Potential for Significant Snow Sunday/Monday, But Very Tough Forecast

We all remember the “Ides of October” windstorm on October 15, 2016. The one that sprung out of Supertyphoon Songda and was feared to be a historic Pacific Northwest windstorm that would long be remembered. Models were originally showing a major windstorm that would pack 100+ mph gusts to the coast and 80 mph winds to the interior, but as the days progressed and the event came closer, it became clear that the storm would be very compact, with small changes in track and intensity having a huge effect on the forecast outcome. The storm ended up tracking just a little further west than modeled, giving much lower winds than forecast to the entire region.

Though this potential for this was clearly illustrated in the models, it could have been communicated better to the public. The Seattle NWS did a fantastic job communicating uncertainty, but their forecasts were overshadowed by hyperbolic, clickbait articles that warned of 150 mph winds, 50 foot waves, and multiple feet of rainfall.

A similar forecast is on tap for Sunday through Tuesday, but this time the threat will be snow, not wind. A narrow plume of moisture will enter into an airmass just on the cusp of freezing, and small changes in the intensity, track, and timing of this plume will have tremendous impacts on the forecast outcome.

Now that we’ve got that disclaimer out of the way, let’s take a look at what is causing such a buzz in the meteorological community.

We currently have a deep upper-level trough off the northern tip of Vancouver Island that is directing periods of rain and snow into the area. Temperatures have moderated above freezing in the lowlands now, but because they were near freezing or slightly below when this precipitation entered the area, we saw a wide variety of winter weather throughout the Pacific Northwest. The Portland metro area saw an extended period of strong easterly winds through the gorge with sleet and freezing rain last night and this morning, and the Seattle area saw all snow due to colder temperatures aloft.

As the weekend goes on, this trough will drift to the SW and then begin to pivot back E midday Saturday. By 7 am Sunday morning, it is just off the coast of Central Vancouver Island and is directing a strong 125-knot jet stream into the area at 300 mb (approximately 30,000 feet, or cruising altitude for airplanes).

As this trough moves south, it will help direct a pool of colder, continental air from British Columbia into our region. But with southwesterly flow aloft, we will have a dramatic shift in temperatures as we head further south.

An area of large temperature change in the atmosphere along a constant pressure surface is called a baroclinic zone. Because extratropical cyclones derive their energy from horizontal difference in temperatures, baroclinic zones are favored areas for storm development. And as the next frames show, a storm will indeed develop along this zone. By 4 pm Sunday, a storm is developing off the coast of Northern California and heading northeast into Oregon.

On the north (cool) side of this baroclinic zone, a plume of moisture will develop. Note the particularly heavy precipitation at the southern edge of the plume where the mild southwesterlies and cool northerlies converge.

And since the track of this storm is parallel to the orientation of this band of moisture, this plume will reside over roughly the same area for all of Sunday.

The location of this plume is critical. Too far south, and Portland gets very light or no snow. Too far north, and Portland gets much warmer temperatures and no snow while Seattle gets pounded. The highest totals will likely be along this area of enhanced precipitation because (1) there’s more precipitation (duh) and (2) heavier precipitation cools temperatures because melting snow takes heat energy out of the atmosphere.  To make the forecast even more difficult, temperatures up at 2,500 feet are predicted to be near freezing. This means that even if the plume is located over Portland, downtown could see a rain-snow mix while the West Hills get over a foot of snow. Finally, the air mass over the area will already be nearly saturated, meaning we won’t get the effects of wet bulb cooling to further cool us off.

Now, the graphic you’ve all been waiting for: the 24-hour snow accumulation over the Pacific Northwest from 4 am Sunday to 4 am Monday.

As you can see, the Willamette Valley gets absolutely pounded in this scenario with nearly a foot from Portland down to Eugene. This model assumes that temperatures will be cold enough for snow. If temperatures were just a teeny bit warmer, we’d see a rain-snow mix and the lowest elevations would get nothing.

To summarize, this is a very difficult forecast because small changes in the location of this plume of moisture and the temperatures around the region will have drastic effects on the outcome. Another factor to consider is the timing of the storm – if it arrives in the early morning hours when temperatures are the coolest, there is a better chance for snow than if it were to arrive in the afternoon.

This is definitely a storm to watch, and I’ll keep you posted with updates!

Thanks for reading!
Charlie