A storm in Northern California has thrown so much water into the area that roads have turned into ditches and isolated two California cities. It's a kind of storm that experts could project if climate change continues – which could mean even worse flooding for California.
In recent days, parts of Northern California were drenched with rainfall records of 19459005 . North of San Francisco, the Russian river rolled over its shores to surround the cities of Guerneville and Monte Rio of Sonoma County. The press democrat reports. Yesterday, thousands of people were told to evacuate. Today it was too late for some: "Guerneville is officially an island," announced the Sheriff of Sonoma County on Facebook. "Without a boat you can not enter or leave the city today."
The storm that tapers is an atmospheric river. These warm, moist strips of air roll over the Pacific, hit the coastal mountains and cool California – it rains every winter with rain and snow. "It has released amazing amounts of rainfall, which has unfortunately caused severe flooding along the Russian river and some other rivers," says Marty Ralph, director of the Center for Western Weather and Water Extreme at the Scripps Institution of Oceanography.
This is because the atmospheric river has come to a standstill, lingering over wet soils and swollen rivers and increasing flooding. "This firehose was just stuck in Sonoma County and was sitting there," says Daniel Swain, a climate scientist at UCLA's Institute for Environment and Sustainability.
These floods show why atmospheric rivers are double-edged swords for California. Each year, in just a few days, they drop between 25 and 50 percent of the rainfall in the thirsty state. According to a 2015 report, they are also responsible for an estimated 81 percent of the dam injuries in Central Valley, California. And in 2017, consecutive atmospheric rivers accelerated the devastated outflow of the Oroville Dam.
To capture this balance between benefit and danger, Ralph proposed a new scale published earlier this month in the Bulletin of the American Meteorological Society . The atmospheric rivers are classified according to the amount of water they transport and the length of their stay. The scale ranges from a "weak" category 1, which leads mostly helpful rainfall to an "exceptional" category 5, which is far more dangerous than helpful. Based on earlier measurements, Ralph estimates that the California that is drenching California falls into a category 3 or 4 – somewhere between strong and extreme.
While Ralph can not go into the effects of climate change on this atmospheric river he says that we may expect climate change to produce worse atmospheric rivers. Warm air can absorb more water vapor, which means more rain. However, it is expected that the wind, which is also important for atmospheric flux formation, on average, will decrease as temperatures rise at the poles. "These two trends are competing for long-term projections," says Ralph. "One says weaker, one says stronger."
That could generally mean slightly less atmospheric rivers, but it could also mean that some of those we see are likely to be bigger and stronger, a recent study by Ralph Laboratory predicts. "An average AR has a water vapor transport value of about 25 Mississippi rivers," says Ralph. "A stronger AR would be like adding an extra steam flow in Mississippi."
UPDATE MI 13:45: The river continues to rise! This is the Guerneville Bridge (Hwy 116). Stay safe.
This is likely to cause more flooding, especially as rising temperatures mean more rainfall that can lead to rivers and less snow sticking to the slopes , "The extremes are becoming more extreme," says Kelly Mahoney, a research meteorologist at the National Oceanic and Atmospheric Administration.
The potential for devastating floods must be considered by California's water managers, as the state's water reservoirs face the conflicting tasks of flood control and water storage. "People have been working on it for some time – the paradigm of the feast or famine, flood or drought that we've seen for many years," Mahoney says. "And the fact is, the predictions apply to this pattern."