weaker than average for the rest of December, with some models indicating a full-on stratospheric polar vortex breakdown in early January. The two main NWP (Numerical Weather Prediction) models that will be looked in this blog, ECWMF and GFS, forecast the stratosphere winds and temperatures, but differ between them, for now, on the likelihood on a SSW.

If the sudden stratospheric warming does happen, the most likely time would be the first 10 days of January, with effects on the weather at the populated mid-latitudes of Europe and North America appearing a few weeks later.

December has so far seen a slightly weaker than average polar vortex, which makes it more susceptible over a prolonged period, to small wobbles that can further destabilize the vortex and break it down completely and this is what is being hinted at by the models in the next few weeks.

The 12z European ECMWF ensembles yesterday showed 29 out 51 ensembles (57%) with a U wind reversal at 60-90 N at the beginning of January. The zonal wind reversal is where westerly winds at 10 hPa reverse direction to easterly, while stratosphere temperatures rise.

So if the SSW does occur in the New Year, which is looking increasingly likely, though not a certainty yet, then there is a strong chance that the UK and northern Europe could see some cold and wintry impacts later in January and perhaps over into February too. We'll keep you posted over the next few weeks on the potential for the SSW and if one happens, if there will be impacts from it on our weather with regards to colder and wintry prospects.