France is experiencing a drastic shift in weather this week, with a parade of storms and a return of colder temperatures. The cause of this change is attributed to the intensity and trajectory of the Jet Stream.

After weeks of mild weather and spring-like conditions, storm Gérard has hit the country with wind speeds reaching over 150 km/h on the northwest coast and record-breaking gusts for the month of January. The storm is followed by another, Fien, which is set to hit the southern regions of France on Tuesday.

In addition to the strong winds, the return of colder temperatures is also expected this week, catching many by surprise as it was thought that winter had already passed. This sudden change in weather conditions is unusual for January, and it is believed that the strong undulations of the Jet Stream are to blame.

Jet Stream causes severe storms in France

The Jet Stream is a powerful corridor of wind that circulates in the atmosphere between 8 and 12 kilometers in altitude, stretching over thousands of kilometers in length. The polar current, which is the most powerful of the Jet Streams, is responsible for reinforcing simple depressions into storms, and also propelling them over France and the British Isles. Meteorologists often refer to this Jet Stream as the “low pressure rail” due to its ability to intensify weather systems.

In December 1999, the “storms of the century,” Lothar and Martin, were exacerbated by an ultra-powerful Jet Stream that circulated at 400 km/h in altitude. As Météo France explains, “not all depressions produce a storm. A depression will develop into a storm all the more easily when the winds of the jet stream are strong and when these winds are subject to significant variations (accelerations, decelerations, changes of direction). The depression widens as it approaches the ends of the jet stream and, on the contrary, loses its activity as it moves away from it.”

Between Sunday evening and Tuesday, the Jet Stream will be circulating over France at a speed of approximately 200 km/h at 9 kilometers of altitude on the west of France. This will intensify a depression that is currently circulating over Ireland and push it at high speed towards the northwest of France, giving rise to storm Gérard. A second depression, Fien, will also be pushed towards the southwest of the country, resulting in a storm on Tuesday.

Strong Jet Stream fluctuations bring cold weather

The Jet Stream is a zone where two masses of air meet, separating cold air from the north and warm air from the south. Its undulations, or meanders, allow these different masses of air to move up and down over the regions of the northern hemisphere. For example, the recent polar cold wave that hit the United States at the end of December was caused by a strong southward ripple in the Jet Stream, allowing freezing air from the Arctic to reach the middle latitudes of North America.

From Wednesday, the Jet Stream will undulate far south of France, extending as far as Spain. This means that cold air from Northern Europe will be moving over all of France, resulting in widespread frost in the morning and temperatures that may be slightly below the averages for January. This cold air will persist as long as this “branch” of the Jet Stream circulates in the south of the country, which is expected to last until the end of the week, or even longer.