New scale ranks atmospheric rivers like hurricanes. Here’s how it’ll help California

Meteorologists just announced the creation of a scale that will rank intensity of atmospheric rivers from Categories 1 through 5, similar to the existing scale used for hurricanes.

The American Meteorological Society in a blog post Tuesday said the AR scale will help water and weather experts track the storms, which are prevalent on the West Coast, and determine whether they are beneficial or hazardous to the regions they hit.

The scale will work like the Saffir-Simpson hurricane scale. Not yet given a more formal name, the AR scale will rank the storms as Categories 1-5 — Category 1 being “weak” and Category 5 being “exceptional,” with 2 through 4 denoted “moderate,” “strong” and “extreme,” respectively.

AMS in its blog post explains that Categories 1 and 2 are mostly beneficial to water health, 4 and 5 are mostly hazardous and 3 is a balanced mix of both.

Benefits include boosts to water supply and snowpack, while hazards include high flood risks.

Atmospheric rivers are most commonly observed in California. One passed through Northern California earlier this year, drenching the Sacramento Valley with record rainfall in mid-January.

AMS defines an atmospheric river as “a long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone.” The term wasn’t officially defined by AMS until 2017, though the storms had been observed before then.

The weather events lead to heavy precipitation; AMS says the average atmospheric river carries more than double the flow of the Amazon.

The society’s blog post gives examples of the scale using previous storms. A January 2017 atmospheric river in the Sierra Nevada that dropped 14 inches of rain and flooded more than a dozen rivers in a 36-hour span was designated Category 4. The types of storms hitting the Sacramento region recently have been categories 1 to 3.

The new scale was unveiled in the February issue of the Bulletin of the American Meteorological Society, a scientific journal. An advance of the report, co-authored by researchers at Scripps Institution of Oceanography, National Weather Service and the California Department of Water Resources, was posted to BAMS online, but appears to have since been removed.

“It can serve as a focal point for discussion between water managers, emergency response personnel and the research community as these key water supply and flood inducing storms continue to evolve in a changing climate,” DWR co-author Michael Anderson said, according to the AMS blog post.

The new AR scale bases its rankings on calculations of “maximum instantaneous integrated water transport” and the duration of those conditions.

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