Puget Sound Overrunning
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Maps created using nationalmap.gov
Air at the low elevations of the Puget Sound, including Seattle, is rarely cold enough to support snow. Most cold fronts move off the Pacific Ocean from which air at the surface is typically above freezing. Air off the Pacific can be cold enough if a storm system moves south out of the Gulf of Alaska then turns east into the Washington coast. A Gulf of Alaska storm sometimes ingests arctic air from southeast Alaska or British Columbia. The warm water still modifies the air but temperatures can be marginally cold enough to support snow.
The best means of getting snow in cities like Tacoma, Seattle, and Everett is for arctic air to establish itself in the Puget Sound BEFORE a low pressure system of cold front approaches. If air overrunning the cold air trapped in the sound is warm enough for rain, then freezing rain and sleet will occur rather than snow. If a large upper tough covers the Pacific Northwest, several shorter wave troughs (upper disturbances) rotating within the main trough can produce two or more snow or ice events in a row. The track of a surface low pressure center is critical. If it passes south of the Puget Sound, the arctic air remains in place so that the next storm can produce another round of wintry precipitation.
Unfortunately for lowland snow lovers, arctic air rarely gets into Puget Sound. The high Cascade Mountains and the Canadian Rockies block the westward movement of dense arctic air to the Pacific Coast. Sometimes however, a particularly deep and cold arctic air mass associated with a strong surface high pressure system, such as 1050 millibars, gets lodged up against the east slopes of the mountain ranges. Through several days the cold air can weave its way westward through the mountain valleys and gorges.
The most notorious entry point of arctic air into the Puget Sound is the valley where the Fraser River exits into the Strait of Georgia at Vancouver, British Columbia. Arctic air enters the strait then fills south into the Puget Sound. The convergence that occurs along the arctic front, aided by a component of convergence induced by the surrounding terrain, can itself produce a brief shot of snow.
Click HERE to see a three day series of surface and upper level charts from November 26, 2006 to November 28, 2006 showing an arctic outbreak which affected Washington State. Note the central pressure of the arctic high pressure system is a little over 1050 mb in the first two surface map images. Also note the position of the upper trough is in favorable position to allow arctic air to try and work southwest into western Washington.
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.