Flooding Rains To Begin the Week

Finally!

In case you haven’t noticed, the weather thus far this storm season has been pretty boring. We’ve yet to see any windstorms, rainstorms, or snowstorms of significance, either in the mountains or the lowlands.

But that will change tomorrow. A truly massive storm stretching all the way from the Gulf of Alaska to the Eastern Hemisphere will bring flooding rains to the North Cascades and Olympics, and gusty winds along Washington’s Coast and Northern Interior could pose a threat to your internet connection during the Cyber Monday mania. We should even see some freezing rain at many of the Cascade Passes north of White Pass, creating treacherous travel conditions there.

I look at the satellite images over the North Pacific every day, and to be honest, I’ve been downright depressed with the lack of meteorological eye candy over the Northern Pacific thus far this storm season. Well, consider me no longer depressed; the above storm is most impressive-looking cyclones I’ve seen in some time. Just take a look at this thing! A huge moisture plume extends thousands of miles from the “parent” upper-level low in the Gulf of Alaska… simply incredible.

Wanna be really impressed? This morning’s surface analysis from the Ocean Prediction Center showed this low at 957 mb, which is on par with a category 2 or 3 hurricane. But even more impressive is the sheer size of this system, with a trailing cold/stationary front stretching all the way past the International Date Line into the western Tropical Pacific.

This front is a textbook example of an “atmospheric river,” which is a long, narrow, and very robust plume of moisture that extends from the subtropics into the midlatitudes. According to a 1998 paper by Reginald Newell and Yong Zhu, the typical atmospheric river transports approximately 180,000 tons of moisture every second, which is comparable to the flow of 23.5 Columbia Rivers. In fact, atmospheric rivers are responsible for almost all poleward water vapor transport even though they typically cover less than 10% of the Earth’s surface at any given latitude (Newell and Zhu, 1998). Atmospheric rivers are awesome and I could write for hours about them, but thankfully I already did that last year in a post titled “Everything You Need to Know About Atmospheric Rivers,” so if you are thirsting for knowledge about atmospheric rivers, check that blog out.

A Freakishly Strong Jet Stream

I’ve seen some strong jet streams in my days, but I don’t know if I’ve ever seen a jet as strong the one currently over the Pacific. If you believe 11/25’s 00Z and 06Z GFS model runs, this storm was powered by a 230 mph jet stream over the Central Pacific.

By the time this storm reaches our region Monday evening, the jet stream will have weakened to “only” 190 mph over the Pacific, which is still incredibly strong. You can bet that eastward trans-Pacific flights will be taking advantage of these tailwinds.

Forecast: 

Alright, I’ll stop geeking out now and give you the forecast that you all came for. Monday morning will start off dry for the lowlands, but the North Cascades, Olympics, and Vancouver Island/BC Coastal Range will see steady, heavy precipitation as the moist air associated with this atmospheric river encounters terrain and rises.

The front will make very slow eastward progress throughout the day, and by 11 PM, the bulk of precipitation should still be over the Olympics/North Cascades while Puget Sound sees much lighter precipitation and much of the Willamette Valley remains dry.

The front will finally push through early Tuesday morning, resulting in steady precipitation throughout the I5 corridor that should switch to showers after sunrise.

The Olympics and North Cascades could see up to 10 and 7 inches in spots, respectively, over the next 48 hours, with approximately an inch likely for Seattle and a little less than that in Portland. Though this system is expected to cause minor flooding on the Nooksack River in Whatcom County and the Skokomish River in Mason County, it is not expected to be a major, region-wide flood event.

The remainder of the week looks seasonably unsettled, with weak systems on Wednesday and Friday bringing lowland rain and mountain snow to the area. Models diverge for the weekend, but ensembles hint that we’ll be in a cooler and wetter-than-normal pattern as we head into December, helping the Cascades build up their snowpack. Hopefully it won’t be too long before the ski resorts open and I can head up to the slopes!

Thanks for reading all, and have a great week!
Charlie