Cars drive through a flooded street after a storm dumped heavy rain in Los Angeles on Feb. 2. (Mark Ralston/AFP)

Atmospheric rivers, the weather phenomena that blast a fire hose of tropical moisture over the West Coast, are a relatively new topic of study for meteorologists. In fact, only now have experts developed a scale to characterize the strength and potential impacts of these systems.

On Tuesday, when California was drying out from the latest deluge sourced from these rivers in the sky, a consortium of researchers announced the debut of a simple 1 to 5 gauge to clearly delineate the punch delivered, from “weak” to “exceptional,” with “moderate,” “strong” and “extreme” in between. The scale is based on, intuitively, how much airborne moisture is present above any given point on the map and for how long the atmospheric river (AR) flowed over that point.


The Atmospheric River Scale categorizes events based on how much airborne moisture is present above any given point on the map and the duration of the flow over that point. (Center for Western Water and Weather Extremes)

ARs, often previously referred to as the “pineapple express” because of their source near Hawaii, are responsible for almost all (92 percent) the West Coast’s heaviest three-day rain events, according to past research. So what was needed, said F. Martin “Marty” Ralph, director of the Center for Western Water and Weather Extremes (CW3E) at the University of California at San Diego’s Scripps Institution of Oceanography, was a “situational awareness support tool.”

With this scale, forecasters can better warn the public about how damaging an AR-fed storm might be. Looking backward, researchers can rank past events and sift through collected weather data to find long-term trends.

The first atmospheric river event to be retroactively rated was the major storm that struck California on Saturday and Sunday, which caused cloudbursts over wildfire burn scars near Santa Barbara and Los Angeles while dumping more than seven feet of snow on Lake Tahoe’s ski resorts. At landfall on the Central Coast, the AR carried a max IVT (Integrated Water Vapor Transport) of 908 kg/m/s while conditions lasted 45 hours — an AR Cat 3, or “strong,” event.

Previous analysis has shown that on the West Coast, Oregon receives the most atmospheric rivers categorized as “extreme,” averaging about one a year. Washington state gets an extreme AR every other year, the Bay Area about every three years, and Los Angeles once a decade.

The 1996-97 atmospheric river that caused California’s worst flooding since 1950 is categorized as an “exceptional” storm. But in a typical winter, the strongest ARs hitting the Southern California coast are AR Categories 2-3.

ARs can wreak havoc on western coasts in the mid-latitudes worldwide, such as when Britain and the Iberian Peninsula are socked by moisture imported from the Caribbean. But most are beneficial, providing 22 percent of total global runoff through rains and snowpack; on the U.S. West Coast, that figure can rise to 50 percent. Long-term absences of ARs can lead to parching droughts.


A computer model simulation of integrated water vapor transport (IVT) from an atmospheric river that hit California in January 2007. (Scripps)

In the past, meteorologists ranked ARs’ strength on the scale of weak to extreme simply by the amount of water vapor detected. But the innovation with this scale is the time element. If an atmospheric river lasts in an area for less than 24 hours, it is demoted by one category; if it lingers for more than 48 hours, it is bumped up a place (but not past AR Cat 5).

Considering an event’s duration when developing the scale, researchers said, better accounts for the damage that can be done by a low-intensity system hanging around for a long stretch.

“We baked it in from the very beginning,” Ralph said.

In this way, it’s an improvement over the familiar Saffir-Simpson scale for hurricanes, in which a tropical storm’s winds may be below the threshold to rate — but that’s not stopping it from dropping buckets of rain for days on end, like Hurricane Harvey did to Houston in 2017.

Although Scripps led the effort to create the scale, assisting organizations included the National Weather Service, the California Department of Water Resources and the U.S. Army Corps of Engineers.

Does this new scale have a name? Not yet, although it would seem natural to credit its primary creator.

“People’s reaction to the idea of the scale often comes with a remark like, ‘Cool, it’ll be the Ralph scale,’” Ralph said Tuesday. “I reply, ‘That’s for others to decide.’ ”

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