Polar vortex is collapsing at a record early date: December outlook

The stratospheric polar vortex over the Arctic is weakening at a record-early date: in early November, the westerly winds around it dropped from the usual 25 m/s to just a few metres per second. If the wind reverses, it will be the earliest major sudden stratospheric warming in the history of observations.

November normally sees the opposite pattern. As the polar regions cool sharply, the vortex gains strength, and the westerly winds that encircle it and keep very cold air locked over the Arctic pick up as well. This year's scenario runs against that pattern.

What the forecast models show

Most models expect the westerly flow to give way to an easterly one: some calculations say as soon as this week, others next week. That would be a record-early major sudden stratospheric warming (SSW). The previous record was set in late November 1958.

The breakdown of the vortex itself is becoming more and more likely. The consequences for weather in Europe, however, remain highly uncertain: nothing like this has ever happened in early November, so there are no past examples to rely on.

What kind of winter to expect: an assessment based on simulations

To gauge the possible impact, meteorologist Andrey Flis of Severe Weather Europe went through thousands of simulated winters. He selected cases in which the vortex was significantly disrupted in early November, alongside a strong El Niño and the westerly phase of the quasi-biennial oscillation (QBO) in the equatorial stratosphere, just as this year. He then compared the results with similar simulations in which the vortex stayed stable.

“For December, we see an interesting signal: combined with a strong El Niño and a westerly QBO, it points to a tendency toward colder weather in some parts of the US and Europe,” Flis wrote on X.

According to the simulations, air pressure over Western Europe in December could be higher than normal, and lower over the northern and eastern parts of the continent. Such a setup can sustain northerly winds and bring colder air to Europe. Still, this is only a model-based estimate, not a forecast: it is not even clear yet how exactly the wind will behave in the coming days.

Around Prague, based on reporting by novinky.cz