Editor’s Note (11/16/21): Washington State and British Columbia have been inundated by a relentless series of atmospheric rivers that have shed up to three feet of rain in some locations over the past four weeks. In 2019, scientists released a new scale to rate the severity of these events, which is explained below in our original story from that time. It is fair to say that the ongoing Flood is classified as Category 4 or 5.
Several times a year an atmospheric river – a long, narrow conveyor belt of storms pouring incessantly in from the Pacific Ocean – drops inches of rain or feet of snow on the US west coast. Such a system triggered floods and mudslides in central and southern California last weekend. But only in the last few years have weather forecasts been able to predict roughly how strong the impending storms will be.
Now they have a new scoring system that will help them – and the public – understand what an incoming atmospheric flux could cause; On Tuesday, researchers unveiled the “atmospheric flow scale”. It ranks severity and impact from Category 1 (weak) to Category 5 (exceptional). “Without a scale, we really had no way of communicating objectively what a strong or weak storm would be,” said Martin Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, who led the work.
Atmospheric rivers flow through the sky about a mile above the surface of the sea and can stretch for a thousand miles of the ocean to the coast. Some bring routine rain, but the more intense systems can carry as much water as 15 Mississippi rivers. The series of storms that hit land can arrive for days or occasionally for weeks. They encounter west-facing coastlines around the world, although the US learns more about them than most of the other national coastlines.
Satellites can see atmospheric rivers grow over an ocean, but the resulting data is not detailed enough to make an accurate prediction. Since 2012, scientists and engineers have been gradually building a system of ground-based sensors, including radars, which look at the sky during the first storm on land, in order to supplement weather models with moisture and wind data. The combination of all this information can improve predictions of how much precipitation can fall and for how long. Still, weather forecasts and contingency planners didn’t have a solid way of assessing how severe potential flooding could be – or sometimes whether rain or snow would actually be beneficial. “Scientists, the media and the public viewed atmospheric rivers primarily as a hazard,” says Ralph, “but the weaker ARs are definitely beneficial.” When the soil is dry or the snow cover or water reservoirs are low, an atmospheric river can wither plants revitalize and strengthen urban water supplies. “Water managers made it clear to us that a rating scale would be helpful,” he adds. For example, when reservoirs are high and heavy rain comes in, managers can drain water behind dams so they don’t get stressed or overflowed later.
The scale, published on Tuesday in American Meteorological Society bulletin, arranges atmospheric flows on five levels:
- Category 1: Weak—primarily beneficial
- Category 2: Moderate—mostly beneficial, but also a bit dangerous
- Category 3: Strong—Balance between useful and dangerous
- Category 4: Extreme—mostly dangerous, but also beneficial (if the drought persists)
- Category 5 – Exceptional –mainly dangerous
Together, Ralph’s Scripps Center, the National Weather Service, and other agencies will determine which category a storm will fit into. Having such a simple scale for assessing a complex weather pattern like an atmospheric river is valuable, according to James Doyle, a senior scientist at the U.S. Naval Research Laboratory who was not involved in the work. “Many terms in meteorology are difficult for the public to understand – or easy to misunderstand,” he says, such as atmospheric flux or polar vortex. “The scale gives a clear picture of the types of conditions that can arise.”
The scale has no formal name like the Enhanced Fujita Scale for tornadoes or the Saffir-Simpson Scale for hurricanes. And it differs on one major point: these scales focus on hazard, but not potential benefit – although it is unclear whether a hurricane or tornado could have a constructive effect. In addition, unlike the others, the new scale regards the storm duration as a fundamental factor. After Hurricane Harvey stalled over Houston and Hurricane Florence stuck over the Carolinas – both causing much more flooding and damage than expected – the Saffir-Simpson scale was criticized for reducing the risk of hurricanes, which increased upon landing moving slowly, not reflecting adequately.
Researchers are wondering if they can possibly predict whether there will be many atmospheric rivers in the coming year, which Ralph says can mean the difference between states like California with a drought or flood year. A seasonal or annual forecast could help farmers decide what type of crops to grow and community planners to decide how to limit water use to keep water levels safe.
Seasonal forecasts would be invaluable, but “that’s a big challenge,” says Doyle. Scientists would need to better understand how atmospheric rivers fit into the larger weather systems around them. And models need to improve how they integrate important processes, such as the constantly changing state of clouds and the exchange of heat and moisture between the air and the ocean, he says. “But,” he adds, “it’s a great goal.”
In the short term, Ralph is hoping local weather forecasters could add an atmospheric river symbol to their television and online graphics. A few years ago he was having breakfast in a San Francisco restaurant when he saw a weather forecast on TV that showed small sun and cloud icons at the bottom of the screen. They indicated that it would rain. “I knew there was a strong atmospheric flow coming,” he says. But “this little shower icon didn’t say anything about what would happen. Perhaps there could be an atmospheric river symbol indicating the category – how good or bad an approaching storm might be. “