A storm in northern California dumped so much water on the region that roads turned into moats, isolating two California towns. It’s the kind of storm that experts project might become more extreme as climate change continues — which could mean a future of even worse flooding for California.

Over the past few days, parts of northern California have been drenched with record levels of rain. North of San Francisco, the Russian River rolled over its banks to surround the Sonoma County towns of Guerneville and Monte Rio, The Press Democrat reports. Yesterday, thousands of people were told to evacuate. By today, it was too late for some: “Guerneville is officially an island,” the Sonoma County sheriff announced on Facebook. “You will not be able to get into or out of town without a boat today.”

The storm, which has tapered off, is an atmospheric river. These bands of warm moist air roll in off the Pacific, hit the coastal mountains, and cool down — showering California with rainfall and snow every winter. “It dumped amazing amounts of rainfall that have sadly created serious flooding along the Russian river and some other rivers,” says Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography.

Russian River keeps rising. Photos from Guerneville. See interactive flood map and more info at https://t.co/3E8qX9nNBl. Be prepared to evacuate if you live near the river. pic.twitter.com/ARtWup3EeG — Sonoma Sheriff (@sonomasheriff) February 26, 2019

That’s because the atmospheric river stalled, lingering over already wet soil and swollen rivers and increasing flooding in the process. “That firehose just got stuck on Sonoma county and sat there,” says Daniel Swain, a climate scientist at UCLA’s Institute of the Environment and Sustainability.

These floods show why atmospheric rivers are double-edged swords for California. They drop between 25 and 50 percent of the thirsty state’s precipitation over just a few days every year. But they’re also to blame for an estimated 81 percent of levee breaches in California’s Central Valley, according to a 2015 report. And in 2017, back-to-back atmospheric rivers accelerated damage to the crumbling Oroville dam spillway.

To capture that balance between benefit and hazard, Ralph proposed a new scale published earlier this month in the Bulletin of the American Meteorological Society. It ranks atmospheric rivers based on the amount of water they’re carrying, and how long they linger. The scale ranges from a “weak” category 1 that mostly brings helpful precipitation to an “exceptional” category 5 that’s far more hazardous than helpful. Based on early measurements, Ralph estimates that the one that just drenched California falls around a category 3 or 4 — somewhere between strong and extreme.

At Pocket Canyon Road. Our Marine Unit is in the Guerneville area to help rescue people. pic.twitter.com/ptSre6689L — Sonoma Sheriff (@sonomasheriff) February 27, 2019

While Ralph can’t speak to the effects of climate change on this particular atmospheric river, he says we may be able to expect worse atmospheric rivers as climate change continues. Warmer air can hold more water vapor, which means more rainfall. But winds, which are also important for atmospheric river formation, are expected to slacken on average as temperatures at the poles climb. “These two trends compete with each other in terms of the long-term projections,” Ralph says. “One says weaker, one says stronger.”

That could mean somewhat fewer atmospheric rivers in general, but it could also mean that some of the ones we do see are likely to be bigger, and stronger, a recent study from Ralph’s lab predicts. “An average AR has about 25 Mississippi Rivers worth of water vapor transport going on,” Ralph says. “A stronger AR would be like adding additional Mississippis worth of water vapor flow.”

UPDATE WED 1:45pm: The River keeps rising! This is the Guerneville Bridge (Hwy. 116). Stay safe. Posted by Sonoma Sheriff on Wednesday, February 27, 2019

That’s likely to bring more flooding, particularly as rising temperatures mean more rainfall that can swell rivers, and less snow that sticks to the slopes. “The extremes will get more extreme,” says Kelly Mahoney, a research meteorologist with the National Oceanic and Atmospheric Administration.

The potential for devastating floods is something California’s water managers will have to account for as the state’s reservoirs juggle the contradictory jobs of flood protection and water storage. “People have been grappling with this for for a while — the feast or famine, flood or drought paradigm we’ve seen over many years,” Mahoney says. “And the fact is that the projections are for that pattern to continue.”