A rare ice storm warning was even issued for the Willamette Valley in western Oregon, where up to three quarters of an inch of ice accretion is possible.
Low pressure west of British Columbia was rolling east-southeast Friday afternoon, sending moisture into southern Washington and northwest Oregon late Friday morning. A mix of snow and freezing rain was falling in Portland, with freezing rain farther southwest between Salem and the Pacific coast. The Highway 18 and 22 corridors could be especially slick.
Snow hadn’t started yet farther north in northwest Washington, where Seattle sat at 31 degrees at noon.
Precipitation is expected to fill in throughout the afternoon and evening hours, especially from the Cascades westwards, with rain likely near the coast and snow inland. The core of deepest moisture arrives by 9 or 10 p.m. Pacific Time, which is when precipitation rates will spike.
Moderate to briefly heavy snow is possible in Seattle overnight, with snow the predominant precipitation type across most of Washington, including low elevation areas. Seattle only saw 1.2 inches of snow in 2020, and hasn’t measured anything yet in 2021. That’s set to change with this storm, which is expected to drop 3 to 7 inches across the metro area.
“We maybe get a few storms a year, but we haven’t really had anything this season so far,” said Jeff Michalski, a meteorologist at the National Weather Service in Seattle, in an interview. “Our last big winter storm was in February of 2019.”
The long-duration event will eventually dislodge the cold air later in the weekend.
“This storm will affect the region Friday night through Saturday, and then there’s a transition from snow switching over to rain Sunday or Sunday night,” Michalski said.
Farther south in Portland, the forecast is a bit more complicated.
Shallow cold air draining out of the Columbia River Gorge has kept surface temperatures in the upper 20s on Friday. In most of Portland and its northern suburbs, that’ll ensure precipitation remains as snow, with snow levels dropping down to 300 feet. Southern parts of the Portland metro area, however, would see freezing rain, where warmer air at the mid levels cause precipitation to fall in liquid form.
“Near the Columbia River Gorge, they typically have freezing rain a couple times a winter,” said Colby Neuman, a meteorologist at the Portland Weather Service office, in an interview. “In the Portland metro, maybe once a winter.”
Neuman explained the forecast in Portland is a tricky call; anything between a half foot of snow to a half inch of icy glaze is on the table. Oregon City, Lake Oswego, Tigard and Wilsonville are all in that wait-and-see zone. Significant ice has already caused problems to the south of Portland, in the Willamette Valley.
“What will make this event unique is that we had freezing rain overnight south of Portland,” said Neuman. “They actually got a tenth to a quarter inch in the central Willamette Valley near Salem. Freezing rain there is relatively rare.”
Another 0.3 to 0.7 inches of ice is on the way in Salem, where an ice storm warning is in effect.
“I can’t say that I’ve seen significant icing there in my time here,” said Neuman. “I’ve been here since 2012.”
One of the trickiest parts of forecasting in the Pacific Northwest is the “data void” to the west, says Neuman. With no weather balloons and sparse observations in the Pacific Ocean, it’s not so easy to look up the pipeline to see what’s coming.
In the higher elevations of the Cascades, the forecast is less of a challenge: 3 to 4 feet of snow are expected before the system wraps up late Saturday. A second, weaker wave could bring some lighter rain and snow into the region on Sunday.
Meanwhile, another system could bring freezing rain to areas northeast of Portland early next week.