Heavy Snow Likely For Willamette Valley Sunday Night/Monday, Possible for Portland

After so many busted snow forecasts this month, I’ll admit that I’m a little gun-shy writing this post. But now that all global models show a snowstorm affecting at least some part of the Willamette Valley Sunday night into Monday, I can remain silent no longer. It now looks likely that heavy snow will fall somewhere between Longview and the Central or even Southern Willamette Valley, though there is still a ton of uncertainty in the exact location and amount of the heaviest snowfall. Allow me to explain.

Let’s start our journey by looking at eye-candy from the GOES-17 satellite, which finally became operational on February 12. These shots are simply unbelievable and they’ve taken the atmospheric science community by storm (no pun intended, I swear!!!). The old GOES-15 became operational on 12/6/2011, and while it is very impressive in its own right, it’s stunning how much of an improvement the GOES-17 is. I’m hoping both the GOES-16 (which covers the Eastern US/South America/Atlantic Ocean) and GOES-17 will spark an interest in meteorology and environmentalism, much like the Hubble Space Telescope got people fascinated about the universe and science in general.

The image below shows the amount of water vapor in the lowest part of the atmosphere. I love water vapor images because they help you see the movement of separate airmasses in the atmosphere. This is particularly useful when tracking extratropical storms, as extratropical systems derive their energy from the boundaries along different airmasses (fronts).

See that swirl of red and orange several thousand miles west of San Francisco? That swirl marks the center of an extratropical cyclone, and as you can see, it is drawing a ton of moisture northeast from Hawaii into the NE Pacific. This stream of moisture is known as an atmospheric river and is visible in the animated .gif below, which shows the total precipitable water (TPW) in the atmosphere. TPW is the depth of water you would have if all the water vapor in a given column of the atmosphere was condensed into liquid water.

Finally, here’s some visible satellite imagery over the same domain. Visible wavelengths are the wavelengths we see with our naked eye, so visible imagery on satellite corresponds to what we would see if we were in space. Colorized visible imagery is one of my favorite GOES-17 features; the GOES-15 only had black-and-white visible imagery.

To summarize, there’s a nearly stationary upper-level low directing subtropical moisture into the NE Pacific. But in order to get low-elevation snow, you need cold air, particularly if it’s late February and there’s subtropical moisture involved. And thankfully, it looks like we’ll have that as well.

The Forecast! 

Saturday

In the discussion above, I talked about an upper-level low several thousand miles west of San Francisco directing moisture NE from Hawaii into the NE Pacific. But we actually have a similar feature right over our area right now: an upper-level low off the coast of Vancouver Island ushering strong onshore flow into Northern California and Oregon. The TPW model chart does a good job showing how this upper-level low directs moisture onshore.

This upper-level trough and strong onshore flow/low-level jet stream are clearly visible on water vapor satellite imagery. The mountain waves in the lee of the Southern Oregon Cascades and over the Great Basin are indicative of strong, mild onshore flow.

This onshore flow is also spreading increasing precipitation across Western Oregon, with the heaviest precipitation in the Cascades. Note the dramatic decrease in precipitation east of the Cascade Crest due to strong subsidence there, as well as the heavy and much more slowly-moving showers off the Washington Coast.

This precipitation will increase through the rest of the day, and by late tonight, a “firehose” of moisture should be pointed directly at the Willamette Valley.

The Cascades will get pounded by snow with all of this, but everything will fall as rain in the lowlands tonight.

Sunday

This band of precipitation/stationary front will shift south and remain over the southern Willamette Valley for the majority of the day while another “wave” of low pressure forms offshore along this stationary front. This wave will tap into the subtropical moisture I mentioned above and strengthen accordingly, further increasing precipitation rates and taking a slight turn to the north. As this wave approaches the coast and turns north, it will increase offshore gradients and will direct strong northerlies from British Columbia into the Columbia Basin. Because cold air is dense, these northerlies will further increase high pressure over the Columbia Basin (and offshore pressure gradients), and strong easterlies will funnel through the Gorge and into the Portland metro area Sunday night as this developing wave makes landfall and cross-Gorge pressure gradients increase. Precipitation is expected to spread northward back into the northern Willamette Valley and potentially the Portland metro area very late Sunday night into Monday, and with cold, dry easterlies coming through the Columbia River Gorge, precipitation will be in the form of snow.

Though Portland will get the extra “kick” of cold temperatures due to strong Gorge outflow, most inland areas that are more than 50 miles to the north of the center of the stationary front/wave of low pressure should be cold enough to see snow. As the image below shows, this is an exceptionally strong front with a very dramatic wind shift and temperature gradient. Such strong fronts are very uncommon in the Pacific Northwest due to the moderating effects of the Pacific, but so are storms that draw air from both Interior British Columbia and the Hawaiian Islands.

Monday

This wave of low pressure should continue to spread moisture northward throughout the AM hours before tracking into SE Washington/NE Oregon in the afternoon and leaving us with a few post-frontal showers for the afternoon and evening. However, snow could last the whole day if this wave tracks further north than modeled, or it could end quickly or even be nonexistent (gasp!) if the wave is further south than modeled.

Risks to the Forecast:

The main determinant of which regions will see snow – and how much snow they will get – is the exact location of this wave of low pressure. The most recent WRF-GFS had this wave tracking near Cape Blanco and only gave Portland a trace of snow while the Central/Southern Willamette Valley got over a foot. Meanwhile, the European and Canadian models had this wave tracking towards the Central Coast and gave Portland several inches and a foot, respectively. Unfortunately for Portland snow-lovers, recent model runs have taken this low further and further south, increasing the potential that Portland may not see any snow at all. However, there is still plenty of time for this to change, so stay tuned.

Finally, the moment you’ve all been waiting for: my snow forecast.

Portland Metro: trace-4 inches
Central Valley: 2-10 inches
South Valley: 0-6 inches (likely rain Eugene south).
Western Gorge: 3-16 inches, higher east
Coast Range: 4-12 inches south of Garibaldi, trace-4 inches North, highest south.
Cascades: several feet, highest Central/Southern Oregon Cascades.

Thanks for reading! We’ll know a lot more when we can see this wave developing on satellite, so I’ll keep you posted with updates on my Facebook page, and I’ll definitely try to get a blog out tomorrow as well.

Charlie