Insights & News

Potential for a Cold Finish to 2020 in the UK?

27th October 2020

In their October update, the National Ocean & Atmosphere Administration (NOAA) stated that La Niña conditions were present in the tropical Pacific Ocean.

This event is characterised by anomalously cold waters along the equator from near the Dateline eastward. Overall, it’s become stronger as October has progressed. Forecast models are predicting a moderate to strong event spanning at least the next four months. This phenomenon has a powerful influence on weather patterns across the Americas and even out across the Atlantic Ocean. This can have knock-on effects for the weather in the UK and western Europe.

It’s one of the dominant players among more than a dozen that MetSwift consider for the identification of ‘analogue’ years. These are historical years which are expected to be most relevant to look at for guidance as to what will happen during the weeks, months, even years ahead.

A Chilly Feel to Nov-Dec 2020

The MetSwift analogues for the final two months of 2020 are characterised by mostly above-normal cumulative heating degree days. These CHDD provide a visualisation of heating requirement based on how much below 18°C the daily mean temperature is. When the temperature is above that threshold, CHDD are unaffected. This makes them useful for gauging the number and intensity of colder spells of weather within a period of interest.  It doesn’t matter how mild it is in between, which is a big deal when looking at the UK’s typically very variable weather. Below, the typical CHDD in the MetSwift analogues has been compared with those of all years 1981-2010.

Map showing how typical cumulative heating degree days compare between the MetSwift analogues and all years 1981-2010.

As you can see, most locations typically observed at least 10 more CHDD during the MetSwift analogue years. Many saw an additional 30-50, a few even more than that.

This suggests a larger number of cold spells of weather, with heightened snow and ice risks. This is true in places UK-wide; from the Lerwick (Isle of Skye) to Camborne (Cornwall). Only a few exceptions exist, which are mainly to the south of high terrain. This makes sense when considering the weather patterns behind these CHDD differences, which I’ll get to later.

Concerning During COVID Times?

A tendency for more cold spells than usual might seem to be the last thing anywhere needs during the battle against COVID-19. You may instinctively think of people huddled up indoors with windows shut to keep in warmth from central heating or blazing fires. Not only is there poor air circulation, that heating dries out the air and in turn the mucus we produce. That makes it less sticky, meaning infections have an easier time entering our bodies.

Sounds bad – and it is – but when you think about it, in the UK, when it’s not cold, it’s often cool and unsettled… and that entices the same behaviour in many of us.

“On a cold but sunny day, you tend to see more people out for walks than during cool, rainy conditions.”

In fact, when it’s cold, there’s a better chance of dry and bright weather. You see, cold air tends to come from somewhere between the north and east, where available moisture is relatively low (especially when it’s from the east). That means more chance of clear skies rather than clouds and rain. On a cold but sunny day, you tend to see more people out for walks than during cool, rainy conditions.

Then there’s the ace card up cold weather’s sleeve: snow. Unless it’s dangerously heavy or there’s a blizzard raging, this can draw a lot of people outdoors. For some, it’s a great mood-lifter. Provided social distancing is observed (restrain snowball-pelting to house members), a covering of snow could be a positive event overall for affected communities.

So, Will It Snow Then?

Alas, away from very high ground, snow is typically hard to come by in the UK. Not every year, though! I’m sure many readers recall the extraordinary, widespread snowfall of late 2010. Let’s check out what MetSwift analogues suggest for lying snow days a decade on from then:

Maps showing the typical number of days with lying snow, in all years 1990-2019 (left) and in the MetSwift analogue years (right).

For lowland parts of the UK, I’m afraid this is not good news for most of those seeking ‘snow days’. From the West Country to the Southeat and northwest from there, lying snow days are around one below normal. What’s more, we’re talking about a normal that’s mostly only one or two days to begin with.

By contrast, when looking at high ground areas, one or two extra lying snow days are suggested widely. Perhaps even three or four more in southern Wales (the Black Mountains and Brecon Beacons).

Connecting to Weather Patterns

Overall, we have indications for a chilly Nov-Dec, but generally not cold enough for lying snow to low levels.

