28 Nov 2016

Cold start to winter more likely than normal this year

The Met Office released its long-range outlook today that covers the whole winter – from December until February.

Interestingly, it continues the trend that was highlighted last month, whereby a cold start to winter is more likely than normal this year for the UK.

Bare tree in snowy field

This is significant, as it moves away from the mild, stormy and wet winters that we have experienced during the past few years, with colder, drier weather more likely through December and January.

As ever, it’s worth emphasising that these long-range forecasts are focused on general trends, rather than the day to day detail that forecasts on TV and apps provide.

Nevertheless, they are a useful tool that take into account the state of the major ocean and atmosphere influences around the world and suggest in which direction this winter’s weather may lean.

Why is cold weather more likely than normal?

As mentioned above, when pieced together like a jigsaw, the state of various parts of earth’s oceans and atmosphere give an indication as to what type of weather is more likely in the longer term trend.

One major difference this year is the state of the stratospheric polar vortex, an area of low pressure 10-50km up in the atmosphere that contains a vault of cold air over the Arctic.

It is much weaker than normal this year, which tends to weaken the westerly jet stream over the Atlantic Ocean, making mild, wet and windy weather much less likely than recent winters.

Also, sea surface temperatures in the eastern Pacific Ocean are colder than normal, nearing the threshold for La Nina.

In La Nina conditions, winters in the UK tend to start colder than normal, before turning milder and wetter later on.

Finally, Arctic sea ice extent is at a record low level for this time of year, with the largest deficits in the Barents and Kara Seas.

Arctic sea ice levels graph
Arctic Sea ice extent: NSIDC

Recent research suggests that a lack of sea ice in the Eurasian part of the Arctic may increase the likelihood of blocking weather patterns, bringing a greater chance of high pressure and cold winds from the north and east.

How confident is this long-range forecast?

Long-range forecasts always have a notable level of uncertainty associated with them because they are looking so far ahead and are incorporating complex ocean-atmosphere processes.

What the long-range forecast doesn’t tell us is how any potential cold weather will manifest itself during December and January.

For example, it could come in the form of persistent slightly colder than normal weather, or it could come in spells of severe cold with milder interludes.

Therefore, if you read any newspaper or online stories that claim to know how cold it’s going to be for how long, as well as how much snow there is going to be, it’s nothing more than a guess that has no credible scientific basis.

I’ll be keeping a close eye on any signs of wintry weather in the coming weeks. In the meantime, you can get the latest forecast on the Channel 4 Weather website. I’ll also be posting updates on Twitter – @liamdutton

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