“Several rounds of moderate to heavy rainfall into this weekend,” wrote the National Weather Service office in Seattle. “Rivers are expected to rise, with some rivers reaching flood stage as early as this [Friday] evening. Snow levels will be around 6,000 to 8,000 feet through tonight. Heavy rainfall may also cause periods of minor urban flooding. The exact placement of heaviest rainfall this afternoon into tonight remains uncertain and will have a direct influence on the magnitude and placement of flooding.”

The atmospheric river affecting the Northwest coast is a stout one, rated 4 out of 5 for its intensity, with its moisture originating as far away as Hawaii. Atmospheric rivers are concentrated strips of exceptionally humid air that deviate from the tropics northward. Where this tongue of moisture laps at a landmass, heavy rain and flooding are possible.

It’s been a waterlogged month for Seattle, which has had rain every day in January except New Year’s Day. The city will likely flip the page on a January with over 9 inches of rain (its average January rainfall is 5.57 inches).

Atmospheric rivers store their moisture in the lowest 10,000 feet of the atmosphere. But most higher-end rainstorms have clouds that tower well above that. The secret to squeezing out a deluge from an atmospheric river is terrain.

When an atmospheric river surges ashore, its jet of juicy air is forced upward by coastal mountains. In the case of this weekend’s setup, the Cascades will prove culpable.

Air that rises in the atmosphere cools, eventually down to its dew point. When that happens, an air parcel reaches saturation — meaning it can’t hold any more water. Any additional cooling beyond this point requires a release of water, leading to heavy rain as long as the air is moving uphill. In the lee of the mountains, the air can subside, drying up and leaving an often thirsty “rain shadow.”

That’s why the heaviest precipitation will fall primarily along the coast and on the west slope of the mountains. By Saturday, an approaching cold front could flip rain to snow above 1,000-2,000 feet.

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By the time this first heavy precipitation event draws to a close late this weekend into the start of the workweek, up to three feet of snow locally are possible in the mountains. The mountain pass along Interstate 90 could feature some dicey travel.

Mount Rainier, meanwhile, could see up to 80 inches of snow through Sunday along with very strong winds.

Where it’s too warm to snow, up to 8 inches of rain could fall on the “upslope” side of mountains. Seattle could wind up with 2 to 4 inches of rain through Sunday. Rain totals realized will be very specific to local topography.

Strong winds gusting upward of 50 mph are also possible for mountain areas, along with the risk of tree damage and power outages.

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The potency of atmospheric rivers, and the ratings they earn, are a product of their moisture content and speed.

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Remember that hilarious scene from “I Love Lucy,” in which Lucy and Ethel find themselves wrapping candies in a chocolate factory?

How overwhelmed the pair became was proportional to two factors: the density of chocolates per unit area on the belt, and the speed at which the conveyor belt moved. That’s similar to how an atmospheric river works.

The density of the moisture so to speak is called the PWAT. That stands for “precipitable waterer index.” It’s a measure of how much water vapor is stored in a column of atmosphere if every last drop was to rain out. PWAT values will be between 1 and 1.2 inches with this first atmospheric river affecting the Pacific coast, indicating plenty of moisture to work with.

But the speed at which the atmospheric carries moisture in is important, too: The moisture works in and it’s converted to rain, before another batch has arrived. The faster the current in an atmospheric river, the more rapidly the humidity in the air mass is refreshed. That in turn lends itself to more plentiful rainfall.

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The intensity of the atmospheric river blasting the Pacific Northwest through the weekend is rated a 4 out of 5 by the Center for Western Weather and Water Extremes at the Scripps Institute of Oceanography.

By Friday night, the core of the atmospheric river will shift slightly north into the extremities of northern/western Washington and Southwest Canada before drifting south and weakening some throughout the day.

After a brief lull on Monday, yet another atmospheric river will target the same areas late Tuesday into Friday, bringing as much as several more inches of water (and more heavy snow in the mountains) to the region.