Snow episode on the plain: what are the ambiguities?

TO Cyril Duchen, meteorologist

While snowfall can be seen with certainty circulating in many regions between Friday and Saturday, there is still uncertainty about whether snow will remain on the ground, projected snow depth and snowfall intensity.

A drop in Arctic air will affect France and Western Europe this weekend, causing wintry conditions with freezing northerly winds and snowfall to hit about 60% of the area. Dryer weather will follow on Sunday and Monday morning with a significant risk of frost.

Why is snow often difficult to predict and what are the uncertainties?

The key element in predicting the arrival of snow is temperature. Snow is most often observed at temperatures close to 0°C, but can also fall at freezing temperatures (as a rule, this is very fine snow). It can also be observed under conditions of strong instability up to a temperature of 3-4 ° C, during the famous wet snow (often these are large flakes loaded with moisture or melted snow).

The difficulty in predicting snow lies in the fact that with an accuracy of one degree, you can go from snow to rain or vice versa. Therefore, temperature prediction is very important. If the observed temperature is 1-2 degrees higher than the predicted one, then there is no snow episode.

Added to this is whether snow remains on the ground. For snow to stick to the ground, it must be cold, i.e. with a temperature close to 0°C or below zero, or that the precipitation is heavy enough to keep the snow.

This Friday, temperatures predicted for the episode are typically between 0 and 2°C. They will allow snowfall to be seen, but there is a risk of differences between rural areas, where it is colder, and urban agglomerations. In Île-de-France, an urban heat island associated with heating and building density can be large enough to bring rain or melting snow to the capital and its immediate suburbs, while some of the highlands of Île-de-France turn white.

Altitude and geography of places are also important. An altitude of 100 m above sea level can allow snow to hold on, as the temperature drops by about 0.6°C for every 100 m. In Ile-de-France, it happens that the snow does not hold in some valley floors, and the surrounding plateaus turn white.

Finally, precipitation intensity is also an element to be taken into account. In certain weather conditions, when precipitation intensifies and cold air at altitude reaches the ground (an isothermal phenomenon), snow can quickly hold. When the snowfall is light, if the ground is not very cold and the roads are wet, it is difficult for the snow to hold on. In this Friday’s episode, it is between the Nord-Pas-de-Calais, the western part of the Paris Basin and the hills of Normandy that we expect the most intensity with a significant risk of running aground. Snowfall is expected to be less and temperatures are expected to drop slightly in the suburbs of Paris and in the eastern Paris Basin, making it difficult for snow to stick to the ground.

How does snow in spring not have the same effects as in winter?

In spring, the length of the day and the number of sunny days are greater than in winter. The slightest rarefaction heats the lower layers of the atmosphere more easily than in winter, when the sun is low on the horizon and much less powerful. In the spring, snow that has fallen overnight or in the morning rarely lasts until evening. Days without a thaw are also extremely rare in the plains, and in some regions they do not exist at all. Only the heaviest snowfalls with low daytime temperatures can allow the snow to stay forever.

This snowfall will be followed by a significant frost risk. Is the situation reliable and what values ​​should be expected?

Yes, in a general weather context, the situation is valid with a temporary return of drier and colder weather on Sunday and Monday, influenced by a slight anticyclonic surge. When the sky clears at night, quite severe frosts (from -1 to -4°C) can be expected in many regions. Monday mornings should be the most dangerous for crops and fruit trees due to vegetation growth associated with the mildness of the last days. A situation reminiscent of last year, when devastating frosts were on April 6-7, 2021.

You are announcing a marked cold snap in the middle of next week: is spring set for good?

If temperatures rise to seasonal levels, the weather will remain quite turbulent with periods of rain on the plains and snow on the mountains. Therefore, we will not find long-term good spring weather, as it was in the month of March.

Feel free to consult our trend for the next four weeksincluding Easter holidays. In this case, the snow that returns to all our mountains will certainly delight holidaymakers at winter sports resorts.

Leave a Comment