A car sits underwater in a flooded neighborhood on Feb. 27 in Forestville, Calif. (Justin Sullivan/Getty Images)

Considering atmospheric rivers didn’t even have a rating system until earlier this year, science is making up for lost time in studying this meteorological phenomenon.

The latest research puts a price tag on the havoc wreaked in the Western United States by atmospheric rivers (ARs): a yearly average exceeding $1 billion. Atmospheric rivers are akin to airborne veins that connect the midlatitudes with the moisture-rich heart of the tropics. These channels, which can stretch for thousands of miles, contain large quantities of water vapor and deliver it in highly concentrated doses.

Atmospheric rivers are capable of carrying more than twice the volume of the Amazon. And for parts of the West, ARs bring up to half of their annual precipitation totals.

But what really caught the attention of researchers who cross-referenced four decades of insurance losses against weather records is the degree to which ARs are responsible for flooding.

Across the West, nearly 85 percent of all flood damage suffered between 1977 and 2017 was the work of ARs, rather than weaker winter weather systems lacking that source of moisture or summer thunderstorms. In coastal areas of Northern California and Oregon, atmospheric rivers cause almost all flooding, a new study finds.

“Maybe the most important policy response this research suggests would be providing post-disaster assistance to people not to rebuild, but to move to a safer community,” said study co-author Tom Corringham of the Center for Western Weather and Water Extremes (CW3E), part of the Scripps Institution of Oceanography at the University of California at San Diego.

The study, which also involved the U.S. Army Corps of Engineers, was published Wednesday in the journal Science Advances.

When conditions are right, ARs can funnel tropical moisture toward the West Coast for days on end — potentially turning even a weak AR into a flood producer if pointed at the same place for long enough.


Map of atmospheric water vapor, showing an atmospheric river aimed at California on Wednesday. (University of Wisconsin)

That’s exactly what happened in the San Francisco Bay area and eastward a few days ago, when a midlatitude cyclone, brought along an AR, stalled out over the northeast Pacific Ocean. The event’s water transport wasn’t much, but its long duration (66 hours) boosted the Ralph scale rating to a 3, which is classified as strong. On the ground, more than 5 inches of precipitation fell in the Coast Ranges and Sierra Nevada, with higher elevations receiving more than two feet of snow.

Damage was limited to closed highways and roads, sporadic power outages and downed trees.

The AR, following days of little movement to the north or south, has finally slid down the coast, setting up for a wet Wednesday in the Southland. Los Angeles was expecting 0.75 to 1.5 inches of rain, perhaps double that in the mountains; San Diego’s forecast called for a spritz — less than 1 inch.

The National Weather Service expects road travel in Southern California to be only slowed and not halted, as happened last week in the hours before Thanksgiving.

And once this clears, the Golden State anticipates yet another AR — perhaps also rated as strong — arriving Thursday.

According to the study, over the past 40 years, flooding from all causes across the West has led to nearly $51 billion in damage. This was tallied by combining National Flood Insurance Program loss data from 11 states with measures of total damage from an NWS data set and a catalogue of more than 600 West Coast ARs.

Of the $42.6 billion in damage that can be pinned on ARs, nearly half that total, $23 billion, was due to just 10 such events.

Corringham also was surprised to learn the power of the relationship between an AR’s strength and how much harm it does.

“I had a feeling it wouldn’t be linear,” Corringham said — and it wasn’t.

Instead, historical averages show the difference to be exponential. For example, the difference between the median flood damage of an AR2 (Moderate) and an AR4 (Extreme) isn’t a doubling, but a 50-fold increase, from $400,000 to $20 million.

There’s reason to be concerned about the future of ARs, too, given studies showing that they are likely to carry more moisture as the world’s oceans and atmosphere continue to warm.

“We know from other research that the intensity of these storms is increasing due to climate change and is projected to keep increasing over the coming century,” Corringham said. “So even if it were a linear relationship, it would be quite significant.”