'Atmospheric river' forecast to blast Pacific Northwest with heavy rain and feet of snow

Several storms will blast the Northwest over the next few days, courtesy of multiple atmospheric rivers that are forecast to dump heavy rain and feet of snow across the region.

"A series of storms will come ashore in the Northwest late this week," said AccuWeather senior meteorologist Heather Zehr. "Additional storms will carry on even into next week."

Coastal and lower elevations of Washington and Oregon will see moderate to locally heavy rain, the National Weather Service said, with the heavier rainfall more likely Friday and then into the weekend.

The weather service in Seattle said "multiple atmospheric rivers look to impact our region this weekend and into next week with possible heavy rain and breezy conditions."

"Heavy snow likely adding up to at least a few feet is expected across the Cascades, including many mountain passes. Travel will be difficult to hazardous due to both heavy snow and blowing snow," the weather service said.

What is an atmospheric river?

AccuWeather defines an atmospheric river as a "phenomenon where a flow of moisture from the Pacific Ocean resembles a fire hose that delivers near-constant rain and higher-elevation snow to the western United States or British Columbia."

Made visible by clouds, these ribbons of water vapor extend thousands of miles from the tropics to the western U.S. At 250 to 375 miles wide, they provide the fuel for massive rain and snowstorms that can cause flooding along the West Coast.

In general, atmospheric rivers pick up water vapor from the warm, moist air of tropical regions and then drop the water over land in cooler regions as rain or snow.

These "rivers in the sky" are responsible for up to 65% of the western USA's extreme rain and snow events, a 2017 study said. And on average, up to 50% of the annual precipitation on the West Coast occurs in just a few atmospheric river events.

Speaking about this week's incoming storms, AccuWeather senior meteorologist Mike Doll said "this particular atmospheric river setup extends for more than 7,000 miles from all the way across the Pacific Ocean to the northern part of the Indian Ocean basin."

'Rivers in the sky': What exactly is an atmospheric river?

Feet of snow possible

"These storms will be accompanied by cold air at first," Zehr said. "Through the weekend, snow levels will be down below 2,000 feet at times, leading to accumulations of 2-3 feet and major travel impacts in the Cascade passes."

Moisture will also spread inland through the Great Basin and Northern Rockies, with a rain/snow mix for lower elevations and moderate to heavy snow in the regional mountain ranges, the weather service said.

"Heavy snow is likely to reach into several mountain ranges from Idaho and Montana to Colorado," the weather service said. "The highest peaks are forecast see as much as 1 to 3 feet of snow."

Heavy rain could lead to flooding

"Total rainfall amounts through Wednesday from these systems of several inches over the lowlands to upwards of 10 inches or more in the Cascades and Olympics will lead to rapid rises on rivers, especially combined with melting as snow levels rise Sunday into the first half of next week," the weather service in Seattle said.

Several inches of rain will fall on both the Seattle and Portland metro areas throughout the next few days. A flood watch was in effect for both metro areas.

"River flooding will be possible on more susceptible rivers such as the Skokomish River in Mason County, but may also include other rivers in the Olympic and Cascade basins," the weather service said.

Not all bad news: Storms could ease drought

"The incoming moisture, while carrying several threats, is not all bad news – it will help to relieve drought conditions in the region," said Bill Deger, AccuWeather senior meteorologist. According to the latest U.S. Drought Monitor, much of western Washington and western Oregon are enduring moderate to severe drought conditions.

This article originally appeared on USA TODAY: 'Atmospheric river' to blast Northwest with heavy rain and feet of snow