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. This is associated with a dramatic weakening, or even reversal, of the strong westerly winds surrounding the polar vortex, disrupting its circulation. The earliest recorded major SSW, occurred on 28 November 1968, after records began 1958. Current forecast models strongly favour a significant weakening of the vortex, but a complete reversal of the winds required for a major SSW remains uncertain.

Such a development would mark a notable early-season disruption, with potential implications for weather patterns into early winter. The likelihood of colder, more settled conditions across northern Europe could increase, although a spell of severe cold appears unlikely over the next two to three weeks.

Meanwhile, an area of low pressure that deepened in the Mediterranean Sea last week is expected to bring heavy rain and strong winds to central and southern Italy on Monday, prompting warnings being issued for these areas. More than 100mm of rain is forecast to fall in just 24 hours across higher ground in southern Italy.

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The system is expected to move eastwards across the Ionian Sea, before making its way to Greece, which is still recovering from exceptionally wet and unsettled weather last week.