This week, the American NWS forecasts significant precipitation, marked accumulations and local flooding due to a depression subject to an explosive cyclogenesis process. The hardest hit areas would be California and Oregon, where the effects of a major atmospheric river will be felt.
What is an atmospheric river and what are its categories?
Atmospheric rivers (ARs) are relatively narrow, water vapor-rich incursions or transport bands that advance from tropical areas to midlatitudes, driven by powerful winds. They are potentially precursors and generators of persistent, sometimes intense, precipitation in land regions that can lead to regional flooding.
Atmospheric rivers (AR) are ranked from 1 to 5 and define the categories “weak”, “moderate”, “strong”, “extreme” and “exceptional”. The classification is based on the quantities of water vapor transported within an atmospheric river, with a standard duration of 24 to 48 hours as a reference.
The scale classifies ARs as follows:
+ RA Cat 1 (Weak) : Mainly beneficial. For example, an AR on February 23, 2017 reached California, lasted 24 hours on the coast, and produced moderate rainfall.
+ RA Cat 2 (Moderate) : Mainly beneficial, but with some danger. An atmospheric river from November 19 to 20, 2016, affected Northern California, lasted 42 hours on the coast, and brought several inches of rain, helping to replenish water supplies after a drought.
+ RA Cat 3 (Strong) : Balance between benefits and dangers. An atmospheric river on October 14-15, 2016 lasted 36 hours on the coast, bringing significant rainfall that helped replenish reservoirs after a drought, but also caused rivers to rise to just below flood stage.
+ RA Cat 4 (Extreme) : Mainly dangerous, but also beneficial. For example, an atmospheric river on January 8-9, 2017, which persisted for 36 hours, produced significant precipitation in the Sierra Nevada, causing at least a dozen rivers to rise to flood stage.
+ RA Cat 5 (Exceptional) : Mainly dangerous. For example, an atmospheric river that lasted from December 29, 1996 to January 2, 1997, more than 100 hours on the central coast of California. The intense rainfall and associated runoff caused more than $1 billion in damage.
Powerful depression and its associated atmospheric river: heavy precipitation and flooding
A powerful depression will develop off the coast of the western United States, in the northeast of the Pacific Ocean, during a process of explosive cyclogenesis, with very intense winds in the lower layers of the atmosphere.
The explosive development of the storm will occur during this week.
The powerful depression will contain within it an intense and long-lasting atmospheric river, capable of bringing up to 600 l/m² of rain. Experts predict the formation of a potentially destructive Category 4 atmospheric river. Some evolutionary scenarios suggest that it could reach category 5.
It is worth remembering that the categories of atmospheric rivers are based on their intensity and duration. Categories 4 and 5 are considered “potentially dangerous”.
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