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Snow Enhancement from the Puget Sound Convergence Zone

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


Introduction

The Puget Sound Convergence Zone is a zone of wind convergence that occurs above the Puget Sound of Washington State when winds from the north to northwest and winds from the south to southwest converge somewhere in 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 enhances precipitation. 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, convection can be triggered to further enhance precipitation intensity. If temperatures in the winter are cold enough at the surface, several inches of snow can fall. Unfortunately, temperatures are rarely cold enough to support snow in the lower elevations of the Puget Sound. Seasonal mean snowfall generally ranges from 5 to 12 inches from Olympia at the south end of the sound, north to Tacoma, Seattle, and Everett. The higher totals occur farther north and in the higher hills.

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 and 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. This discussion of the Puget Sound Convergence Zone focuses on wintertime scenarios that produce Puget Sound Convergence and can contribute to the production of snow. For a more comprehensive discussion of Puget Sound Convergence, please refer to the articles in the "References" section.

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 Vancouver Island to the north. Vancouver Island is also covered with mountains, known as the Vancouver Mountain Ranges, with the four highest peaks at or a little above 7000 feet. 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 region. To the north of the Puget Sound, 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 Cascades and other adjacent mountain ranges.

Common wintertime Puget Sound Convergence scenario



Puget Sound Convergence occurs when W to NNW winds near the surface impinge on the northwest bulge of the Olympic Mountain Range. Based upon research presented by articles in the reference section below, the best wind direction to help trigger convergence is WNW since that direction results in the best split of wind around the Olympic Mountains. The air splits into two primary streams, with some of the air continuing southeast to rise over the mountains. One air stream is forced east through the Strait of Juan de Fuca between the Olympic Mountains to the south and the Vancouver Mountain Range to the north. The other branch of winds is forced south-southeast, then enters the western end of the Chehalis Gap, and then is forced east between the Olympic Mountains to the north and the Willapa Hills to the south. The north-south wall of the Cascade Mountains to the east of the Puget Sound 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 to turn north. The air converges near the middle generally from Tacoma to Seattle and Everett and the adjacent low lands of the sound. Also, note that the nature of flow around an island-like barrier such as the Olympic Mountains causes air to wrap around the backside of the barrier. As a result, convergence is increased. A switch in winds behind a Pacific cold front to come from the west-northwest is one way to provide favorable winds conducive to Puget Sound Convergence.

Puget Sound Convergence due to arctic frontal passages

An east to west oriented Arctic front passing from north to south can help produce convergence when north to northeast winds behind the front collide with winds from the south. South winds ahead of a front moving east or ahead of a low pressure system can increase the convergence along 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 region. Since the arctic front is typically a shallow boundary, much of the lift occurs along and north of the surface front. As a result, precipitation falls through the cold air behind the front and reaches the ground as snow if the rest of the atmosphere's vertical temperature profile is also cold enough to support snow. The amount of snow will be depended on the availability of moisture and the presence of other atmospheric processes to contribute to the production of precipitation.

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.

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