The stratospheric polar vortex over the northern hemisphere is facing the prospect of an unusually early weakening. Forecast models suggest it could weaken significantly over the next 10 days, when it would typically be strengthening.

A major sudden stratospheric warming (SSW) event is possible. This could influence weather patterns in the following weeks, increasing the likelihood of persistent areas of high pressure across the mid-latitudes and colder conditions weeks later.

The polar vortex is a large area of low pressure and cold air high above the Arctic. It weakens during the summer before strengthening during autumn and into winter. Its strong anticlockwise circulation helps contain the coldest air near the pole, although disturbances can allow colder air to spill southwards into the mid-latitudes.

An SSW event occurs when temperatures high in the stratosphere rise rapidly, often by as much as 50C in just a few days.…