Our Head Meteorologist investigates the likelihood that disruptive cold weather will affect Europe and/or the eastern USA during Nov-Dec 2021.
www.metswift.com
Welcome to the ‘light’ version of MetSwift’s look ahead to the risk of disruptive cold weather hitting the Eastern USA and/or Europe in Nov-Dec 2021. For a more in-depth read, please see the full article here.
The Northern Hemisphere’s autumn is barely a third done, yet already, the thoughts of some are turning to the cold season ahead. This is because it pays to size up severe winter weather risk and plan accordingly.
This year, more than most: I’m writing this blog in the wake of sharp rises in wholesale power and gas prices. This has put some energy companies at risk of collapse, including Bulb, the 6th largest in the UK.
Customers, faced with higher prices or being ‘left in limbo’ as their suppliers fold, will hope for a kind end to the year; one that demands less artificial heating than usual (i.e. has low heating degree days; HDD).
Are November and Dec going to ‘play nice’, or make matters worse? Let’s see what we can learn from history.
A NATURAL LEADER: THE NORTH ATLANTIC OSCILLATION (NAO)
Analysis here focuses on a major North Atlantic circulation pattern known as the NAO. It varies between positive and negative states, which in northern Europe and the eastern USA are broadly associated with an increased frequency of mild and cold spells of weather, respectively.
Britannica provides a good illustration of it here. At MetSwift, we take it to the next level, by sub-dividing it into ‘East’ (ENAO) and ‘West’ (WNAO) variants. Associated weather tendencies are illustrated below.
A negative ENAO state is mainly distinguished from a negative WNAO state by the fact that it brings unusually warm, not cold, weather to the eastern USA. It also doesn’t have to be as strongly negative to bring widespread cold weather to Europe.
FREQUENCY DISTRIBUTIONS
To obtain guidance on how the NAO is likely to behave, I have filtered daily NAO observations to ones on which three key driving forces of weather patterns (teleconnections) were behaving in the same way as they’re expected to be during the upcoming months.
Those teleconnections are:
The El Niño Southern Oscillation (ENSO): Variations in sea surface temperatures (SSTs) across the tropical Pacific from the Dateline eastward. Once every 1-3 years, these become strong and coherent enough to be classified as La Niña (low SSTs) or El Niño events (high SSTs).
Pacific Decadal Oscillation (PDO): Another SST cycle, but occurring in the northern Pacific Ocean, with predominantly positive and negative stages that usually last a decade or two.
Quasi-Biennial Oscillation (QBO): A cycle of westerly and easterly winds occurring in the stratosphere above the tropics.
All three are expected to be in negative phases (but the QBO specifically in the lower to mid-stratosphere).
That filtered data has then been compared with unfiltered data, using frequency distributions. This shows us what historically suggests the WNAO and ENAO are most and least likely to be doing.
NOVEMBER WNAO: PROBABLY NOT UP TO MUCH
The distribution of the filtered Novembers suggests the number of snowfall events could be a little above average, with mild and wet winter weather less prevalent than usual, especially in north-eastern USA.
NOVEMBER ENAO: PROBABLY WEAK ONE WAY OR THE OTHER
For the ENAO frequencies, we again see an indication that November will be dominated by ‘average’ conditions, especially in Europe. However, there is certainly some scope for notably mild or cold spells of weather (just not in a big way).
DEC WNAO: POSITIVELY INCLINED…?
The filtered distribution here has a strong positive skew. This points toward a wetter, windier than usual month for the north-eastern USA. However, a very negative WNAO also has a raised frequency compared to average. There could be a few more unusually cold, snowy days than usual in the eastern USA, perhaps Europe too.
DEC ENAO: UNUSUALLY PRONE TO BEING VERY NEGATIVE
Last but far from least, we come to December’s ENAO filtered frequency distribution. Cold, potentially snowy weather is suggested to be much more common than usual in Europe. This corresponds to more unusually mild days than usual in the eastern USA.
It’s possible that this signal is connected to something called a ‘sudden stratospheric warming’ (SSW).
SSWS TRIGGER SEVERE COLD SPELLS… AND ONE MAY OCCUR BY DEC
Within a 1-3 weeks of one of these events, weather patterns change drastically. Cold air, usually ‘bottled up’ over the Arctic regions, spills south across one or more regions. Weather systems become slow-moving, prolonging the impact of the cold weather where it occurs. Meanwhile, adjacent regions can experience unusually high temperatures, sometimes rainfall too.
Broadly, these can be sub-classed as ‘major’ and ‘minor’ SSW events. As you may imagine, the major ones tend to affect the largest combined area and have the longest-lasting impacts.
