River of Rain Threatens to Unleash Pacific Northwest Mudslides

Flooding rains will leave as much as 10 inches (25 centimeters) of rain throughout the Pacific Northwest, sending mud and debris down hillsides burned by wildfires last summer.

The storm, called an “atmospheric river,” will be one of the strongest “in recent memory,” said Rich Otto, a forecaster at the U.S. Weather Prediction Center. Rain could fall at a rate of 0.5 inches to 0.75 inches in Oregon and Washington through Wednesday. Landslides are possible in areas where the deluge meets burns scars, he said.

Atmospheric rivers can flow thousands of miles, carrying enough water to rival the average flow at the mouth of the Mississippi River, according to the National Oceanic and Atmospheric Administration.

Flood warnings and advisories are posted from the coast of Washington and Oregon to parts of northern Idaho, according to the National Weather Service. While atmospheric rivers often bring heavy snows, Otto said this will be mainly a rain event.

A U.S. Army Corps of Engineers study last year found that such events caused 84% of flood damage across 11 western states over a 40-year period. They cost $1.1 billion in damages annually.

©2021 Bloomberg L.P.

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