Two atmospheric rivers are forecast to pound parts of Oregon and Washington on Friday and Saturday, arriving after two earlier storms already dropped multiple inches of rain on both states this week, NPR reported.
"What's really problematic is when you get compounding impacts from multiple atmospheric rivers," said Josh Weiss, a meteorologist with the National Oceanic and Atmospheric Administration's Weather Prediction Center. Those impacts include flash flooding, urban flooding and even landslides, he said. This year's atmospheric river season, which runs from October to March, is off to a fast start, Weiss said.
An atmospheric river is a narrow band of water vapor pushed by the wind. Loaded with moisture, it dumps rain or snow. These rivers can be up to 2,000 miles long and 600 miles wide, said F. Martin Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography. "It's literally a river in the sky," Ralph said. About 50 to 60 percent of rain in California comes from atmospheric rivers, Weiss said.
King tides, El Niño and a 20-foot swell risk
The storms are arriving as El Niño's warm ocean waters contribute to rising sea levels and fuel storm systems, and as annual high tides known as king tides coincide with the rain. That combination leaves coastal and inland waterways along the West Coast at risk for flooding, NPR reported.
Ralph said that according to 40 years of data from the Federal Emergency Management Agency, 90 percent or more of all flood damage in the Western United States is from atmospheric rivers. This weekend, Jeff Michalski, a meteorologist at the National Weather Service in Seattle, said Washington's coast could be hit by significant 20-foot swells.
People often know atmospheric rivers by nicknames such as the Pineapple Express or Rum Runner Express, which describe where the moisture originates. A Pineapple Express that came across the Pacific from Hawaii in late October caused devastating flooding and multiple deaths in the Southwest's Four Corners region, NPR reported. Atmospheric rivers are not only a West Coast phenomenon. Some larger nor'easters on the East Coast can pack a "nasty" atmospheric river, Ralph said.
They don't simply follow El Niño
Ralph once theorized that El Niño's warm waters would make atmospheric rivers stronger. His research, dating to 2015, showed the opposite. "[Atmospheric rivers] don't dance to the tune of El Niño," he said. Climate scientists also say there is not enough data to connect the weather pattern itself to human-caused global warming, although a warmer world does mean more water vapor in the atmosphere.
"We've got to keep our eyes" on atmospheric rivers, Ralph said, "whether it's an El Niño year or a La Niña year or a No Niño year." For California, Oregon and Washington, the practical warning is simpler: several storms in a row matter more than any single wet day, and the ground is already soaked.