Lowland Snow Review

There are many well-known portmanteaus of snow and catastrophic descriptors, with Snowzilla, Snowmageddon, and Snowpocalypse being some of the primary offenders. But what about a more pedestrian snowfall that still inspires joy in the hearts of adults and children everywhere? I prefer “conversational snow” – not only is it an extremely accurate description (yes, a single snowflake in the Portland city limits sets Twitter ablaze), but it puts a lid on the “hype” that often comes with a marginal lowland snow forecast in Portland. I proselytize the importance of forecast communication on this blog, and there is no forecast tougher to communicate than the subtleties of lowland snow in the Portland metro area – particularly when the chances of snow are hyped by cherry-picked model charts that make the rounds on social media or misleading news headlines that exaggerate the snow threat.

However, this event slightly exceeded my expectations and the expectations of most meteorologists across the region. I thought that the precipitation would be too light and the air would be too warm to see any sort of sticking snow below 500 feet other than a slushy trace on cars and grassy surfaces, and I thought that we’d just have rain or a rain/snow mix below 200 feet. Well, I’m at 155 feet in the Hollywood District near Grant High School, and although we didn’t exactly see a crippling blizzard, we did see a trace of accumulation on cars and grassy surfaces.

Meanwhile, areas over the Western metro saw much more snow – significantly more than forecast. Here’s a photo taken by John Rinier in Forest Grove with nearly 4 inches at 220 feet. Forest Hill and parts of the West Metro were expected to see higher amounts, but I still think you’d be hard-pressed to find anybody who would forecast 4″ of snow below 250 feet.

Meanwhile, folks in the eastern metro in places like Gresham and Troutdale saw less snow. Areas further in the Gorge tend to get more snow during patterns like this, where a system from the Pacific is spreading precipitation inland while cool, easterly flow is pouring through the Gorge into the Portland Metro Area. Why was this time so different?

The reason: the air over the Columbia Basin wasn’t all that cold. Take a look at the observed temperatures at 3pm (right when snow was beginning to fall over Portland). Temperatures east of the Cascades in the Columbia River Gorge are in the upper 30s or even the low 40s, while temperatures in the Willamette Valley are much cooler; 34 in Hillsboro and Corvallis, and 32 in Salem. Portland had a temperature of 37 at the time with easterly winds gusting to 31 mph.

But the fact that there were mild temperatures east of the Cascades doesn’t explain why temperatures got so low over the Willamette Valley and Western Metro. Instead, Western Oregon cooled due to heavy snowfall melting as it fell into warmer air near the surface. This phase change from solid to liquid takes energy (just like it takes energy to melt an ice block on your stove), and this results in a cooling of the atmosphere and lowering of the freezing level.

With easterly winds over the lowlands, the West Hills and eastern foothills of the Coast Range saw substantial upslope flow, resulting in heavier precipitation, cooler temperatures, and surprisingly high snow totals. The extremely high-resolution UW WRF did a great job with this forecast. – we had the general idea (including very light snow for the eastern metro) but underestimated the effect of upslope flow over the Western metro and the heavier snow that resulted from it.

I gotta head to bed now but I’ll have more to write about in the coming days. There is a MASSIVE storm impacting California with up to 10 FEET of snow in the Sierras and ridgetop winds up to 100 mph (no I am not joking!), and we should see a fair bit of snow in the Cascades this weekend. I hope you all enjoyed the snow as much as I did!

Charlie