New intensity scale ranks California’s ‘atmospheric river’ storm events just like hurricanes


Image: The long white cloud band is an atmospheric river hitting California in January 2017. Credit: Jesse Allen, NASA Earth Observatory/ VIIRS/ Suomi satellite

Remember the series of reservoir filling and snowpack building winter storms California experienced mid-January? It was the result of an “atmospheric river” driving a series of Pacific storms onshore, just like the event that busted the Oroville Dam spillway in 2017, but not nearly as strong.


The January 8-9 2017 AR event that was a major driver behind the Oroville Dam spillway crisis. Credit: The SSMI satellite sensor of Defense Meteorological Satellite Program.

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 University of California San Diego ranks the strength and impacts of these “atmospheric rivers” (AR) 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 AR 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 AR will be beneficial, hazardous, or both.

The new intensity scale ranks ARs 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.

From the paper:

For example, in California, there was the “Great Flood of 1862″ which was an AR that continued non-stop from late December 1861 to mid-January 1862. It flooded downtown Sacramento. That storm would be categorized as a Cat5, or “Exceptional”. It is the largest flood event since California was settled.

Downtown Sacramento following the Great Flood of 1862. The city remained underwater for months, triggering a massive reconstruction project to raise the downtown area 10 to 15 feet. Photo Courtesy NWS/NOAA

An example of a Cat 4 (Extreme) AR that would be mostly hazardous, but also beneficial occurred in 2017 on January 8-9. That storm continued for 36 hours and produced up to 14 inches of rain in the Sierra Nevada and causing many rivers to reach flood stage. It was a major contributor to the Oroville dam spillway crisis.

Dozens of other AR 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 ARs, 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.





via Watts Up With That?

February 13, 2019 at 12:07AM

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