Atmospheric rivers eye Pacific Northwest and British Columbia, heavy rain expected

A frontal system will begin to approach the Pacific Northwest by Wednesday evening, October 18, 2017, with precipitation increasing throughout the day and into tonight. The same system will equally affect Canada's British Columbia. Heavy rain will continue falling through the rest of the week.

"A fascinating weather pattern is in place in the Northern Hemisphere this week. It’s a river in the sky that spans the full width of the Pacific Ocean from Asia to North America. The phenomenon is remarkably obvious — a plume of clouds with a few areas of low pressure embedded, transported at shocking speeds across the planet’s largest ocean via the jet stream," Capital Weather Gang's Jason Samenow said.

"Plainly spoken, it’s a train of rain on its way to Washington State," Samenow said. "Over the coming days, this so-called ‘atmospheric river’ will unload tremendous amounts of rain and snow over the Pacific Northwest."

"This event looks like a pair of significant atmospheric rivers over 4 to 5 days," F. Martin Ralph, an expert on atmospheric rivers at Scripps Institution of Oceanography, told Samenow in an interview. "The details of where they make landfall and how long they last will determine where the heaviest precipitation and impacts will occur."

By Thursday, October 19, the strong cold front will begin pushing eastward over the Pacific Northwest and toward northern California with precipitation spreading farther east as a result. 

Once the front moves into the Central Great Basin, rainfall will also follow suit; however, due to onshore flow, showers and higher elevation snow will continue across the Pacific Northwest.

There is a marginal risk for flash flooding in western Washington and northwestern Oregon on Thursday as a result of the heavy rainfall, the National Weather Service warns.

On Thursday, snowfall will expand throughout the Washington and Oregon Cascades and parts of the northern Rockies – with some light snowfall in higher elevations of northern California. By Friday, October 20, snow will continue across the Cascades and throughout the northern Rockies.

This system will affect both the northwestern United States and British Columbia.

"Into the weekend, a new storm with resurgence of rain and strong winds will arrive on the coasts of British Columbia and the Pacific Northwest every 24 hours," AccuWeather Senior Meteorologist Jack Boston said.

"Each storm looks to be stronger as the week progresses," he said.

The news is good for Northern California which will benefit from rainfall this week that will aid firefighters given the recent large wildfires.

Featured image credit: NOAA/GOES-16. Acquired: October 18, 2017