Biblical Rains Thursday Through Saturday

Meteorologically speaking, I can’t remember ever seeing a more boring November & first half of December in my life. I was looking through some climate stats, and the last time Portland saw at least a half inch of rain was on September 17, when 1.01 inches of liquid sunshine fell at the airport. The last time we saw a gust greater than 45 mph was on February 26, and to add insult to injury, I was out of town for the dusting of snow we saw on Thanksgiving! Obviously I’ve cherry-picked these stats, but you get the idea. This has been a pretty underwhelming winter so far.

But starting tonight and continuing through Saturday morning, Mother Nature will make up for our humdrum winter by inundating us with heavy mountain snow, strong winds (especially for the Coast), and extraordinarily heavy rain for Western Washington and NW Oregon. All of this weather will be courtesy of a slow-moving, strong atmospheric river that will direct deep, subtropical moisture right into NW Oregon and SW Washington.

The foreboding storm first showed its teeth Tuesday morning, when the denizens of NW Oregon and SW Washington woke up to a truly spectacular sunrise. I instantly recalled one of my all-time favorite weather quotes. “Red sky at night, sailors’ delight. Red sky at morning, sailors take warning.”

This idiom first arose in the New Testament (Matthew 16:2b–3) and is one of the most accurate “weather sayings” out there. A red sky in the morning means that there is clear weather to the east but increasing clouds to the west, and because the prevailing wind in the mid-latitudes is from the west, it means that thicker clouds – and potentially rain – are headed our way. And sure enough, it’s raining outside as I type this blog.

But tonight’s rain is just a prequel to the Biblical amounts expected tomorrow afternoon and Friday as an extraordinarily wet atmospheric river stalls right over NW Oregon and SW Washington. The new GOES-17 satellite provided an amazing image earlier today of this atmospheric river and the deep Gulf of Alaska low driving it.

Atmospheric rivers are long, narrow regions in the atmosphere that transport moisture from the subtropics to the mid-latitudes (see this post for more detail). The image below shows the amount of “precipitable water”  – i.e. the amount of liquid water you would have if all the water vapor in a column of the atmosphere was condensed to liquid – at 4pm Pacific Time on Thursday. As you can see, NW Oregon and SW Washington are in the bullseye of this atmospheric river.

Forecast:

We’ll see periods of light rain tonight as a moderate front ahead of this atmospheric river moves through the area, but the real heavy stuff will arrive mid-morning tomorrow as the leading edge of this atmospheric river moves onshore.

Rain should persist through late afternoon tomorrow before lifting north into Western Washington by the evening.

Note the extraordinarily heavy precipitation off the Oregon Coast – this shows the exact location of the cold front associated with this atmospheric river and is due to convergence and uplift along this front. You can clearly see the location of the front in the wind speed forecast below – the front separates the gale force winds from much weaker winds offshore and is right over Astoria at this time. Whoever spends a lot of time under this intense rainband will get a ton of precipitation from this event.

By Friday morning, the heaviest rain shifts to NW Oregon.

But by Friday evening, the river moves back north over Washington. It also begins to develop a more north-south orientation in response to an intensifying surface low-pressure system along the atmospheric river. Features like these are often referred to as “waves” riding along the atmospheric river.

The UW WRF model below shows this wave bringing sustained winds of 45 knots to the South/Central Oregon Coast. I’m excited to see how high the winds will be at Cape Blanco, an exposed headland between Coos Bay and Gold Beach that is the western-most point of Oregon. They could easily see 80 mph gusts at late Fri/early Sat.

The atmospheric river slowly traverses the region Saturday, and we should finally dry out by early Sunday.

By 4am Sunday, storm totals could reach 2-6 inches for the Western Washington and Oregon lowlands, with the highest amounts in SW Washington. Much more precipitation will fall in the mountains, with 4-8 inches likely in most spots and isolated 10+ inch amounts near Mt. Rainier and Mt. St. Helens.

Impacts: 

Rain/Flooding:

With 2-6 inches throughout the Western Washington and NW Oregon lowlands and even higher amounts in the mountains, some rivers are likely to flood. However, any flooding should be minor, as river levels are much lower-than-average for this time of the year and snow levels are expected to remain relatively low for an atmospheric river event, peaking at 6,000 ft for NW OR and 4,000 feet for the Central WA Cascades, highest Thursday night and Friday. The biggest threat of flooding will be on uncontrolled (dam-free) rivers/streams; most reservoirs are below-average and will be able to capture a lot of the rain that falls.

Mountain Snow/Avalanches:

This series of storms will being a TON of snow to the Central/Northern WA Cascades above 3500-4500 feet and the Southern WA Cascades above 5000-6000 feet. We’re talking 2-5 FEET for those spots. Timberline may only see 15-30 inches, but I’m sure they aren’t complaining (even if they may be a bit jealous). The forecast for the summit of Mt. Rainier is comical – the NWS is calling for 132-170 inches of snow there by Sunday. This is overdone, but I think Paradise Ranger Station should easily pick up 4-5 feet of snow during that time. Avalanche Danger will also be high tomorrow and Friday due to the heavy/wet snow and changeover to rain, especially for the Southern WA Cascades and Mt. Hood area.

Wind and Waves:

The Oregon Coast in particular should see two very windy periods with waves riding along the front: one Thursday night/Friday morning, and another Friday night/Saturday morning. These winds should gust to 60-70 mph on beaches and headlands, and I think the strongest winds will occur early Saturday morning along the Southern Oregon Coast. The Willamette Valley and Western Washington lowlands should see 15-25 mph sustained winds with gusts to 40 Thursday afternoon through Friday morning, but winds will be much lighter later Friday. Waves could reach 20-25 feet on the coast, so don’t go fishing (it sucks this time of the year anyway).

I’ll end with a fun fact: As of 12/17, Portland has only seen 4.81 inches of rain since the beginning of our “water year,” which is October 1. If this series of storms ends up being wetter-than-modeled, the Wednesday-Saturday period could be wetter than the 2 1/2 months combined!

Thanks for reading, and stay dry!
Charlie

Featured image retrieved from pixels.com