Winter Forecast 2020/21

Welcome to my winter forecast for 2020/21. Winter is here. And my winter forecast is here too.

Where to start? I’ve written enough of these over the years that it should be second-nature. Let’s start with reminding you that seasonal forecasting is still fairly experimental – it can be difficult to forecast details 3 days away sometimes, let alone 3 months away.

Some of this forecast will be wrong. There are varying factors at play, some with more importance than others and I could easily misread something. And background signals can strengthen or weaken unexpectedly. Not to mention the atmospheric-resetting that a sudden stratospheric warming event can do – which cannot be predicted more than 2-3 weeks in advance – more on that later.

My autumn forecast wasn’t the best. September was a disaster – I forecasted a mid-month unsettled spell and an end of month very warm spell – and it was the opposite way around. Probably my error, though hurricane season does make September more difficult to predict, especially exceptionally busy seasons. My October forecast was closer and November was reasonably accurate, for a long-range forecast.

Also this is your chance to say thank you. A share, retweet, invite your friends – I don’t ask for much. I don’t ask for anything really. Just some form of sharing every 3 months.

Finally, thank you to Miranda for the winning photograph – it sums up expectations for this winter – there’s a bit of snow on the ground but I cannot be sure if it is raining or sleeting? It’s dark yet has lots of light. It might not be what you would have picked, but for me I like the story it tells. I did come very close to choosing photographs by Lisa, Georgia and Chris too.

A £10 donation will shortly be made to CALM. Which is a fitting charity given the troubles that 2020 has given.

Background Signals

I’ll start with some background signals.

La Niña is the main game in town this winter. Weak La Niña winters tend to be quite cold and snowy, with high pressure tending to be a strong feature in the Atlantic, and north-westerly or northerly winds more regular than normal. Strong La Niña winters tend to be mild and wet. Unhelpfully the strength is somewhere in between weak and strong, but is likely to strengthen. To my eyes, this favours a cold start to winter but becoming mild and wet.

The Quasi-Biennial Oscillation is a pattern of winds in the stratosphere – which is about 50km above us (we are in the troposphere). The wind direction switches between west and east roughly every 28 months – it should be in easterly phase now which enhances the chance of a cold winter, but is weirdly more in westerly phase or neutral which enhances the chance of a wet and mild winter – it is complicated to explain, but it isn’t behaving as expected. I’m not taking too much account of it this winter, other than as slightly reducing the chance of cold weather – but there are suggestions that it could properly go into easterly phase by the end of the winter.

We are just coming out of a sunspot minima which some people seem to believe enhances the chance of cold weather in winter. I get less convinced of this having an effect every year.

I don’t especially follow the Madden-Julian Oscillation, but from a brief read from people in the know, there are suggestions that this could go into phase 6 towards the latter part of December, which tends to increase the chance of blocking highs either to our west or north – which means a good chance of cold weather. This is more a medium weather-forecasting signal.

The polar vortex in the troposphere is a bit all over the place, and more towards the opposite side of the globe, so in the medium-term this enhances the chance of high pressure and potential cold weather. There is also a polar vortex in the stratosphere, and when the troposphere and stratosphere polar vortex connect with each other – we tend to end up in long spells of wet and windy weather, normally mild, just like last winter. This is a very difficult pattern to break down as proved last year – though there is a wild card.

So, the wild card – a sudden stratospheric warming event (SSW). These are kind of the holy grail when you are stuck with a strong polar vortex, enduring weeks of wind and rain. The temperature in the stratosphere (50km above us, remember!) shoots up and the wind direction reverses. Over the course of 2-4 weeks, this pattern drips down into the troposphere (our level) and cold weather spills down from the Arctic into many more mid-latitude levels of the globe such as the UK.

February 2018 and March 2013 are two classic SSW events, with very cold weather and snow that followed, and stayed for weeks rather than days. An SSW does not guarantee cold or snow for the UK, but is around a 65% chance.

