Portland, OR is located in the north end of the Willamette Valley and along the south side of the Columbia River. Vancouver, WA is on the north side of the river from Portland. The area is dominantly a maritime climate keeping air generally too warm at the surface in low elevations to support extended periods of ice and snow. The Cascade range to the east blocks cold air from easily flowing west into the area if cold air is already sitting in the flatter portion of the Columbia Basin from Eastern Washington southwest into adjacent North Central Oregon. The Rocky Mountains extending from Southeast British Columbia and Southwest Alberta into Northeast Washington and south through Idaho block cold air from moving southwest out of the Western Canadian Prairies as a cold high pressure system slides south from the prairies into the Western Plains of the United States. Dense cold air must somehow squeeze its way through the mountain valleys to get into the basin.
Maps from nationalmap.gov
Once cold air gets into the area it is difficult to dislodge. The cold air is trapped between the Cascades to the east and the coastal hill and mountain ranges to the west. Any storm off the Pacific Ocean will likely produce one or a combination of snow, sleet, and freezing rain depending on the strength and depth of the cold air. Not unusual to get a series of snow or ice events before the air at the surface warms above freezing.
One way for cold air to enter the Portland area is through the Columbia River Gorge. If cold air already resides in the Columbia Basin, a high pressure system to the northeast can push low level cold air west through the gorge. A tightening pressure gradient caused by the approach of a low pressure system from the west can help suck the cold air through the gorge.
If cold air is not already positioned east of the Cascades then it has to somehow get through the Rocky Mountain barrier. The Columbia River Valley provides one path for cold air to enter the basin. The Columbia River extends east then turns abruptly north-northwest through central Washington, then northeast through the Rocky Mountains of northeast Washington. It then extends into Southeast British Columbia where meanders a bit. If a strong high pressure system with cold air pushed up against the Canadian Rockies sits to the east or northeast long enough, cold air will flow through the valley and eventually enter the Columbia Basin. From there it can spread south and west into North Central Oregon. Some other points of entry include the
The Pend Oreille River Valley Extends from Northern Idaho into Northwest Washington and the Canadian Border. The Oreille River connects to the Columbia River at the border with British Columbia.
The Okanagan River Valley extends from South Central British Columbia into North Central Washington. The Okanagan River connects to the Columbia River about one-third of the way south into Washington along the east edge of the Cascade Mountain Range.
Cold air can also spread south from the Puget Sound of Western Washington. The cold air is channeled south through Western Oregon between the Cascade Range to the east and the coastal ranges to the west. The cold air often originates from Southern British Columbia. In a process similar to the Columbia River Valley, cold air works its way through the Southern Canadian Rockies along the Fraser River Valley. The Fraser River flows into the Strait of Georgia at Vancouver, British Columbia. The Strait of Georgia connects to the Strait of Juan De Fuca and Northern Puget Sound. Cold air enters the Strait of Georgia then flows south into the Puget Sound region, including Bellingham, Seattle, Tacoma, and Olympia.
Columbia River and Columbia Basin Information - Wikipedia
Fraser River Information - Wikipedia
Climatology of the Interior Columbia River Basin - Sue Ferfguson, Forestry Sciences Laboratory, Seattle, WA
Garth, K. F., C. F. Mass, G. M. Lackmann, and M. W. Patnoe, 1993: Snowstorms over the Puget Sound Lowlands, Wea. Forecasting, 8, 481-504.