ARI SHAPIRO, HOST:
Powerful storms are dumping water on Northern California. There is widespread and dangerous flooding. Today's storms and several others hitting the state are being caused by atmospheric rivers - sinuous plumes of moisture that travel up from the tropics. A single one can carry more water than the Mississippi River. As NPR's Christopher Joyce reports, new research on these airborne rivers shows they do a lot more than drench us.
CHRISTOPHER JOYCE, BYLINE: The atmospheric rivers that soaked California this winter ended an epic drought in the state. That was the good news. The bad news, as reported by The Weather Channel, was the flooding.
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UNIDENTIFIED WOMAN: We continue to get jaw-dropping images out of California - just day after day after day of heavy rain, rockslides, mudslides.
JOYCE: This winter, the Western U.S. experienced more than 10 atmospheric rivers. These weather systems are famously wet, but atmospheric scientist Duane Waliser has some new research that shows they're also remarkably windy. Waliser studied two decades of storms outside the tropics. When he focused on the very windiest, he found this.
DUANE WALISER: Atmospheric rivers are typically associated with 30 and even up to 50 percent of those very extreme cases.
JOYCE: Winds during an atmospheric river are typically twice the speed of the average storm. Waliser says emergency responders need to know that.
WALISER: Not only do they come with this potential for flooding hazard, they also come with potential for high impact winds and extremes that can produce hazardous conditions.
JOYCE: In fact, the atmospheric river that hit California January 8 knocked over a famous and gigantic sequoia tree in the state park. Waliser, who's with NASA's Jet Propulsion Lab in California, says the combination of water and high wind is especially costly. Over the last two decades, for example, Europe experienced 19 storms that each did at least a billion dollars in damage.
WALISER: And so out of these 19 storms, we associated atmospheric rivers with 14 out of 19.
JOYCE: Waliser's research appears in the journal Nature Geoscience. He says his next project is to find out if climate change will make atmospheric rivers more frequent. Christopher Joyce, NPR News.
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