A category 4 atmospheric river is forecast to drench the Pacific Northwest early this week

On the atmospheric river scale a category 4 is characterized by “mostly hazardous” but also “beneficial” in terms of the amount of rain expected, according to the Center for Western Water and Weather Extremes (CW3E).

Atmospheric rivers can be both good and bad. On the good side, they are the main contributor of the West Coast water supply, however they are also the source for the region’s most impactful flooding events.

And this week is no exception in terms of heavy rain and flooding across Washington and Oregon.

“Starting late Sunday night a strong atmospheric river will spread into western Washington and far northwest Oregon,” the National Weather Service (NWS) office in Portland, Oregon, said.

The heaviest rain is expected on Monday through Tuesday morning, with 24-hour rainfall totals of 3 to 6 inches forecast along portions of the Washington and Oregon coasts. Isolated higher rainfall amounts are possible across portions of the Cascades during this same time span.

This heavy rain event is coming off of a relatively dry start to February. As of Sunday morning the Seattle-Tacoma airport had only received just over 1 inch of rain this month, which is more than a 2.5 inch deficit for the month of February.

While the mountains will first experience snow, temperatures will slowly rise through the higher elevations, leading to a changeover to rain. As this transition occurs rain will lead to additional snow melt which could cause flash flooding and river flooding over the next few days.

“With this much rain and rising snow levels, area rivers will rise, with some forecast to flood,” the NWS office in Seattle said. “The current forecast suggests potentially most area rivers off the Cascades reach minor flood stage Monday night into Tuesday, with moderate river flooding possible on the Skokomish, Snoqualmie, Nisqually and Cowlitz.”

As this system pushes onshore, it will bring gusty winds to the coastal communities and beaches, where gusts of 40 to 55 mph are forecast.

Also along the coast, sneaker waves are possible on Sunday, which means waves can travel much farther up the beach than normal, including over rocks and jetties.

“Sneaker waves can suddenly knock people off of their feet and quickly pull them into the ocean which may lead to serious injury or drowning,” the NWS office in Portland said.

Due to the high tides along the coast pushing inland and the swollen rivers bringing excess runoff, minor flooding is also possible along the coast Monday and Tuesday.

By Tuesday night, the Pacific Northwest could see a brief break in the heavy rain before another round of rain returns to the region on Wednesday.

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