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Puget Sound Overrunning

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

Discussion

Air at low elevations of the Puget Sound region, 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 southeast out of the Gulf of Alaska or south from the Yukon and northern British Columbia, then turns east into the Washington and Oregon coast. This type of storm ingests colder air from western Canada into its circulation. The water modifies the air but temperatures can still be marginally cold enough to support snow in the lower elevations of the Puget Sound.

The best means of getting snow in cities like Olympia, Tacoma, Seattle, and Everett is for cold air to establish itself in the Puget Sound before a Pacific low pressure system or cold front approaches from the west. If air overrunning the colder air trapped in the sound is warm enough for rain, then freezing rain and sleet will occur rather than snow. If an upper level trough dominates the atmosphere over the Pacific Northwest for an extended period of time, multiple snow and ice events can occur. The track of a surface low pressure center can be critical. If it passes south of a location in the Puget Sound, cold air is more likely to remain in place so that the next storm can also produce more snow and ice. A good example of multiple storms producing snow occurred from February 3 to February 12, 2019 when a series of four storms dumped 20.2 inches at the Seattle-Tacoma, WA Airport. Cold air initially invaded the Puget Sound on February 3. A reinforcing shot of cold air moved into the region on February 8.

Unfortunately for lowland snow lovers, strong outbreaks of cold Canadian or Arctic air are infrequent in the Puget Sound. The various mountain ranges including the Rockies and the Cascade Mountains block the westward movement of the cold air. Sometimes a deep cold air mass with a strong surface high pressure system center, such as 1050 millibars, gets lodged up against the eastern slopes of the Rockies for several days and the cold air has time to weave its way westward through the mountain valleys and gorges. The high pressure center does not always have to be that strong but is favorable. An upper level trough over the region, as can be analyzed at 500 mb, is also favorable to allow cold air to move west.

The most notorious entry point for cold air to get into the Puget Sound is where the Fraser River empties into the Strait of Georgia at Vancouver, British Columbia. The air enters the strait from the Fraser River Valley then spreads south into the Puget Sound. The convergence that occurs along the front, aided by convergence enhanced by the surrounding terrain, can produce a brief shot of snow along and behind the front as it passes.

References

National Weather Service Forecast Office Seattle/Tacoma, WA - Area Forecast Discussions issued over the years

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.