Remember the series of winter storms we experienced mid-January? It was the result of an “atmospheric river” driving a series of Pacific storms onshore.
Now there is a new scale to characterize strength and impacts of atmospheric river type storms, just like hurricanes. The scale is useful because atmospheric rivers often have a significant impact on California, bringing large amounts of snow, rain, and sometimes, catastrophic flooding. They are also a significant source for our water supply.
A new study, in the February 2019 Bulletin of the American Meteorological Society, and published by Scripps Institution of Oceanography at the UC San Diego ranks the strength and impacts of these “atmospheric rivers” type storms, sometimes called a “Pineapple Express” due to some of the moisture originating as far away as Hawaii.
Just like we have with hurricane strength categories, this new scale assigns five categories to atmospheric rivers from 1 to 5 and labels the categories “weak,” “moderate,” “strong,” “extreme” and “exceptional.” The categories consider the amount of water vapor the carried by the storms and the duration at a given location.
While the scale might be fine for helping people understand the strength of the storm when reported in the news, the real value is helping to determine if the atmospheric river will be beneficial, hazardous, or both.
The new intensity scale ranks atmospheric rivers like this:
Cat 1 (Weak): Primarily beneficial.
Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous.
Cat 3 (Strong): Balance of beneficial and hazardous.
Cat 4 (Extreme): Mostly hazardous, but also beneficial.
Cat 5 (Exceptional): Primarily hazardous.
For example, in California, there was the “Great Flood of 1862” which was an atmospheric river that continued non-stop from late December 1861 to mid-January 1862. It flooded downtown Sacramento, and Chico experienced flooding from Big Chico Creek. That storm would be categorized as a Cat 5, or “Exceptional.” It is the largest flood event since California was settled.
An example of a Cat 4 (Extreme) atmospheric river that would be mostly hazardous, but also beneficial occurred on Jan. 8-9, 2017. That storm continued for 36 hours and produced up to 14 inches of rain in the Sierra Nevada, causing many rivers to reach flood stage. It was a major contributor to the Oroville Dam spillway crisis.
Dozens of other storm events throughout California history can now be ranked by this new system.
In the study, researchers noted that 80 percent of levee breaches in California’s Central Valley are associated with atmospheric rivers, so this new scale will be helpful to water resource managers and emergency planning personnel in determining if the next storm coming our way will be helpful, hurtful, or both.