To understand how this is likely to come about, we can look at the mean sea level pressure (MSLP) for each of the two months. Specifically, how that differs between the MetSwift analogues and the long-term average:

Maps showing mean sea level pressure for November (left) and December (right), for the MetSwift analogue years.

For November, we see a distinct dipole anomaly pattern. The MSLP is much higher than average across a wide area to the west of the UK. This means stronger, more persistent areas of high pressure than usual in that region. Meanwhile, the opposite is indicated for the area northeast of the UK (Scandinavia and Barents-Kara Seas). This corresponds to low pressure systems than usual, with attendant rainy or snowy, often windy weather.

For the UK, this setup tends to mean that winds come from the northwest or north more often than usual. Typically, northwest is more common than north. From that direction, the air is often chilly but not quite cold enough to bring snowfall below around 200 m elevation.

Northerlies, on the other hand, have been known to bring lying snow more widely. However, in the past decade, northerlies have been trending less cold overall. The chance of snow is not as high as it once was. This is a consequence of a reduction in Arctic sea ice coverage to the north and northeast of the UK.

Another complication this year could be that the polar vortex is looking on the strong side as we end October. A strong polar vortex makes it harder for winds from the northwest or north to affect the UK for long at a time. That means less time for cold air to make its way across before the wind changes direction and pushes it away.

“The pattern shown favours very changeable conditions, with brief windows of
opportunity that usually struggle to put down more than a brief covering.”

Looking on to December, I’m afraid the indications aren’t very interesting snow-wise, unless perhaps you’re keen on some winter skiing in the mountains of Scotland.

The pattern shown, of low pressure to the north and high pressure to the south than usual, favours very changeable conditions, with temperatures varying between mild and somewhat cold. That tends to mean little to no lying snow at low levels, with chances restricted to whenever a low pressure system is moving away to the east of the UK. Brief windows of opportunity that usually struggle to put down more than a brief covering.

For high ground, however, it can be a different story. Especially in Scotland, which has the coldest ocean waters to its west. So, it could be a decent month for the ski season in that country… or at least, not poor!

 

A Brief Look into Early 2021… Mild & Wild?

I’m going to keep this short and… well, only sweet if you’re not a fan of cold UK weather!

When there’s a moderate to strong La Niña event, UK winters have a habit of becoming predominantly mild by early January. This can be quickly demonstrated via use of a measure known as the North Atlantic Oscillation (NAO).

Broadly, positive values of around 1 or higher are often associated with mostly mild winter weather in the UK. Likewise, negative values of around -1 or lower (negative phase) often correspond to cold winter weather. Exceptions do occur, especially with the negative phase, but overall, it’s a decent guide to work with. Especially if you focus on more extreme values, e.g. above +2 or below -2.

The graphs below show how the frequency of daily NAO values compared between a group containing all years 1950-2019 (climatology) and a group featuring only those with a moderate to strong La Niña event.

Graphs showing how the frequency of daily NAO values compared between a group containing all years 1950-2019 (climatology) and a group featuring only those with a moderate to strong La Niña event.

On the left, we see more negative NAO days and less positive ones. This ties in with the colder weather chances in November 2020. It’s not a strong signal, though, which likely has a lot to do with the polar vortex strength. As mentioned earlier, it’s looking a bit strong this year, which may work against seeing many negative NAO days.

To the right, a very different picture is painted. This is a strong signal for a positive NAO to dominate proceedings in Jan-Feb 2021.

Arctic Accomplice?

The next pair of graphs have applied the same approach to compare low Arctic sea ice extent years with climatology. I’m showing this because the signal for Jan-Feb is strikingly similar to that from looking at the La Niña group. As of writing this, the sea ice extent in 2020 is at record low by over 850,000 square kilometres!

As with the previous two graphs but for low Arctic sea ice years versus climatology.

The two working together could mean a wild ride for the UK in early 2021, especially in the northwest.

This is not for certain, mind. There are rare but high-impact wildcards that can come into play. Most notorious is known as a sudden stratospheric warming event. We should have a shot at that this December – but don’t hang your hat on it!

 

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo by Petr “Dragonovski” Shevelko is licensed under CC BY-SA

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