Sound The Alarms! Regionwide Snowstorm Likely Friday night/Saturday.

Snow is in the forecast once again, and this time, Portland should get more than an inch.

But before I dive into the snow potential this weekend, let me first touch on the surprise snow that Portland saw Monday night/Tuesday morning. I have to admit, I was pretty jealous when I saw the surprise snowmaggeddon that buried Western Washington on Monday. So when I woke up to a slushy inch at 4 am yesterday morning, I was absolutely ecstatic. The walk into work along the Willamette was serene and well worth the (lighthearted) vitriol thrown my way at work for not forecasting this event.

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But while this snow was a nice “consolation prize” for us Rose City folk who missed out on the blizzard that buried Western Washington, Portland too could see a moderate or even major snowstorm on Saturday. And those in Seattle could get a second helping of the white stuff.

Upper-Level Analysis:

Let’s start by looking at the holy grail of weather charts – the 500mb height chart (with overlaid infrared satellite and 500mb temperatures for good measure). This is a good large-scale pattern for colder-than-average weather over the Western US, with a large ridge centered by 150 degrees W and a large trough downstream over the Intermountain West. You can also see a cutoff low near 140 W and a ridge axis near 130 W, both of which are not ideal for lowland snow but will move out of our region by Friday.

Over the next several days, an upper-level trough containing extremely cold arctic air will drop south into the Pacific Northwest. Take a look.

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Here’s an alternate, more colorful view that shows the actual temperatures at the 500mb level, along with the wind speed and height lines.

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Surface Analysis and “BC Sliders”

Now that we’ve gone over what’s happening at the upper-levels, let’s take a look at the surface. And to do this, we’ll look at the University of Washington’s wonderful WRF-GFS model. In the interest of time and my monthly Amazon Web Services S3 storage bill, let’s start the graphics at 4 am Friday, which is when the surface low affiliated with the aforementioned upper-level trough is expected to begin developing off the coast of Vancouver Island. As Friday goes on, this low will strengthen and slide SSE along the Washington Coast, spreading precipitation from north to south in the process.

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This is a classic example of what is called a “BC Slider.” A BC Slider may sound innocuous… heck it sounds like some kind of hipster hors d’oeuvre you’d find in one of the many rapidly gentrifying neighborhoods of Portland. And to be honest, their overall storm structure is downright pitiful compared to the warmer, Pacific storms that barrel into the Northwest throughout November and December. They have a much higher central pressure, move much more slowly, and are far more compact than the stronger, maritime storms that affect us then. On the flip side of the coin are the warm, windy, and very wet storms we call “atmospheric rivers,” which you can read more about here in a dissertation-length blog post I wrote a couple years back.

But for all their shortcomings, BC Sliders can be extremely high-impact events that cripple a region for days. The BC Sliders in December 2008 not only brought Seattle nearly two feet of snow, they brought numerous reinforcing shots of frigid air through the Fraser River Valley into the I-5 corridor, ensuring that the snow stuck around for over a week and that Seattle mayor Greg Nickels would not get elected to a third term. Additionally, even though the Sliders themselves may have weak central pressures, they are often surrounded by extremely high surface pressures over Interior British Columbia and, to a lesser extent, the Columbia Basin, leading to downright ferocious gap winds in NW Washington (from the Fraser River Valley), the western Columbia River Gorge, and occasionally the Cascade foothills.

Forecast Timeline:

Finally – the part you’ve all been waiting for! There’s still a bit of uncertainty in the details (particularly with regards to snowfall amounts), but we now have a general idea of what is going to happen. I’ll do this forecast in a “timeline” format.

Friday morning: This BC Slider will begin to strengthen off the coast of Vancouver Island and will spread some light precipitation into Northern Washington (both west and east of the Cascades). With temperatures well-below freezing, this precipitation will fall as snow, but rates will be light and steady, accumulating snow won’t begin in earnest until the afternoon.

Friday afternoon/evening: A band of precipitation will likely slide south through Western Washington. Areas north of Everett should be cold enough for all snow, but areas south may initially see a rain-snow mix or non-accumulating, wet snow at sea-level. Expect precipitation to transition to snow for the entire Puget Sound region by the evening.

Meanwhile, the infamous northeasterly Fraser River outflow will begin to pour into Northwest Washington, with Whatcom and San Juan Counties getting a particularly good blow. Expect 30-40 knot sustained winds in exposed locations by late Friday night. These winds are expected to peak Saturday morning and persist through Sunday morning and will bring sub-zero windchills to the region. Point being: unless you are some sort of masochist, I’d avoid any trips to NW Washington.

Saturday morning:

This same band of precipitation should spread into the Portland metro area late Friday night/early Saturday morning. Now, it’s worth noting there is some disagreement between forecasters here. Due to persistent southerly winds throughout the day on Friday, some folks believe that temperatures in the Portland metro area will be above freezing when this precipitation first arrives and will fall below freezing later in the morning. However, I think that any period of rain or a rain/snow mix should be very brief, as the high-resolution models show cold easterlies beginning to filter through the Gorge early Saturday morning and this should lower temperatures below the magic 32 degree mark by daybreak. It may take a little longer for temperatures south of Clackamas to fall below freezing since they are not exposed to Gorge flow, but they too will do so later Saturday as this push of arctic air continues to move southward.

Saturday midday-evening: Confidence decreases here, as some models show one or multiple “deformation zones” (the same feature that brought the surprise snowfall to Puget Sound on Monday) forming over the I-5 corridor. Also, the “East Wind” will really start ramping up through the Gorge midday Saturday and will persist through at least Sunday morning.

To summarize, we’ve got a pretty classic snow setup on our hands, with a “BC Slider” ushering in a very cold airmass and dropping some precipitation into the region. However, due to (1) uncertainty in the timing of a switchover from rain to snow and (2) uncertainty in precipitation amounts with both the leading bands of this BC Slider and any deformation zones that may form later Saturday, snow amounts are highly uncertain. That being said, many folks from Portland north look like they could see a couple inches out of this system, with the potential for more.

Stay tuned! I’ll keep you posted! You can always comment here if you have any questions or give me a call at (206) 450-6810 if you want a forecast, or just want an excuse to chat.

Thanks for reading and do some snow dances!

Charlie