Weather News: Atmospheric river: a potentially dangerous natural weather phenomenon – METEO CONSULT – DETAILED 15-DAY WEATHER FORECAST

Severe weather is currently underway in California with heavy rain and flooding, heavy snow in the mountains and strong winds along the coasts. This prolonged bad weather is due to the “atmospheric river” phenomenon that Weather Channel experts are predicting for you…

What is an atmospheric river?

The term “atmospheric river” has only been used for about ten years in meteorology. We speak of an “atmospheric river” when we have a rapid circulation of warm and humid air in the form of a relatively narrow corridor (200 to 400 km on average), from tropical latitudes to temperate regions. Tropical air, which contains a lot of moisture, finds itself pushed by a strong high altitude jet stream (Jet Stream). Encountering colder air, a depression deepens and causes an active disturbed system to dump prolonged heavy rainfall over a relatively geographically limited continental area.

The “Pineapple express” or the “Rum express”

While atmospheric rivers can affect large geographical areas, some regions are more regularly concerned, especially the western seaboard of North America and Europe because of the prevailing oceans in these regions. For the North American continent, meteorologists use the term “Pineapple express” or “pineapple road” in connection with the rapid circulation of tropical air coming from Hawaii.

In Western Europe, meteorologists use the term “Rum express” when warm, humid air rises from the Antilles to the European continent.

What are the consequences of a river in the atmosphere?

Atmospheric rivers play an important role in transporting and distributing moisture from the tropics to temperate and polar latitudes. They are in some cases beneficial by bringing regular rain for nature. They can also cause more or less bad weather with heavy rain or snowfall, floods and floods, violent winds, causing damage to property and people.

At the end of last December, a tropical air mass with high humidity content affected Brittany for several days. Finistère and Morbihan are on alert for heavy rains, floods and floods, which have suffered an attack of moderate, but long-lasting rains. In southern Finistère and western Morbihan, it fell up to 130 millimeters in 3 days, equivalent to 3 weeks of rain. These rains lead to flooding and flooding such as in Quimperlé where the Leita river flooded part of the city.

Some atmospheric rivers are more active such as in November 2021 when the Pineapple Express caused severe flooding in British Columbia, especially in the Vancouver area. In 3 days, almost 300 mm fell in the town of Hope, equivalent to a month and a half of rain. The city also broke a daily rainfall record with 174 mm, the old record from December 2, 1975 with 133 mm. Many floods and floods occurred following this heavy rain that caused 6 victims and damages worth several billion dollars.

At the beginning of January, California suffered heavy rains, heavy snowfalls in the Sierra Nevada reliefs and violent winds along the coasts as they passed. of a depression bomb. Flooding continues and a state of emergency has been declared in California.

Will atmospheric rivers become more frequent with global warming?

The warmer air masses are, the more moisture they can hold. According to IPCC scientists, an atmosphere that is 1°C warmer can hold 7% more water vapor. Thus, air masses coming from tropical latitudes pushed towards temperate latitudes have the capacity to contain more and more moisture and condensable water. According to the latest IPCC report from 2021, we can expect an increase in the intensity and duration of precipitation events associated with atmospheric rivers on the western fronts of the European and American continents.

France may therefore experience more extreme phenomena with more frequent droughts associated with rising subtropical high pressure and anticyclonic blockage, but also periods of heavy rain when the atmospheric rivers that it originates from tropical latitudes.

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