Unprecedented early disruption high above the North Pole, though a cold Belgian winter is far from settled

The polar vortex, a large mass of very cold air in the stratosphere 10 to 50 kilometres up, is weakening exceptionally early this autumn. Normally it strengthens through the autumn and only weakens from January or February. Meteorologist Bram Verbruggen told VRT NWS that such an early disruption has not been seen in 75 years of records. There are also signs that the vortex is splitting in two.

During a disruption, part of the stratosphere can suddenly warm sharply. The westerly winds that hold the cold air around the pole then weaken, meander or even reverse. After a few weeks this can affect the jet stream that steers Europe's weather. Weer.nl notes that the effects at ground level usually appear only one to four weeks later, and that weather models still diverged widely at the end of October on how far the weakening would go.

According to both sources, a cold spell in Belgium is anything but certain. The cold air could just as easily flow towards Canada and the United States, and this early in the season little cold has built up over the pole. Forecasters expect no unusual weather for the next two weeks, and next week will even be milder. Why the vortex is already faltering is unclear. Verbruggen thinks the strong El Niño and climate warming may both play a part.