Predicting them more than 2-3 weeks in advance is not really possible – they tend to happen more in easterly QBO winters than westerly but I’m not sure there is much significance there. I also read that they happen more often in La Niña winters with a westerly QBO – hello 2020 winter! But I’m not sure of the source of this claim.

I know I am going on a bit now, but the current pressure patterns in the troposphere are known precursor patterns to sudden stratospheric warming events. The signs suggest to me that we are more likely to have an SSW than not this winter (they happen roughly every other winter) – maybe early to mid January, and with the 2-4 week timeline for this to affect the troposphere (us!) then February could be very interesting from a cold perspective.

I hope I’ve explained that without going too far down a technical rabbit hole – yet without dumbing down too much for those in the know, and I’m sure there are a handful reading that know much more than I do.

Phew. The forecast.

There seems more hope for cold weather, if that is your bag, then last winter – with multiple potentials. But overall in the 3 months, I expect a milder and wetter winter than normal.

December starts fairly cold. Low pressure arriving from the north-west will bring outbreaks of showery rain at times – with small chances of some sleet or wet snow. Not expecting anything especially noteworthy but the chance of a surprise heavy snowfall is there. Generally cold with overnight frosts when clear skies permit – fog possible also.

Mid-month is more uncertain, but a greater chance of somewhat milder conditions and also staying changeable – some rain at times, some dry weather also. Overnight fog possible, frost less likely.

Then for the latter part of December, overall quite varied with some chilly, sunny weather with overnight frosts, some brief milder unsettled weather possible but most notably a fairly good chance of a north-westerly or northerly plunge, bringing a chance of snow (assuming I am reading the La Niña and MJO signals correctly!).

Which does mean that a White Christmas is actually possible this year. Been a few years since I’ve said that, but remember snow is always difficult to achieve in the south of England and is a nightmare to forecast, so don’t expect anything other than “possible” until nearer the time.

Overall temperatures below average, sunshine around average and rainfall slightly above average.

75% confidence level – main uncertainty around mid-month, and also whether late-month cold plunge will occur.

January looks like it will become unsettled, quite deeply so at times. Perhaps still chilly, sunny and frosty to start, but becoming mild for most of the month.

The low pressure track often over Scotland, fairly traditional westerly flow with strong winds at times, heavy rain at times and low pressure systems moving through fairly swiftly. Generally mild, with very mild south-westerly flows possible though mildish westerly more likely. Overnight frosts very few.

There will be the chance for some brief north-westerly or northerly flows – 1-2 day cold snaps will be possible, so this may allow for some wintry precipitation, for example snow turning to rain as weather fronts meet cold air.

85% confidence level.

February is a tricky call – I am going to assume that an SSW does not occur, even though I believe it is more likely to occur than not. This is because the date of the potential SSW, and how long it takes to effect our weather, and whether it would affect our weather are all known unknowns.

I think February will more likely start unsettled, but dry conditions will slowly take over as high pressure to the south has more influence.

More likely dry and colder as the month goes on as high pressure navigates closer to the UK, allowing more in the way of overnight frosts and colder temperatures.

Though if the SSW occurs in January as I currently think very possible, then expect this to be wrong – with cold air flooding down from the north or east, and snow becoming very possible.

Overall I expect around average temperatures, below average rainfall and slightly above average sunshine.

Confidence level 50% – the SSW possibility really does cause a major forecasting headache this far in advance.

Done.

Phew, I can go make my dinner now.

So a quick summary. December starts cold, fairly mixed from mid-month though good chance of a notable cold spell later. January mild, wet and windy. February dry – but uncertain due to potential SSW.

Now your time to share!

Early suggestions for spring include a cold start to March, a dry and sunny April (could be warm or cool) and a showery May. But very, very early signals.

Have a good winter.

One thought on “Winter Forecast 2020/21

Leave a Reply

Your email address will not be published.