Headline: Temperatures around average, rainfall average or a little below, greater chance of snowfall compared to last winter but a severe winter unlikely.
It’s that time of year again when Twitter weather accounts like myself dish out our long-range thoughts on winter in forecasts that you’ll either never hear about again (if it doesn’t go to plan) or you’ll never hear the end of. In reality, even the professionals and the Met Office cannot confidently predict the weather one month ahead – let alone three – so whilst a lot of thought goes into these forecasts concerning background signals and so on, it is pretty much just for fun.
Before getting to the main forecast summary at the end, I will run through a bit of info on some of the background signals considered within the forecast outcome. I will *try* and make it as simple and brief as possible, so not to drag my own time or yours! I will also point out as ever, I am only an amateur – not a meteorologist – and I don’t know everything!
Long-range seasonal models
Several different meteorological agencies release long-range anomaly charts for the months/seasons ahead, indicating a general synoptic trend. For the UK, the key thing to look out for is whether they’re more suggestive of a +NAO or -NAO pattern, this means looking at height anomalies across the North Atlantic from the Azores up to Iceland/Greenland alongside trends in Europe. Whilst it isn’t totally black & white; +NAO generally means ‘zonal’ – so Atlantic / mild / wet – whilst -NAO generally indicates ‘meridional’ – so a greater chance of colder & drier weather and increased snow risk.
The chart posted above is the Met Office GLOSEA5 November update showing predicted height anomalies for the period December to February (D/J/F). Whilst it isn’t as strong a signal as last winter, with positive anomalies over the Atlantic and into Europe coupled with negative anomalies over Iceland & Greenland, it is indicating a +NAO winter overall.
Posted above is the same as discussed before but this from the ECMWF seasonal forecast – again showing anomalies for the D/J/F period. This one is a little more interesting… yes it does have the anomalous heights over the Atlantic/Azores and into Europe but this also extends to the east/north-east of the UK over East Europe/Scandi. It also has only a very weak negative anomaly over Iceland/Greenland with largely no strong signal. I wouldn’t consider this a great chart but it isn’t the worst you’ll ever see.
More interestingly, when we push it forward to the January to March period (J/F/M) (therefore dropping December) – the signal becomes somewhat more strongly +NAO (not that the previous wasn’t also +NAO)… this suggests the ECMWF is going for a ‘front-loaded’ potential, with December seeing a high pressure influence before zonality increases into January and February.
I won’t post & discuss all of the seasonal models but the general theme does appear to be indicating a +NAO winter. Last winter these models were on the money, that doesn’t mean they’ll be right every year but they also shouldn’t be dismissed.
ENSO (La Nina)
The equatorial Pacific is currently in a La Nina state and has been for some months since late summer / early autumn when the 3.4 region SST anomaly dropped below -0.5. This is the ‘cold phase’ with El Nino being the ‘warm phase’. It looks like this La Nina will persist into 2021 and likely throughout our winter. Its link with UK winter weather is often subtle and indirect but historically there are some correlations, the most of note is a weak/moderate La Nina favoring a ‘front-loaded’ winter with an increased chance of high pressure/blocked patterns.
Looking at the Nino 3.4 region SST anomaly index, you can see there was a period of steep cooling in late October and into early November with the index dropping below the -1.5 ‘strong La Nina’ threshold, peaking at near -1.7. A strong La Nina is considered a poor signal for UK winter weather. Since then, waters have warmed a little and in recent weeks we have plateaued comfortably within the ‘moderate La Nina’ range (-1.0 to -1.5) which is considered more favorable. I wouldn’t like to say for certain just yet that the -1.7 in early November was the ‘peak’ of La Nina cooling but at the moment it appears a moderate La Nina winter is more likely than a strong one. The latest index value 05/12/2020 is -1.246.
Always worth throwing in a mention to solar activity and the recent solar minimum, we very recently came out of the cycle 24 solar minimum (as you’d easily guess, indicating very low solar activity) and we now know we are entering solar cycle 25. These cycles take around 11 years so we expect solar activity to increase over the next few years towards solar maximum and the next minimum is over a decade away!
It’s also worth noting just how weak cycle 24 was, in fact it was one of the weakest cycles ever recorded (although not as weak as the Dalton Minimum cycles 5 & 6). Cycle 25 is expected to be just as weak if not slightly weaker.
For the UK and winter – there is a credible link between the solar minimum period (especially the year or two after) and increased amounts of high latitude blocking with meridional ‘wavy’ jet stream – this opening up the likelihood of more anomalous weather (possible reason behind the frequent hot plumes & record temperatures in both summer 2019 & 2020?). That all said, it is only an increase in probability – suffice to say we won’t have continuous high latitude blocking for the next 24 months!
Quasi-biennial Oscillation (QBO)
Another background signal we look at is the QBO, essentially it’s a cycled variation in the winds that circulate high up above the equator. Once every 14-ish months, these winds switch direction and this change can have an impact on the jet stream. When QBO is positive/westerly, the UK is at a greater risk of Atlantic zonality with a stronger jet – meaning more in the way of mild & wet winter weather. When QBO is negative/easterly, the jet stream can become somewhat weaker which allows for a greater chance of cold weather, less rain and also a heightened risk of PV (polar vortex) disruption and/or an SSW (sudden stratospheric warming) event.
