Atmospheric river to wallop Pacific Northwest
- The heavy rain will create areas of flash flooding, with urban areas, roads and small streams the most vulnerable.
- "There is also potential for debris flows over and near the burned areas in the Cascades."
- Atmospheric rivers extend thousands of miles from the tropics to the western U.S.
Another round of heavy, potentially flooding rain is forecast across portions of the Pacific Northwest on Thursday and into Friday thanks to an atmospheric river that's forecast to hit the region.
The heavy rain will create areas of flash flooding, and urban areas, roads and small streams will be the most vulnerable, primarily in Washington and Oregon, the National Weather Service said.
The storm could drop nearly a month’s worth of rain in some areas.
AccuWeather chief broadcast meteorologist Bernie Rayno said the moisture from the incoming storm could be traced all the way to the tropical Pacific.
"This is going to be a potentially very wet system and a very warm system," Rayno said. Widespread rainfall totals of 1-3 inches are expected across the low-lying valleys of the Pacific Northwest from this storm, AccuWeather said. At higher elevations, rainfall amounts could ramp up significantly, perhaps to over a half of a foot along the western-facing slopes.
The firehose of moisture is expected to wobble northward into Washington through the day on Thursday, potentially expanding the flooding threat into places like Olympia, Tacoma and Seattle, AccuWeather said.
"Those traveling during the event should expect to encounter heavy rain, which will reduce visibility as well as ponding on roads," the National Weather Service in Seattle said. "If you are going out, give yourself plenty of time and give other drivers space."
Because of the already saturated soils this added rain will put western Washington under the threat of landslides, the weather service warned. The weather service in Portland, Oregon, said "there is also potential for debris flows over and near the burned areas in the Cascades."
Debris flows – torrents carrying massive boulders, soil, trees and other objects – are considered more dangerous than mudslides or landslides.
Made visible by clouds, the ribbons of water vapor known as atmospheric rivers extend thousands of miles from the tropics to the western U.S. They provide the fuel for the massive rain and snowstorms and subsequent floods along the U.S. West Coast.
Though beneficial for water supplies, these events can wreak havoc on travel, bring deadly mudslides and cause catastrophic damage to life and property, the National Oceanic and Atmospheric Administration said.
One well-known nickname for an atmospheric river is the "Pineapple Express," which occurs when the source of the moisture is near Hawaii.