Climate Science you can take to the bank:
We don't get too many severe Arctic outbreaks here in Western Oregon. Our typical weather in the Winter is influenced by the Pacific Ocean, so it is cool and wet most of the time. We've been in a major Arctic Outbreak for almost a week though, and everything is frozen solid, although the thermometer just topped 32 degrees for the first time in days.
It's almost the rule though, that when the air-mass changes from Continental Polar to Maritime Pacific, that a transition period of winter precipitation occurs from the overrunning of the Polar air by the wetter, milder Maritime air.
That winter precipitation is made worse in the Portland area by the micro-climate caused by a feed of cold air down the Columbia River Gorge. My house sits within 4 miles of that gorge mouth.
The basin between the Cascade Mountains immediately to my East, and the Rocky Mountains about 350 miles to my East forms a reservoir of cold air, and the major outlet of that cold air, which behaves like any fluid and drains to lower places (to Portland, through the sea-level gap caused by the Gorge), causes a constant supply of below-freezing air to cause frozen precipitation of either freezing rain, sleet, snow pellets or snow. Depending on air mass pressure differences, this cold outflow through the Gorge can take a week to modify, and if there is constant precipitation coming in from the West, that's a week of misery for us dwellers in the mouth of the Gorge.
The conditions are prime for a major mess in the Gorge. The last time this happened, about 10 years ago, there was a week of snow pellets, and they flow like tons of ball bearings, always seeking the lowest ground. I-84, the major East-West route out of Oregon, and the BNSF & UP-SP railroads were halted by 30-foot drifts of the heavy pellets. Clearing the drifts takes days, not just one pass with a rotary snow-blower, because more pellets fall down to fill in where the tracks/roads were just cleared. Blizzard conditions can prevail for days in this micro-climate. It's probably a bigger weather challenge to transportation than the Donner Pass.
I go out in a few minutes to top off provisions, and will be prepared to winter in, albeit with some hardship, since I had to shop my Plasma big-screen today (waaaaah!).