Atmospheric River Set to Strike US: Heavy Rain and Snow Forecast

Forecasts indicate that an atmospheric river will bring heavy rain and snowfall to much of the Pacific Northwest starting late this week and lasting through the weekend.

With persistent heavy rainfall, flash flooding could become a concern for residents across the region.

Why It Matters

While atmospheric rivers vary in size and intensity, those carrying the most water vapor and strongest winds can produce extreme rainfall and flooding, especially when they stall over flood-prone watersheds, according to the National Oceanic and Atmospheric Administration (NOAA).

These events can also lead to travel disruptions, trigger mudslides and potentially cause damage to life and property.

What To Know

A cold front could bring rain along the Pacific Northwest coast and snow in the mountains from Friday into Saturday, according to the Weather Channel.

Behind this front, rain may transition to or mix with snow in Portland and Seattle from Saturday night into Sunday.

The highest rain totals were anticipated in Northern California, the channel said.

By Friday morning, the core of the moisture plume is expected to focus near the Oregon-California border, the San Francisco Chronicle said. Steady rain will spread along Northern California’s coast, including areas like Crescent City and Eureka, where rainfall totals could surpass an inch by Friday evening.

The NOAA says atmospheric rivers are long, narrow bands in the atmosphere that transport large amounts of water vapor outside the tropics. Their size and strength can vary significantly, but on average they carry a volume of water vapor comparable to the flow of the Mississippi River at its mouth.

Exceptionally strong atmospheric rivers can transport up to 15 times that amount. When they make landfall, they typically release this moisture as rain or snow.

What People Are Saying

Weather Channel meteorologist Jim Cantore said on X (formerly Twitter): “Pretty significant AR (atmospheric river) for the west starts Friday night and lasts through early Tuesday for northern CA and southern OR.”

The National Weather Service (NWS) office in Boise, Idaho, said on X: “Snow and rain coming as an atmospheric river brings LOTS of moisture. As the system approaches we will continue to look at the effects of terrain, how quickly we mix out mtn valley cold pools, and where the AR sets up exactly. Exact amounts will change, but expect wetness.”

The NWS office in Pocatello, Idaho, said on X: “Quiet/dry weather will continue through Thursday, but changes are on the way starting Friday. Following an extended dry stretch, increasing confidence supports atmospheric river moisture moving in Friday through the weekend, bringing widespread rain, snow, and gusty winds.”