Western WA weather: First atmospheric river of the season on its way

Mother Nature is shaking up an atmospheric river with heavy rainfall and breezy winds for this weekend’s meteorological cocktail.

The atmospheric river had already churned in its early morning alchemy Friday, dragging in high concentrations of tropical moisture to British Columbia and Western Washington.

This band of moisture blasted our neighbors up north Friday before it is expected to sag over the weekend, “bringing significant rainfall to the region,” the National Weather Service said.

The Olympic Peninsula and North Cascades will get a good dousing of rain through Sunday night, with the heaviest amounts up to 6 inches.

But for the Seattle area, Mother Nature will serve up our soaking a little neater.

The system will deliver on-and-off showers over the next several days, strengthening to the heaviest on Sunday, to deliver up to an inch of rainfall around Seattle, the weather service said.

Breezy winds will also rush into the region through Saturday morning, gusting up to 50 mph over water, the northern coast and lowlands north of Everett. Elsewhere, gusts will reach up to 30 mph, the weather service said.

The deep moisture advection from over the Pacific will also drag in warmer temperatures. Weekend highs are expected to peak in the mid-60s, keeping the lowlands balmy and the snow elevations in the mountains above 5,000 feet.

While the atmospheric river visits the region for the first time this fall, its terrestrial counterparts are expected to rise but not flood. The weather service advises that water will likely collect on roadways in areas with heavier rainfall, too.

The rain train will run out of steam by Monday, and the region is expected to dry out by Tuesday. The skies will follow soon after, clearing out to deliver sunshine once again by the middle of next week.

What is an atmospheric river?

An atmospheric river — a term coined in the 1990s — is a long, narrow stream in the atmosphere that transports moist air outside of the tropics, according to the National Oceanic and Atmospheric Administration.

Atmospheric rivers
Atmospheric rivers are long, flowing columns of condensed water vapor. Like a conveyor belt, they carry vapor for thousands of miles from out over the ocean. When an atmospheric river hits the West Coast, it can generate a series of storms, with each storm producing inches of rain or feet of snow.

Sources: NOAA, scientificamerican.com (Mark Nowlin / The Seattle Times)

You can think of it like a river in the sky.

Atmospheric rivers function much like a river on land, but they can carry even more water than the Mississippi, the NOAA said.

They form near the equator, where warm temperatures cause water to evaporate and rise into the atmosphere. Some of the water vapor is pulled away from the equator, forming a narrow band that transports the moisture to other regions like a conveyor belt.

Once the atmospheric river reaches land and the moisture-laden air lifts over terrain, it cools and condenses the water vapor, like wringing out an atmospheric sponge. It can produce heavy rain and snowfall, depending on the elevation of the land.