Snow Enhancement from the Puget Sound Convergence Zone



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Introduction

The Puget Sound Convergence Zone is a zone of low level convergence that occurs in the Puget Sound of Washington State when winds from the north and winds from the south converge in the middle of the sound. The higher terrain surrounding the water and lowlands of the Puget Sound force the air into convergent directions. The zone can migrate north or south depending upon the relative strength of the converging winds. The upward motion forced by the convergence can enhance snowfall in cities like Seattle and Everett. The convergence zone most commonly occurs over the central and northern Puget Sound from the north side of Seattle northward through Everett. If the stability of the air in the vicinity of the convergence is low enough, the triggering of the convection can further enhance snow intensity. Temperatures however are rarely cold enough for snow in the lower elevations of the Puget Sound. The Seattle-Tacoma Airport only averages 11.4 inches per season based on 52 years of data though 2002 saved at the National Climatic Data Center (link). The key is to somehow get the cold air into the sound at the same time the convergence occurs.

General Information

The Puget Sound Convergence Zone can occur at any time of the year and under a variety of circumstances. The highest frequency is in late spring and early summer. At that time diurnal processes such as land an sea breezes are more likely to affect its behavior. Precipitation associated with the convergence is enhanced in a narrow zone. The compensating sinking air to either side of the upward motion produces a tight precipitation intensity gradient. The convergence zone does not always have to be associated with precipitation. This discussion of the Puget Sound Convergence zone will focus on wintertime scenarios that produce Puget Sound Convergence and can thus contribute to the production of snow. For a more comprehensive discussion of Puget Sound Convergence, please refer to the articles in the references section from which much of this discussion is drawn.

Applicable Terrain Features


The Puget Sound is bounded by the Cascade Mountains to the east and the Olympic Mountains on the Olympic Peninsula to the west. The Strait of Juan de Fuca separates the Olympic Peninsula from the mountainous Vancouver Island to the north. The Chehalis Gap separates the Olympic Mountains from lower mountains and hills to the south which include the Willapa Hills. The gap extends from the Pacific Ocean to the southern end of the Puget Sound. To the north of the Puget Sound, the water extends north into British Columbia as the Strait of Georgia. The Strait of Georgia separates the east side of Vancouver Island from the northern extension of the Canadian portion of the Cascades.

Common Wintertime Puget Sound Convergence Scenario


Puget Sound Convergence occurs when W to NNW winds in lower troposphere impinge on the northwest bulge of the Olympic Mountain Range. Based upon the research presented by the articles in the reference section, the best wind direction to help trigger convergence is WNW since that direction results in the best split wind around the Olympic Mountains. The mountains force the air to split into two primary streams. Some of the air also continues southeast and rises over the mountains. One stream is forced east through the Strait of Juan de Fuca by the Olympic Range to the south and the Vancouver Mountains to the north. The other branch of winds is forced south-southeast, then enters the western end of the Chehalis Gap, and turns east between the Olympic Mountains and Willapa Hills. The north-south wall of the Cascade Mountains forces the northern branch of winds in the Strait of Juan de Fuca to turn south and the southern branch of winds in the Chehalis Gap area to turn north. The air converges near the middle generally from Tacoma to Seattle and Everett including adjacent lowlands on both sides of the sound. Also note that the nature of flow around an island-like barrier such as the Olympic Mountains causes the air to wrap around the back side of the barrier. As a result, the convergence is increased. A switch to winds from the west-northwest behind an east or southeast moving cold front is one means to provide favorable winds to trigger airflow conducive to Puget Sound Convergence .

Another Puget Sound Convergence Mechanism

An arctic front passing from north to south can help produce convergence when north to northeast winds behind the front collide with air being funneled to the north. Winds ahead of an approaching low pressure trough from the west or northwest can increase the convergence into the arctic front. The arctic air will typically enter the Strait of Georgia from the Fraser River Valley at the city of Vancouver, British Columbia then spread south into the Puget Sound. Since the arctic front behaves as a shallow boundary, lift due to the convergence occurs along and north of the surface position of the front. The structure of the front positions the cold air with the lift making the arctic front an effective snow producer. The amount of snow will be highly depended on the availability of moisture and interaction with other precipitation producing weather systems.

Cold air into the Puget Sound from off the Pacific Ocean

Getting air cold enough into the Puget Sound to support snow is a difficult task. In the post cold front scenario, the air off the Pacific Ocean is simply too warm in most cases. Sometimes storm systems that drop south out of the Gulf of Alaska may have enough cold air near the surface to marginlly support snow. The communities in the higher hills such as those east of Seattle, have the better chance of getting measurable snow. The melting and evaporation/sublimation processes associated with heavier showers also increase the odds of snow reaching the ground.

Getting arctic air into the Puget Sound

The highly elevated Cascades Mountains block the westward movement of dense arctic air. If the cold air can be pushed up against the mountains long enough, it can spill over the lower elevated breaks between the mountain chains and weave its way west through the valleys and gorges. A deep cold upper level trough positioned over southwest Canada, the Pacific Northwest, and the adjacent Pacific Ocean waters is favorable to allow and arctic air mass to move west toward the coast. The air mass however usually needs to be deeper and colder than average. A strong arctic high pressure system of 1050 millibars central pressure or stronger, lodged up against the east slopes of the Rockies, presents a favorable situation for cold air to move west to the Pacific Coast.

As mentioned earlier, one of the best entry points of arctic is the mouth of the Fraser River that enters into the Strait of Georgia at Vancouver. The Fraser River is the longest river in British Columbia and wiggles its way deep into the mountains. The river's path makes it one of the best means for arctic air to get through the mountains. Call it the "Arctic Passage" into the Puget Sound. The air then moves south from the Strait of Georgia into the sound.

If winds are crossing the Olympic Mountains from the west, the development of a leeside low pressure trough on the west side of the Puget Sound produces a pressure gradient between the trough and the arctic high pressure ridge. The difference in pressure can help suck cold air into the sound from the various openings between the mountains. A similar pressure difference can be attained when a low pressure system or trough approaches from the west as the cold air tries to spill into the sound.

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 slightly above 1050 mb on the first two surface maps. Also note the favorable position of the upper trough. The closed part of the trough moved from southern British Columbia and the adjacent Pacific Ocean waters to eastern Washington and northern Idaho.


Formal References

William, M. W., R. L. Doherty, and B. R. Colman, 1993: A Methodology for Predicting the Puget Sound Convergence Zone and Its Associated Weather, Wea Forecasting, 8, 214-222.

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.

Clifford M., 1981: Topographically Forced Convergence in Western Washington State, Mon. Wea. Rev., 109, 1335-1347.