An unusually early weakening of the northern hemisphere's stratospheric polar vortex could increase the likelihood of colder weather in northern Europe in the coming weeks. Forecast models suggest the circulation may weaken substantially in the 10 days following 2 November, at a point in the year when it would normally be gaining strength.
The forecasts raise the possibility of a major sudden stratospheric warming event, known as an SSW. Such a disruption could affect weather patterns over subsequent weeks, making persistent areas of high pressure across the mid-latitudes more likely and potentially bringing colder conditions later. Whether the weakening will meet the requirements for a major event remains uncertain.
The polar vortex consists of a large zone of cold air and low pressure high above the Arctic. After weakening through summer, it normally strengthens during autumn and into winter. Its powerful anticlockwise circulation helps keep the coldest air close to the pole, but disturbances can allow some of that air to move south into the mid-latitudes.
During an SSW, temperatures high in the stratosphere climb rapidly, often rising by as much as 50C within a few days. This warming is associated with a sharp weakening of the strong westerly winds around the vortex. Those winds can even reverse direction, disrupting the circulation.
Records began in 1958, and the earliest major SSW recorded occurred on 28 November 1968. Although current models strongly favour a substantial weakening of the vortex, they are less certain about a full reversal of its winds, the condition needed for a major SSW.
The projected weakening would represent a notable disruption so early in the season, with possible consequences extending into early winter. Northern Europe could become more likely to experience colder and more settled weather. However, a period of severe cold appears unlikely within the next two to three weeks.
Elsewhere, a low-pressure system that intensified over the Mediterranean Sea last week is forecast to bring heavy rainfall and strong winds to central and southern Italy on Monday, 2 November. Warnings have been issued for those areas, with more than 100mm of rain expected over higher ground in southern Italy within 24 hours.
The system is then expected to travel east across the Ionian Sea towards Greece. Its arrival would follow exceptionally wet and unsettled conditions there last week, from which the country is still recovering.
