Look ahead to Europe and the US’s festive weather,
while learning a little more about what makes a snow day…
Forecasters in Northwest Europe will be especially familiar with (if not tired of!) the phrase…
‘Will we have a ‘white Christmas’?’
Where seasonal snow is frequent, adaptations are readily available, such that snow is mainly a metaphorical icing on the Christmas cake.
Where it’s not, even a little can cause travel chaos, due to low investment in icy weather preparedness. That is, if it settles.
Rather than focus on the one ‘big day’, I’ve looked at the whole festive week and bordering weekends.
Ideally, I’d cover the whole Northern Hemisphere, but that would make for a truly immense amount of data processing… So I’ve focused on Europe and the contiguous U.S.
Even doing that, I’ve had to process data for over 800 weather stations! I hope you’ll agree that the results (Figures 1 & 2) are worth the effort. They illustrate where you’re most likely to observe at least 1 cm of lying snow 21st-31st Dec.
So you can see, for example, the UK’s mere 0-2 days being outdone by almost all places north and east of Turin*. In Belarus and Ukraine, Minsk and Kiev experience lying snow on more than half the days 21st-31st Dec. This is impressive; there are only a few other major cities that can compete with that in Europe. Meanwhile, even the snowiest of those labelled in U.S. aren’t quite on par (1-2 fewer days).
Figure 1: Map of Europe showing the long-term average mean number of days within 21st-31st Dec that have at least 1 cm of lying snow. (Note: There are no snow depth reports for the far north of Scandinavia, hence the cutoff).
Figure 2: Same as Figure 1, but for the contiguous US.
Why and Where? The Averages
…Snow Prefers a Cold Turkey
There are several key factors that determine how likely a given location is to see lying snow.
You won’t be surprised to read that all these factors relate to how cold it tends to be…
But we’ll look at some more specialist info than that.
It’s generally colder the further north you go (reducing sun strength and day length), but also the further inland you look. So, New York sees more lying snow than Atlanta, but Detroit is even snowier.
It’s also colder with elevation – check out the Alps bordering Italy, for example.
For both Europe and the U.S. Eastern Seaboard as far north as the Pamlico Sound, there’s also the influence of the Gulf Stream to consider**. Model experiments suggest it adds at least 10°C to temperatures in western Europe! Without it, the region would likely see very large winter season snowfall, much of it brought by North Atlantic storm systems.
‘For both Europe and the U.S. Eastern Seaboard as far north as the Pamlico Sound, there’s also the influence of the Gulf Stream to consider. Model experiments suggest it adds at least 10°C to temperatures in western Europe!’
There’s also a weaker warm ocean current that parallels the west coasts of the Oregon and Washington states. Not only does it discourage snowfall there, the warmer water evaporates more, providing additional moisture for the light rain and drizzle that the area’s well-known for. On the flipside, it keeps the heating bills down a bit!
Does Moisture Matter?
Speaking of moisture, it typically decreases with distance inland. Yet the number of days with at least 1 cm lying snow do the opposite.
This seemingly counterintuitive pattern can be explained by considering what it takes to saturate the atmosphere, resulting in precipitation…
The amount of moisture required to saturate the atmosphere reduces as temperature drops. So, when it’s at least a few °C below freezing, it only takes a little moisture to cause precipitation. This means snow falls often – but is very light.
Such conditions are typical for winter in northeast Europe and western Asia, for example.
So to summarise… from coast to inland, we generally see a transition from less cold with more moisture but rarely as snow; to colder with less moisture but easier saturation leading to frequent but light snow.
This is All Very Well… but Will it be Average This Year?
Reliably forecasting snow is difficult at the best of times, but there are some strong signals afoot for 2019.
In fact, for 21st-26th Dec, they are suggesting that it may be among the warmest on record in Europe. It’s also looking unusually warm for much of the contiguous US (but not as extremely so).
‘There are some strong signals afoot for 2019.
In fact, for 21st-26th Dec, they are suggesting that it may be among the warmest on record in Europe. It’s also looking unusually warm for much of the contiguous US (but not as extremely so).’
Figure 3: Maps of Europe (left) and the contiguous US (right), showing the 5-day mean of sea level pressure for 21st to 26th Dec 2019, as predicted by the Global Ensemble Forecasting System (GEFS). Associated major movements of unusually warm air are represented by orange arrows. Maps source: tropidaltidbits.com
Little or no settling snow is being indicated for the whole of Europe, excepting the highest mountains. Away from the UK and Scandinavia, it may feel more like October than December!
Below-normal settling snow is being indicated for the east half of the US, especially south of the Great Lakes. The other half looks closer to average. So, perhaps Idaho to North Dakota will be the states most likely to see festive snowfall.
It remains to be seen whether conditions change much after Boxing Day. Forecast models are hinting at a cooldown to near-normal temperatures in Europe and the north-eastern US, but without much confidence.
2019 brought many dramatics, but for many, snow isn’t looking to be one of them as the year draws to a close. Some readers may find themselves doing the strange British thing of declaring a ‘white Christmas’ based on a single flake of snow!
Whatever the weather, MetSwift wish you a happy end to 2019, and will continue to provide detailed weather-based analysis and insights in the New Year.
James Peacock MSc
Head Meteorologist at MetSwift
* Bonus fact: The UK’s obsession with a ‘white Christmas’ has roots in the Charles Dickens novels written during a run of unusually snowy Decembers. While it’s a lot rarer these days, that makes it more noteworthy when it does happen. It’s become a truly special event (whether you like it or not!).
** This ocean current transports warm water from the Gulf of Mexico up the Eastern Seaboard until it glances off easternmost North Carolina and traverses the North Atlantic to pass between the UK and Iceland.