PORTLAND VANCOUVER SNOW AND ICE

Cold Air to Support Snow and Ice in Portland, OR and Vancouver, WA

* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database

Introduction

Portland, OR, and the surrounding metropolitan area is located in the north end of the Willamette Valley and along the south side of the Columbia River. Vancouver, Washington 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 at low elevations to support extended periods of ice and snow. The Cascade range to the east blocks cold air from flowing west into the area when air is sitting in the Columbia Basin to the east of the Cascade Mountains. Getting cold air into the Columbia River Basin, which extends from the north-central part of Oregon into the central part of Washington, is itself sometimes a task. The Rocky Mountains and other highlands extending from Southeast British Columbia and Southwest Alberta into Northeast Washington and south through Idaho can restrict cold air from moving into the basin when a cold high pressure system resides to the east in the southern high prairies of Canada and the adjacent high plains of the United States. Dense cold air must spill over the lower mountain passes and squeeze its way through the mountain valleys to get into the basin.

When cold air does get into the Portland and Vancouver area it is difficult to dislodge. The cold air is trapped between the Cascades to the east and the coast ranges to the west. A 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.

The Columbia River Gorge

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 or east can push cold air west through the gorge. A tightening pressure gradient caused by the approach of a low pressure system from the west can also help suck cold air through the gorge.

The Columbia River Valley of northeast Washington and southeast British Columbia

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 a path for cold air to enter the eastern part of Washington State and Oregon. The Columbia River, as seen on the Wikipedia map, extends south from the southeast part of British Columbia into Northeast Washington State, then wiggles south to the Oregon border. The river then flows west through the Columbia River Gorge and separates Portland to the south and Vancouver to the north. If a strong high pressure system pushes cold air up against the eastern slopes of the Canadian Rockies, especially for an extended period of time such as several days, cold air can spill into the river valley near the border of Alberta and eventually work its way to Portland and Vancouver.

Additional entry points for cold air to enter the Columbia Basin

The Pend Oreille River Valley and Okanogan River Valley (also called Okanagan in Canada), as noted on page 8 of "Climatology of the Interior Columbia River Basin" by Dr. Sue A. Ferguson (see reference below), are also favored entry points for cold air to get into the Columbia Basin.

The Pend Oreille River flows northwest from near Bute and Missoula in Western Montana to Northern Idaho, then through extreme Northeast Washington State. The river then extends just slightly north into British Columbia, then turns sharply west to connect to the Columbia River just barely north of the international border. A Wikipedia map highlights the river's location and its flow into the Columbia River.

The Okanogan River (called Okanagan in Canada) flows from South Central British Columbia into North Central Washington State. The Okanogan River connects to the Columbia River about one-third of the way south into Washington State east of the Cascade Mountain Range. A Wikipedia map highlights the river's location and where it merges with the Columbia River.

The Puget Sound

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 coast ranges to the west. The cold air in the Puget Sound typically originates from Southern British Columbia. Cold air works its way through the Southern Canadian Rockies through 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 of Washington, including Bellingham, Seattle, Tacoma, and Olympia.

References

Columbia River and Columbia Basin Information - Wikipedia

Fraser River Information - Wikipedia

Ferguson, Sue A., March 1999: Climatology of the Interior Columbia River Basin. Gen. Tech. Rep. PNW-GTR-445. U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, Portland, OR.

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.