Snow arrived early last week. What are the trends for the month of December? Can the snowflakes appear on the plains again between now and Christmas? Elements of response
What trends for the month of December?
Temperatures
According to the American model, temperatures in December would be generally above seasonal norms in France. It would seem that the thermal excess compared to the averages for a typical December is clearer in the northern regions while it would be modest in the south, where the month would be more or less seasonal. This trend therefore evokes a relatively mild month of December, without any major surplus, which could therefore allow the cold to pass through episodically.
Weekly thermal anomalies expected in December 2024 – American model via tropicaltidbits.com
The European model, for its part, is a little less gentle. If we envisage a mild first week in the southern half, that of December 9 to 15 would be neutral. A small thermal surplus would return during the week of December 16 to 22 then temperatures would return to normal levels during Christmas week. With such a scenario, the cold could therefore appear episodically and the holiday period will not be safe.
Weekly thermal anomalies expected in December 2024 – European model via ECMWF
Precipitation
For it to snow, there obviously needs to be instability. According to the American model, this month of December 2024 could display fairly standard rainfall in France. The week from December 9 to 15 would be rather calm but the second part of the month, including the holiday period, could turn out to be quite humid, especially in the northern part of France. Enough to allow snow?…
Weekly rainfall anomalies expected in December 2024 – American model via tropicaltidbits.com
The European model suggests a wet start to December around the Gulf of Lion but dry in the north. Then, the week of December 9 to 15 would be generally calm, validating the option of its American counterpart. On the other hand, it does not show any clear trend for the second half of December, which would leave the field of possibilities open.
Weekly rainfall anomalies expected in December 2024 – European model via ECMWF
Snow: phenomenon impossible to predict for several weeks
It is essential to remember that it is impossible to predict an episode of snow in the plains more than a few days in advance. The reason is simple: snow is generally a local phenomenon and rarely lasts over time. However, multi-week trends are large-scale trends, smoothing averages and masking epiphenomena. The best example is the snow episode last Thursday, November 21 between Normandy and Alsace. It occurred in a month that was generally milder than normal, showing a difference of +0.8°C as of November 25. Proof that a milder average does not prevent a few cold and potentially snowy days.
Difference from normal in average temperatures in France from November 1 to 25, 2024 – via infoclimat.fr
As with temperatures, long-term trends do not reveal a brief disrupted episode. This month of November 2024 will probably end with a rainfall deficit in France, particularly linked to the very anticyclonic first fortnight. However, this did not prevent the agitation from occurring over the last few days and the snow from falling on the plains last Thursday, November 21. Proof that a month drier than normal can also be the scene of one or more episodes of snow.
Departure from normal rainfall in the first half of November 2024 – via JMA
White Christmas: an increasingly rare fact
You have to go back 13 years to find a white Christmas in the plains of France. The latter brought a culmination to a particularly wintry month of December 2010. In the northern half, a succession of snowy episodes punctuated the month. One of them occurs on Friday December 24, 2010 for New Year's Eve. The north-east of France is the most affected, notably Alsace and Lorraine where snowfall continues throughout Christmas night. On December 25, 2010, the snow cover was often 20 to 40 cm on the plains in these regions, where it had been 40 years since we had seen such a snowy Christmas! The Strasbourg station officially measures 26 cm on Christmas morning!
26 cm of snow officially recorded in Strasbourg on December 25, 2010 – via infoclimat.fr
It has now been almost 14 years since France last experienced a white Christmas in the plains. With warming, the chances of seeing a white coat covering the plains on December 25 are increasingly low, including in the east of France. For example, Besançon has only experienced 2 white Christmases since 1991 (in 2001 and 2010), or only two occurrences in 32 years! This happened more frequently in the past. For example, if we take the same period of 32 years between 1949 and 1981, there were 6 white Christmases in Besançon.
White Christmases in Besançon since 1949 – Weather France
If the occurrence of white Christmas is only decreasing given global warming, these data also show that a Christmas with your feet in the snow has never been a norm in France. There have always been more Christmases without snow than Christmases with it in lowland regions.
