Met Office warns of increased chance of 'cold start to winter'
Forecasters have predicted an increased chance of a colder-than-usual start to this winter due to a change in Arctic conditions and "unusual" tropical rainfall patterns.
The Met Office said the conditions mean there is a 30% likelihood the mercury will plunge at the beginning of this winter - the highest risk of a cold start since the bitterly cold season of 2010/11.
But the agency said it was too early to predict whether it would be a snowy, wet or dry three-month period from November.
The key factors leading to colder temperatures include "disturbed" stratospheric Arctic winds known as the polar vortex, which affects the jet stream, and a La Nina, the opposite of an El Nino, bringing lower temperatures in the tropics.
But the Met Office pointed out there was still a 70% chance of milder temperatures and, following a reversal in expected atmospheric patterns in February this year, the prevailing westward winds could increase the chances of warm and wet conditions.
Forecasters reached the long-range prediction after feeding their analysis of those factors into a computer model, with the result higher than the expected 20% probability suggested by a 30-year rolling average up to 2010.
Professor Adam Scaife, head of long-range prediction at the Met Office Hadley Centre, said: "The risk of a cold start to winter has increased to 30% this year.
"Statistically, however, it is still more likely that the UK will experience a normal start to winter, but there is an increased risk of cold snaps between now and Christmas, although this doesn't necessarily mean we will get large amounts of snow.
"Several factors, including tropical rainfall, are known to drive UK and European winter conditions: following a strong El Nino last year, the tropics are now influenced by a weak La Nina and unusual rainfall conditions in the Indian Ocean.
"Historical weather observations and our latest computer model simulations agree that these factors are increasing the risk of a cold start to winter for the UK, but this is unlikely to persist through winter as a whole."
Meanwhile, a study published in October suggested warming in the Arctic is influencing the jet stream, a high-altitude corridor of fast-moving air.
It is thought this could have caused severe cold snaps such as the record snowfall in New York during the winter of 2014/15, and unusually cold winters in the UK in 2009/10 and 2010/11.
The winter of 2010 saw the UK's coldest December in records stretching back 100 years.
Studies have shown that when the jet stream follows a "wavy" irregular path there are more cold weather fronts plunging south from the Arctic into mid-latitudes, bringing freezing conditions that persist for weeks at a time.