Study projects sequences of atmospheric rivers will increase. Here’s what that means for Sonoma, Napa counties

Atmospheric rivers are a double-edged sword. They’re both a major source of water for North Bay residents and a significant contributor to flooding.

The pros and cons all depend on when the moisture-laden storms appear.

Last winter was marked with back-to-back-to-back storms caused by atmospheric rivers. They helped the North Bay emerge from a yearslong drought, but also brought significant damage and even death to Sonoma County.

In the next week, two atmospheric rivers will move into the North Bay, just a little over a week since a previous storm exited the region.

Unfortunately, the number of these back-to-back atmospheric rivers — which have corresponded with compounded damages — are projected to increase over the next century, according to a recently published study.

The future storm “sequences,” as they are defined in the study, are predicted to be more intense, last longer and potentially occur in places they hadn’t before, said the study’s lead author Corinne Bowers, who recently earned her Ph.D. in civil engineering from Stanford University.

Specifically the “super sequences,” which are longer than 60 days, “drive the projected increase in frequency and present a growing water management threat in California,” according to the study.

Bowers and the other scientists came to this conclusion in the study after finding that when the sequences were occurring, there were also extreme saturated soils, rain and flooding in the areas pinpointed.

Using data from the wet seasons between 1981 to 2021, the team also determined that a storm will cause three times more damage if it occurs in a sequence than if it occurs alone.

“Anecdotally, it's something that Californians are aware of,” Bowers said, referencing the increased damage that comes with concurrent storms. “But the magnitude of the number surprised me.”

One of the key takeaways from this finding, she said, is “considering one atmospheric river on its own in isolation, is not enough to tell the whole story about the potential impact and damage it can cause.”

“We need to get better at forecasting not just one storm,” she added, “but forecasting clusters of them.”

In anticipation of this week’s storm, the National Weather Service issued a flood watch and wind advisory for both Sonoma and Napa counties through at least Wednesday.

Brian Garcia, a meteorologist with the weather service’s Monterey station, said the incoming storms and compounding rains are problematic for the region because the soils are already saturated from previous rainfall.

Some hazards to expect, he said, include flooding, shallow landslides and downed trees.

“If the soils are oversaturated, then their roots are essentially in soup,” Garcia said. “You put any wind on that, and we have trees fall down.”

Garcia said while he does agree that sequences of atmospheric rivers are likely to continue to cause larger amount of damage, he also believes it’s important to factor in other rain sources and not simply the high-moisture storms included in the study.

“While the author talks about how atmospheric rivers amp up California storm damages, it just doesn't matter to me whether it's an atmospheric river or a cold front off the Gulf of Alaska,” he said. “Anytime that we put additional rainfall on top of already swollen rivers, creeks, streams, and heavily saturated soils, we run the risk of significant impacts.”

Brad Sherwood, assistant general manager of the Sonoma County water agency, said he was not surprised by the study’s findings.

For the past 10 years, Sherwood said, he has been involved among a cohort of water managers who have pushed for further investment in atmospheric river forecasting to preserve water and prevent flooding.

“Atmospheric rivers… are only going to increase in their prominence in our region and be a major player in how we live with Mother Nature,” he said. “So as water managers, we're doing everything we can to stay on the forefront of science and data to upgrade and improve our existing infrastructure to work with atmospheric rivers.”

Helping predict storms

Last weekend, the U.S. Army Corps of Engineers increased water releases ahead of this week’s storms at both Lake Mendocino and Lake Sonoma and slowly decreased the releases Tuesday before the storm arrived. One tool that helped guide that decision was an instrument propelled into the air by research teams from UC San Diego’s Scripps Institution of Oceanography.

Late last week and early this week, Scripps’ center for western weather and water extremes attached moisture-detection sondes — battery-operated beacons — to helium-filled balloons in Bodega Bay, field research manager Anna Wilson said. As it drifted through the air, the weather instrument then recorded data – specifically temperature, pressure, moisture, and winds at six times per second – and sent it back in real-time.

The goal was to help experts predict the impending storms and inform decisions at the reservoirs.

Wilson has been contributing to this research and working with atmospheric rivers for at least eight years in the region. During her time, Wilson said she has seen a decrease in the number of weaker storms and an increase in frequency of heavier storms.

“In addition to the risk of having big events back-to-back is there's still increased risks of longer droughts, too,” she said. “The swings are getting wider between wet periods and dry periods.”

Bowers said research for her study also aligned with this idea of California seeing longer and more arid dry seasons, along with more drawn out and soaked wet seasons.

Garcia said some of this climate whiplash can be seen in how prolonged storms during the 2022-23 winters and the early portion of 2017 both resolved prolonged droughts.

“State Climatologist Mike Anderson used to say that we don't get out of a long term drought in one winter,” he said, laughing a little bit. “I haven't heard him say that and probably over five or six years.”

Bowers said she hopes the study will encourage individuals to consider more situational context when reporting, studying or listening to reports on California weather.

She also hopes it propels further research in the area, such was the case after the term atmospheric river was defined in academia.

“Naming it led to an explosion of research, a better understanding of kind of the way that our water gets delivered,” she said. “And I'm hoping that the same thing can happen with sequences and compounding.”

You can reach Staff Writer Madison Smalstig at madison.smalstig@pressdemocrat.com. On X (Twitter) @madi.smals.