Meteorologists use the term to refer to the flow of cold air that comes out of the Fraser River Valley in Vancouver, BC in Canada and flows out over the Strait of Georgia. The air then spreads south into the Puget Sound region of Washington State. If the flow persists long enough, the cold air can work its way south between the Cascade Mountains to the east and the coastal range hills and mountains to the west and get into the Portland, OR area. The Fraser River Valley is one of the primary routes that Arctic air can use to get from the Canadian High Plains, through the mountains, and into the lowlands of the western portions of Southern British Columbia, Washington State, and Oregon. Fraser Flow can cause convergence in the Puget Sound when the cold flow of air collides with southwest or south winds flowing around the south side of the Olympic Mountains. It can also cause orographic enhancement of precipitation along the foothills and mountains of the northeast part of the Olympic Peninsula as north winds associated with the cold flow run into the higher terrain.
Fraser Flow is referenced in the following articles on this website.
Puget Sound Cold Air Outbreaks
Snow Enhancement from the Puget Sound Convergence Zone
Puget Sound Overrunning
Cold Air to Support Snow and Ice in Portland, OR and Vancouver, WA