QBO is usually known for being quite uniform in its cycles – but of course in 2020 it had other ideas. QBO transitioned from westerly to easterly back in January (2020) when the monthly index value went negative (+1.66 in December 2019 to -2.51 in January 2020) and this winter was supposed to be a straightforward QBO-easterly winter. In April the 30mb zonal winds were still negative, down to -5.03, but by June they’d reverted back to westerly and the QBO index reached +0.34. This increase in westerly zonal wind at 30mb has continued ever since – the November index value was +11.15… far from easterly!
There does appear to be some conflicting opinions on where we stand with QBO at the moment. As shown in the chart above, easterly winds in the far upper levels of the stratosphere (shown as those blue colours in the far top right hand corner) are now slowly starting to descend down through the stratosphere again. However, at 30mb where the index is measured (you can see the faint line running across the chart, that marks 30mb) we are still very much seeing westerly winds (as shown by the brown…or yellow?…gold? whatever colour – those are the westerly winds). It looks to me like QBO is westerly and will remain westerly for the duration of this winter – possibly transitioning to easterly late in the season which won’t be of much use. Some are theorising those uppermost level easterly winds between 10mb and 20mb could still have some impact on the jet stream (which has been rather tame in recent months, it’s worth saying) thus still maybe helping increase the probability of less wind/rain/mild and something more blocked this winter.
Sea surface temperatures (SSTs)
Looking at sea surface temperature anomalies – there are as ever three areas of interest; the equatorial Pacific, the north & north-east Pacific and the North Atlantic Ocean.
Not too much to say regarding the equatorial Pacific – it’s displaying the signature La Nina look with cold anomalies extending from South American coasts all the way towards Southeast Asia.
The first thing I noticed which is of some concern going into winter is the presence of notably warm SST anomalies across the north-east Pacific to the south of Alaska and west of Canada. This has been a persistent feature in recent years/winters and is a negative for those not wanting a mild/stormy winter. As things stand, I don’t think the winter as a whole will be dramatically mild/stormy but if it were to end up so, this would likely be taking a lot of the blame. Warmer waters around the north-east Pacific promote higher pressure in the area thus sending the jet stream on a north-west to south-east trajectory through the States, pulling down cold Arctic air into the eastern States which can in turn fire up a more powerful jet exiting into the Atlantic.
The North Atlantic SSTs look rather mixed but there isn’t the presence of the desired ‘tripole’ and colder than average waters sit not far to the south of Iceland & Greenland (although there is some anomalous warmth in the Labrador Sea between Canada and Greenland).
Eurasian snow cover extent
Something we watch closely in October is the expansion of snow cover across Eurasia. It’s believed that the greater the snow extent in October, the higher the chance of a weakened PV and negative Arctic Oscillation (AO) later into winter. The theory is; the more vast the snow expansion in October, the stronger the Siberian High going forward – thus eventually weakening the Stratosphere PV, this propagates down into the Troposphere PV and we end up with a -AO (blocked) pattern.
This October, after a slow start, snow cover expansion was healthy from mid and towards late month but some losses in the final week – including on the final day – left the October 2020 final result at a decadal low. Snow cover across the rest of the Northern Hemisphere was in contrast very healthy – at times sitting at a decadal high.
First and foremost, I want to point out this forecast is based on the assumption we do not see a sudden stratospheric warming (SSW) event this winter. At the time of writing this, zonal mean zonal winds @ 10hPa 60N are very strong compared to the climo average but all modeling is consistent and in agreement for these to weaken towards mid December – possibly to below the climo average. Whilst there is some stratospheric interest going forward, at this stage there is no major SSW (zonal wind reversal) ‘expected’. As I say, this forecast is based on the assumption one will not occur. If one does, more significant anomalous weather – both milder or colder – would be likely in the weeks after.
My forecast is for temperatures around average across the winter as a whole (UK mean anomaly -0.5C to +0.5C) with a very slight favour towards a little above. I’d consider both a significantly milder or colder than average winter (greater than +1.5C or -1.5C anomaly) rather unlikely. I’d lean towards December being a little below average, January closer to average and February possibly milder than average, but not significantly so. Greatest chance of cold spells into January.
Precipitation; average or a little below. December in particular looks to be drier than average – even if things turn a little more zonal for a time. January & February more mixed but possibly not delivering us a deluge. I’d put February as the month to most likely be wetter than average. UK winter as a whole rainfall %….85-105%.
Whilst I don’t think Winter 2020/21 will be historical and remembered for decades to come for severe, harsh & snowy weather – I do think it’ll offer up more potential snowfalls compared to the previous two. In all honesty, that really isn’t a huge prediction to make as the last two winters have been close to snowless. A greater chance of cold spells, hopefully some snow for us all (strongly assuming hear that everyone reading this is a snow lover, apologies if you aren’t!) and hopefully a lot less of the record strong polar vortex that dominated last winter. As for the whole ‘loaded’ thing, I’d stick with saying front-loaded more likely than back-loaded (cold/snow more likely Dec/Jan rather than Jan/Feb) but I don’t think it’ll be overly pronounced on either side – more of a mixed bag.
Thanks for reading… I’m not quite sure I achieved that “as brief as possible” goal but I’m not surprised 🙂 …. Nick