More Snow for Portland Tuesday Night!

We probably ended up with approximately an inch of snow at our place in the Woodlawn neighborhood of Northeast Portland from yesterday’s late-season snowstorm. Fellow WeatherTogether blogger Jeff Taylor ended up with 3.3 inches in Happy Valley at 600 feet, and the weather at Boring, OR (7 miles NW of Sandy, OR at 655′) was anything but boring, as they picked up an incredible 9 inches of snow from yesterday’s storm. You can find a list of snow totals from official weather stations/certified NWS spotters compiled by the Portland NWS office here.

But as Billy Mays (rest in peace) tells us, residents of the Portland metro area should buckle up for another round of snow tomorrow evening/night.

Just a couple days ago, Tuesday was expected to be cold and sunny with highs in the upper 30s as a weak low pressure system paralleled the 135⁰ west longitude line, nary spreading a cloud over the Pacific Northwest. Lows were predicted to sink into the upper teens/lower 20s at Portland Int’l Airport, which would easily surpass the coldest temperature of the winter measured at the airport so far (23 degrees on December 26th). However, subsequent model runs strengthened this low and moved it further and further east so that it was only a few hundred miles off the coast, and the last few runs have had it close enough to spin some snow into SW Washington and Western Oregon. This type of track – with a low pressure system sinking down from the north and directing moisture into the area, is a textbook track for lowland snow, and the fact that we are even mentioning the potential for a widespread lowland snow event in February is a testament to how effectively this track brings both cold air and moisture into an area simultaneously.

Current Conditions:

Let’s start by looking at a surface map faithfully drawn by the meteorologists over at the Weather Prediction Center, which is a sub-agency within the National Weather Service. The system we are watching is that very weak, elongated trough along the BC coast.

Because it is so weak, it is hard to discern on satellite, but you can see a disjointed line of increased water vapor content stretching from the Alaskan Panhandle to the mouth of the Columbia River in the image below. Notice the thick band of clouds over our region; this will prevent temperatures from dropping as much as originally feared. Temperatures will only drop into the mid/upper-twenties instead of the upper teens that were forecast a couple days ago.

This system is stretching the limits of how little precipitation a radar can measure while still technically measuring precipitation. See those super light radar echoes off the coast? Those are exceedingly weak showers from our incoming system. The echoes over the Cascades are just random clutter, and that echo in Eastern Washington is due to wind turbines, some of which may be Avangrid’s (the company I work as a wind energy forecaster for).

Finally, here’s the system as modeled by tonight’s 00Z WRF-GFS. Keep in mind that this is a forecast, not an observation/analysis. Still, it is much easier to read and gives you an idea of the bigger picture.

To summarize, we currently have a super weak system dropping down from the NNW and doing little more than spreading high clouds over the region. But through tonight and into tomorrow, this system will strengthen and further approach the area, coming close enough to throw some snow showers our way.

Forecast

The storm is expected to track SSE and increase in intensity as the day goes on. By 10 am, it should be right off the mouth of the Columbia and beginning to spread moisture into the Coast Range. Because winds along the coast will be from the south during this time, snow levels are expected to remain above 400-800 feet, though I wouldn’t be surprised to see temporary sea-level accumulations with any heavy showers that come through. Meanwhile, offshore gradients will develop through the Columbia River Gorge as air rushes from high pressure to low pressure, and easterly winds will begin to pick up in the Portland metro area Tuesday evening/night. This will deliver a reinforcing shot of dry/cool air to areas in the Gorge and the greater NW Oregon/SW Washington region.

This low is predicted to spread moisture into the Portland area in the late afternoon/early evening. The snow situation in Portland/the Willamette Valley is a bit more complicated than the coast, as it’s not a matter of simply having showers enter the region. Instead, frontogenesis (literally, genesis of a front) is predicted to occur as the precipitation being spun northward off this low interacts with a shortwave trough pushing south through BC, forming a small but heavy band of snow. Where this band sets up and how slowly it moves south will play the big role in deciding who gets the most snow.

As such, different models with slightly different storm tracks give vastly different lowland snow totals; the UW’s WRF-GFS develops this band between Salem and Corvallis and gives the PDX metro area no snow while the UW’s MM5-NAM shows amounts ranging from 2 inches in North Portland to 4+ inches from South Portland to Salem. The ECWMF brings approximately 2 inches of snow to the Portland metro area, and the Canadian model goes gangbusters and gives the metro half a foot. Given all the different models, a trace to 3+ inches appears likely for the Portland metro area, and regions between Centralia and Eugene have the best chance of seeing some accumulation.

Commute

The combination of snowfall beginning in the evening and Portland’s notoriously terrible winter-weather drivers no doubt raises some concerns about the evening commute. The great thing about late-season snowfall is that higher sun-angles do a better job of warming surfaces, so roads are not as susceptible to icing. With road temperatures above freezing during the day, I’m not as concerned about the evening commute, but I’m very concerned about the roads later at night when temperatures drop below freezing. Additionally, if the aforementioned band of snow ends up being a doozy and drops 4+ inches over the metro area, Tuesday night and Wednesday morning will be a mess. Regardless of what happens, bridges will be significantly more icy than roads, so use caution when traveling over them.

I’m working late tomorrow, but I should be back to answer any questions and give a 10:30 pm (+- 1 hour) forecast update if I’m not throwing snowballs at my roommates.

Thanks for reading, and do some snow dances!
Charlie