An arctic blast that has sent the Pacific Northwest into the ice box is about to mix with a strong Pacific storm later Thursday and Friday, bringing a recipe for a significant ice storm across northwestern Oregon and western Washington, including Portland and the Willamette Valley, and Seattle and the Puget Sound region, threatening to grind holiday travel to a halt and knock out power to thousands.
Winter storm warnings are in effect across much of the coastal Pacific Northwest, including the greater Portland and Seattle areas for a myriad of winter woes.
For the Portland area, freezing rain accretions are expected to reach 0.2 to 0.4 inches, wind gusts to 55 mph near the Columbia River Gorge and wind chills dipping as low as zero. Around the Seattle and western Washington area, a few inches of snow may precede ice accretions up to a quarter inch.
Meanwhile, ice storm warnings stretch into the Eugene and Corvallis areas for even greater freezing rain accretions up to 0.5 inches – considered potentially ‘crippling’ levels – with a half inch or even greater accretions likely along the Columbia River Gorge.
Low temperatures in eastern Washington dropped to near or below zero Thursday morning with single digits in eastern Oregon as arctic air continues to pool into the region. Temperatures in northern Oregon and western Washington in the teens and 20s Thursday morning and not budging much during the day.
However, the next weather system heading toward the Northwest late Thursday and into Friday is a much milder storm rolling in off the Pacific Ocean – a more traditional wintertime event for the region bringing air that would typically push temperatures well into the 40s, if not the 50s.
With cold air currently locked at the surface, precipitation will likely begin as snow. As the battle between the warm air intrusion and the stubborn freezing air at the surface takes place over several hours, precipitation will change to a potentially prolonged period of sleet and freezing rain, with significant accretions possible.
“Ice is sometimes worse than snow – especially when it comes to traveling on the roads,” said FOX Weather meteorologist Britta Merwin.
Freezing rain is possible across much of the coastal Pacific Northwest, starting late Thursday night and lasting through Friday afternoon or evening. It could even last into Saturday in the Columbia Gorge and eastern Willamette Valley with ice accretions in the Gorge reaching near 1 inch.
The Portland area and into southwestern Washington could see significant ice accretions ranging from one-tenth to one-half inch, with lighter but still potentially impactful glazes expected in the Seattle area and into northwestern Washington.
Ice accretions of one-quarter to one-half inch are considered “disruptive,” with numerous power outages and some tree damage expected as roads become very slick. Once accretions hit one-half inch or more, a potentially crippling event is underway, creating widespread power outages and tree falls and dangerous travel conditions.
Travel along the Interstate 5 corridor from central Oregon all the way to the Canadian border will be challenging during the ice storm, with icy glazes on road surfaces sapping any hope for traction.
Both Seattle and Portland’s airports could see significant weather-related delays during and immediately after ice storms as planes become covered in icy glazes and airports frantically work through busy deicing operations, adding to nationwide air travel woes this week with more storms in the East.
Portland’s airport was shut down multiple days during an ice storm in 2004, while Seattle’s Sea-Tac Airport was closed for more than a day during a historic ice event in January 2012.
Eventually, the warm air will win the battle, and freezing rain will gradually turn to plain rain late Friday as temperatures warm into the 40s.
Once the cold air scours out, the weather pattern will flip, with above-average high temperatures reaching into the upper 40s and 50s Christmas weekend and the following week with rising snow levels and heavy rains.
The lurking subzero or near-zero temperatures on the eastern side of the Cascades create a strong area of high pressure due to the high density of cold air. Meanwhile, as the deep low pressure approaches the coast from the approaching storm, an intense difference in pressure will draw strong easterly winds through the gaps in the Cascades.
Such gaps include the Fraser River Valley in British Columbia and major passes such as Snoqualmie Pass in Washington.
The greatest conduit for channeling arctic air from the east into the coastal valleys is along the Columbia River Gorge that straddles the Washington-Oregon border and spills into the eastern Portland suburbs.
Gusts through the gorge can reach 50 to 70 mph, with peak gusts roaring up past 100 mph in some of the strongest events. Already on Thursday, Crown Point along the Gorge recorded an easterly wind gust of 86 mph.
Those winds will not only blast the eastern Portland areas with gusts over 50 mph but also act like an icy blow-dryer, continuously spilling modified arctic air into the lower elevations of the Willamette Valley.
As the relentless warm Pacific rains fall into this frozen layer of air trapped near the ground, it freezes on contact creating freezing rain.
Depending on the depth of the cold air east of the mountains and the strength and speed of the approaching storm, the howling easterly winds can last for hours, if not days, leading to thick ice accretions that cripple the Portland area region with widespread and extended power outages as trees and power lines buckle under the weight of the ice, leaving streets impassable.