Recently, numerous long-range weather forecasting computer models have been predicting a SSW, or something close to it, to occur by Dec. Most notably, that of ECMWF, which has one of the best track records.
To gauge how seriously we should take these predictions, I’ve studied the frequency distribution for filtered historical years, as was done for the NAO earlier. A minor SSW is suggested to be likely (73% risk) in November, less so (33%) in Dec.
A CHILLING PROSPECT
In conclusion, there’s a heightened risk (not guarantee) of a substantial spell of cold, snowy weather affecting Europe sometime Nov-Dec. Considering the NAO analysis performed earlier, the risk for the eastern USA is close to the long-term average.
If a minor SSW occurs, probable duration of highly anomalous weather patterns is 3-6 weeks. One small comfort is that we’re not talking about a major SSW here – although I can’t rule that one out, sadly!
Regardless of duration, such an event would see soaring demand place strained energy companies under even more pressure. Certainly, one worth making contingency plans for, just in case.
James Peacock MSc
Head Meteorologist at MetSwift
Gohja, wat moet je hiervan zeggen. Ik weet de betrouwbaarheid er niet van. En ik denk dat het voorlopig nog af te wachten valt, maar ik zou ook zeggen dat het risico op een front-loaded winter (dus early cold, middle & late unsure or milder) toeneemt. Novemberwintertjes, ze bestaan maar ze zijn heel zeldzaam. 1993 is een bekendere. 1919 is een harde novemberwinter. En 1921 had ook wel al wat novemberkou. Recent naast 1993 waren ook 1985 en 1998 koud. Sinds 1998 rond of boven klimaatgemiddelde eigenlijk, dus het is van 1998 geleden. Sindsdien was de temperatuur altijd boven 6.0°C.
In 1858 was de temperatuur voor november 0.8°C. Dat is best koud, want het is al van 2012 geleden dat een maand nog zo koud verliep, toen 0.7°C in de Siberische twee weken februariwinter. In 2018 ook 0.8°C. Het lijkt wel alsof dat de muur is waarop we botsen tegenwoordig over de koudste wintermaand die mogelijk is. En toen gebeurde het in november, maar ondertussen al dik 160 jaar geleden.
Novemberwintertjes worden afgestraft in de echte winter vaak. Ook in de winter van 1858-1859 was dat een zachte winter, zeker voor die tijd enorm zacht. Het is een winter die vandaag perfect zou passen klimatologisch. Ook in 1919, 1921, 1993 en 1998 was het vat al snel af. Maar in 1986 keerde na de koude novembermaand na een zachte tussendoortje in december en januari de kou met een vengeance terug, met als resultaat de koudste wintermaand ooit waargenomen in de laatste 50 jaar (hier) en op ons geheugen grift en een befaamde: "it giet oan". Maar bijvoorbeeld in het winterhalfjaar van 1919-1920 viel de twee koudste weken van dat jaar in de eerste twee weken van november.
Rear-end load winters zijn er ook. De meest recente is 2012-2013, al was de winter an sich ook niet slecht, maar maart 2013 was extreem, en heeft nog steeds één van de grootste anomalieen naar beneden in het moderne klimaat. Dat zou zelfs extreem geweest zijn in het vorige klimaat. De laatste jaren komen dat type winters ook wel wat vaker voor, maar zo extreem als toen. Nee, dat niet. Maar dit jaar had een lange koude lente (al had dat weinig met winterpatronen te maken). 2018 had een blitzeindfebruari winter. In 2012 viel de winter na een heel zachte december en januari in op het slot van januari met twee weken ultieme vrieskou in februari, en de rest dan weer zacht. De winter heeft de laatste jaren wel wat meer het venijn in zijn staart. Enkel afgelopen jaar was redelijk vroeg, maar met een extreem lange aanloop van waar dat het wel koud was, maar niet wou lukken om toch eens (kort) te pieken in januari. En dan die koude april + mei (maar opnieuw, dat is geen winter meer, dat is de lente die niet wou komen, en niets te maken met winter verder).
De laatste front-load winter was in december 2010. Heel koude maand, met heel veel sneeuw. De rest van de winter stelde dan niets meer voor. 2009-2010 is wellicht naar wat je in moderne normen de laatste grote winter kan noemen. 2008-2009 was best koud. 2011-2012 was dan weer zacht maar had die heel geconcentreerde vrieskou in begin februari. En 2012-2013 was best en goede winter, met een heel venijn in de staart. Nadien eigenlijke elke winter overwegend te zacht, maar de laatste winters zijn ietsje beter (met feb 2018 bvb of afgelopen jaar was oké, niet goed, maar oké).