Several storms will blow into the Northwest over the next few days, thanks to multiple weather rivers that are expected to dump heavy rain and feet of snow across the region.
“A line of storms will come ashore in the Northwest late this week,” AccuWeather Senior Meteorologist Heather Zehr said. “Additional storms will continue into next week.”
The National Weather Service said coastal highlands and lowlands in Washington and Oregon will see moderate to locally heavy rain, with heavier rain possible Friday and then into the weekend.
“Multiple weather rivers are looking to impact our area this weekend and into next week with the potential for heavy rain and breezy conditions,” the Seattle Weather Service said.
“At least a few feet of heavy snow is likely across the Cascades, including several mountain passes. Travel will be difficult and dangerous due to heavy snow and blowing snow,” the weather service said.
What is an atmospheric river?
AccuWeather defines an atmospheric river as “a phenomenon in which the flow of moisture from the Pacific Ocean resembles a firehose that delivers near-constant rain and snow at higher elevations to the western United States or British Columbia.”
These bands of water vapor, exposed by clouds, extend thousands of miles from the tropics into the western United States, are 250 to 375 miles wide, and provide fuel for heavy rains and snowstorms that can cause flooding along the West Coast.
In general, atmospheric rivers capture water vapor from warm, moist air in tropical regions and then drop the water to the ground in cold regions as rain or snow.
A 2017 study said these “rivers in the sky” are responsible for up to 65% of heavy rain and snow in the western United States. On average, up to 50% of annual precipitation on the West Coast occurs in a few weather-river events.
Speaking about the storms coming this week, AccuWeather Senior Meteorologist Mike Doll said, “This special atmospheric river formation extends more than 7,000 miles from 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,” Zahr said. “Over the weekend, snow levels will drop below 2,000 feet at times, resulting in 2-3 feet of accumulation and significant travel impacts in the Cascade Passes.”
Moisture will also spread inland across the Great Basin and northern Rocky Mountains, with a mix of rain and snow at lower elevations and moderate to heavy snow in regional mountain ranges, the weather service said.
“Heavy snow is likely to reach several mountain ranges from Idaho and Montana to Colorado,” the weather service said. “The highest peaks are expected to see up to 1 to 3 feet of snow.”
Heavy rain can lead to flooding
“Total rainfall amounts through Wednesday from these systems from several inches over the lowlands to over 10 inches or more in the Cascades and Olympics will lead to rapid rises in rivers, especially as the snow melts as snow levels rise Sunday into the first half of the year.” Next. “A week,” the weather service in Seattle said.
Several inches of rain will fall on the Seattle and Portland metro areas over the next few days. A flood watch was in effect in both metro areas.
“Flooding will be possible on more vulnerable rivers such as the Skokomish River in Mason County, but may also include other rivers in the Olympic Basin and Cascade Basin,” the weather service said.
It's not all bad news: Storms can alleviate drought
“The coming moisture, while it brings many threats, is not all bad news — it will help alleviate drought conditions in the region,” said Bill Deger, AccuWeather's chief meteorologist. According to the latest US Drought Monitor, most areas of western Washington and western Oregon are experiencing moderate to severe drought conditions.