How far in advance can we forecast snow?

If the snow is the joy of those who are on vacation, it is a factor of significant risk on the road and its forecast is essential to be able to anticipate our daily lives.

Reliable weather forecasts up to 5 days

In general, the weather forecast can be described as truly reliable and accurate, in just 4-5 days. The trend weather report is then more or less clear, in particular on the evolution of temperatures, over 10 to 15 days. Beyond fifteen days, it is only possible to identify seasonal global trends based on the influence of the ocean, sea ice and major centers of action in theatmosphere. This means that beyond fifteen days, we cannot predict snow in a specific area in France.

However, we can get a general idea of ​​the average temperatures over the coming month and possibly the weather context, which promises to be disturbed or not. Long-term forecasts are presented as trend probabilities (warmer or cooler, drier or wetter). And above all, beyond 3 months, they are very unreliable, or even absolutely unreliable. To get a slightly clearer picture of the overall winter trend, which officially begins on December 21, you usually have to wait until November.

And it is only after mid-December that the meteorologists Forecasters may begin to have an early answer to the momentous question: ” Will there be snow in the plains at Christmas? “. Four to five days before December 25, the answer is refined on a possible risk. But, as far as the location of snowfall is concerned, in a specific city on a plain like Paris, it is only one day before, or even a few hours before, that the answer can be clear. The trajectories of disturbances can indeed always deviate by a few kilometers at the last moment.

Predict long-term snowfall by looking towards the Atlantic

Regarding the long term (1 to 3 months), it is necessary to take into account the ingredients necessary for the arrival of the snow, that is to say the association of a cold weather with a wet weather and disturbed.

For this, meteorologists study the North Atlantic Oscillation (NAO), a difference of pressure between two points in the Atlantic. This climatic phenomenon has an influence on winter, because it determines the positioning and trajectory of depressions in the northern hemisphere. And to have a snow situation, you need disturbances that are faced with theair cold: in France, south-west, north-west and north-east flows, but also easterly returns, are situations that can lead to snow when these disturbed flows encounter a mass of cold air.

The index NAO varies from November to April and can be in a positive phase or in a negative phase: a positive index is associated with a “zonal circulation”, with winters generally mild, humid and favorable to storms in northern Europe; a negative index, less frequent in recent years, is conversely associated with cold and snowy winters in northern Europe.

It is largely with this index, and other climatic parameters, that meteorologists and supercomputers are trying to determine whether the coming winter will be rather snowy in the plains, or not. Concretely, the NAO index was rather positive from the 1990s to the 2000s, with rather mild winters and a significant snow deficit in the Alps. The Oscillation then swung back into an often negative phase from 2001 to 2011, giving rise to historic cold waves in winter and heavy snowfall in the plains, particularly in 2010-2011. In recent years, the trend has once again shifted towards an often positive index, with rather mild winters, and very little snow in the plains.

However, it should always be borne in mind that these forecasts long term are still in the scientific research stage and their reliability is therefore limited. To know the risk of snow in your city, you should check the weather forecast a week before, then regularly until the last moment, as this forecast is delicate.