Heavy Rain, Flooding This Weekend As Unusual Atmospheric River Hits Western Wash. | KNKX

Heavy Rain, Flooding This Weekend As Unusual Atmospheric River Hits Western Wash.

Feb 2, 2018

Unusually heavy rain and flooding are in the weekend forecast, with the worst hitting Sunday as an atypical atmospheric river sets up from due west and aims its warm plume at western Washington.

KNKX weather expert Cliff Mass says that phenomenon means the rain total by the end of this weekend could be 5-10 inches over the lower slopes of the central Cascade Mountains.

“It’s going to be a wet weekend, particularly on the west slopes of the Cascades,” Mass said.

Rainy Start To The Weekend

On Friday, Mass says it will be rainy as warm, moist air comes through with an upper level trough of atmospheric pressure. He predicts an inch or two of precipitation on the western side of the Cascades.

But at the same time, it will be extremely dry on the eastern slopes of the Cascades. Mass says this is due to the winds coming in from the coast, hitting the mountains. “That’s what we expect when we have westerly flow coming in,” Mass said.

Another disturbance will come through on Friday night into Saturday morning, Mass says, bringing more rain.  

“So it’ll be rainy at least in the start of Saturday,” Mass said. “I think there will be a break later on Saturday afternoon, so I think you’ll be able to get out a bit. Except if you’re on the western slopes of the Cascades, where it will remain wet.”

Flooding Sunday

Sunday is when the big action will happen, Mass says, as the atmospheric river hits from due west.

“A slug of moisture and warmth will come right at the Cascades from the west. And some locations are going to get 2-5 inches on the western slopes,” Mass said. “There’s going to be so much rain that I think that the flooding is inevitable.”

The National Weather Service’s River Forecast Center in Portland is predicting moderate flooding on some of the rivers that drain off the western slopes of the Cascades, from Sunday through Tuesday. These include the Snoqualmie and Skagit, among others.  

“If you look at the total for the whole weekend, we’re talking about some places are going to get 5-10 inches of rain on the western slopes of the Cascades,” Mass said.

Mass says the interplay of local geography with the unusual atmospheric river coming in from the west is what causes such a big downpour. 

“It really optimizes the effects of the mountains,” Mass says.

If the rain gets you down, hang in there. Mass says it will taper off as a series of minor disturbances come through Monday and Tuesday, and then it should yield to some clear skies.

“The ridge of high pressure really builds in as we get into Thursday-Friday. And I expect to have sun,” he said. “It’s going to dry out completely and sun as we get into later in the week.”

Should You Trust The Groundhog Or The Ground Frog?

It’s Groundhog Day, and Punxsutawney Phil saw his shadow when he came out of his burrow in central Pennsylvania. Legend has it this means six more weeks of winter. Cliff Mass was skeptical.

“I don’t know if I would follow a groundhog for my weather predictions,” he said, though he didn’t discount it entirely.

“If it’s clear and it’s sunny there, that generally means high pressure, which means cold conditions, so I can see somewhat of a physical consistency with that,” he said

But Mass said he prefers a more local source.

“Around here, we have a ‘Ground Frog,’” Mass said, referring to Snohomish Slew, which the City of Snohomish celebrates with an annual parade in late January.  

“And it foresaw a warm and wet period ahead. So I think our frog may be on to something,” Mass said.

Temperatures on this wet weekend are expected to be relatively warm, with lows in the mid- to upper-40s and highs in the low to mid-50s.

To hear the full conversation – including a discussion of what makes this weekend's atmospheric river so unusual – you can click on the 'play' icon at the top of this post. 

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.