Rivers in the Sky: Back-to-Back Atmospheric Rivers Bring Heavy Rain to the US Pacific Northwest

It is the wettest time of the year for the US Pacific Northwest. This week, heavy rain, high tides and a rising sea level are hitting the West Coast.

Two atmospheric rivers will pound parts of Oregon and Washington state back-to-back on Friday and Saturday, bringing heavy rain. Two other atmospheric rivers already dropped multiple inches of rain on both states earlier this week.

Josh Weiss, a meteorologist with the National Oceanic and Atmospheric Administration's Weather Prediction Center, said the problem grows when impacts from multiple atmospheric rivers compound. Those impacts include flash flooding, urban flooding and even landslides, he said.

According to Weiss, this year's atmospheric river season, which runs from October to March, is off to a fast start.

An atmospheric river is a narrow band of water vapour pushed by the wind. Loaded with moisture, it dumps rain or snow.

F. Martin Ralph, the director of the Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography, said these rivers can be up to 2,000 miles long and 600 miles wide.

Ralph, one of the leading researchers on atmospheric rivers, said, 'It's literally a river in the sky.' According to him, these rivers are a natural climate pattern.

Weiss said about 50 to 60 percent of rain in California comes from atmospheric rivers.

Residents in the Pacific Northwest are used to heavy downpours. But the atmospheric rivers are hitting as El Niño's warm ocean waters contribute to rising sea levels and fuel storm systems. At the same time, the annual high tides known as King tides are coinciding. That leaves coastal and inland waterways along the West Coast at risk of flooding.

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 US is from atmospheric rivers.

Jeff Michalski, a meteorologist at the National Weather Service in Seattle, said Washington's coast could be hit by significant 20-foot swells this weekend.

People also know atmospheric rivers by nicknames such as Pineapple Express or Rum Runner Express. These names describe where the atmospheric river originates. A Pineapple Express that came across the Pacific Ocean from Hawaii in late October caused devastating flooding and multiple deaths in the Southwest's Four Corners region.