High above the Arctic, powerful winds usually circle the North Pole each winter, helping trap some of the coldest air on Earth.
But this circulation, known as the polar vortex, has weakened and shifted position in recent days.
While such disruptions can occur during winter, scientists say an event this early in November is unprecedented in modern observations.
Similar disruptions in past years have sometimes been followed by colder weather affecting parts of Europe and North America.
However, as this is happening so early in the season, there are few historical examples to compare with and it could throw current winter forecasts into even more uncertainty.
The polar vortex develops every autumn as temperatures fall in the Arctic.
Strong westerly winds form in the stratosphere, roughly 10 to 30 miles (15-50km) above Earth's surface, locking the coldest air near the pole.
Atmospheric disturbances can cause the air high up to start warming which then makes the vortex winds weaken. This sudden stratospheric warming (SSW) can eventually lead to the strong westerlies reversing into easterly winds.
In the following days and weeks these changes can alter the position of the jet stream, increasing the chance of a blocked weather pattern where it can become drier with a colder northerly or easterly wind bringing Arctic air in.
It can also boost the possibility of snow, especially in eastern UK.
If the winds continue to weaken and end up reversing as forecast, this will be known as a major SSW event.
Jeff Knight, lead climate scientist from the Met Office, told the BBC: "It would appear to be the earliest on record looking back over the 75 years we've been observing…with most experts agreeing this is a very unusual event."
The current earliest major SSW event occurred on 28 November in 1969 and 2025.
Reasons for why we're seeing such an early event is not clear. It could be just part of the variability of our weather patterns.
But we also know in a "normal" El Niño year, this can typically bring a weakening of the polar vortex towards the end of winter.
While the warmer waters of the Pacific in an El Niño directly disrupt the weather around tropical areas, pulses of energy in the atmosphere, known as planetary waves, can cause disturbances even at polar regions.
These waves can push the polar vortex out of position and break the circulation up.
With the current record El Niño taking us into uncharted territory in terms of disruption to worldwide weather patterns, Knight suggested it could have "helped load the dice for this SSW event. However, because this is so rare, there's not enough evidence".
A link between a strong El Niño and early onset of a SSW event is unclear and will need further research.
Long-range forecasts issued by the Met Office show that the UK was more likely to see a stormy start to winter, followed by a colder spell to end winter. This is consistent with expectations from an El Niño pattern.
However, this unusually early SSW event was not widely expected.
Professor Andrew Perez-Charlton, meteorologist at the University of Reading, said: "The typical response to a SSW is for colder and drier conditions over Europe and the UK."
"Because we haven't seen a SSW event quite like this before, it is harder to anticipate a cold spell at the moment."
One reason is the timing.
The Arctic is not yet as cold as it becomes later in winter, with the polar vortex itself having only recently developed.
Any influence it may have on weather patterns could be weaker than during a mid-winter disruption.
And because the air isn't at mid-winter temperatures yet it means cold and snowy weather for the UK is far from guaranteed.
Long-term forecasts are based on large-scale climate signals and how they have affected weather patterns in previous years.
Much of the forecast for this autumn and winter has been focused around the influence of the exceptionally strong El Niño, pointing to a stormier start to winter, with the potential to be followed by colder and drier conditions from January.
But this unusually early disruption of the polar vortex and SSW presents meteorologists with a situation for which there are few modern comparisons.
For Perez-Charlton, the "concerning thing would be if this event ends up bringing the typical drier weather pattern and lack of rain" associated with a sudden stratospheric warming over the coming months.
With most of England and Wales still in drought, we need to rely on a wetter winter to fill up extremely low reservoirs.
For now, experts stress that it is too early to know whether recent changes in the polar vortex will change the winter outlook.
