Insights & News

Late 2021 Risk of Disruptive Cold Weather in Eastern USA & Europe

28th September 2021

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

Cover Photo, taken in Dec 2017 by Kris Hirst, is licensed under CC BY

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