Seattle maintains a mild climate due to the maritime polar and maritime tropical air masses which hover over the Pacific Ocean. When Rossby waves dip south from the Arctic Circle they bring with them wisps of continental arctic and polar air into the northern reaches of Washington. The continental polar air mass is usually responsible for any extreme temperatures in Seattle. During the winter, the maritime polar air mass dominates most of the United States’ western coast, from the middle of California up into Alaska. During the summer months, the maritime polar air mass recedes to the southern edge of Canada and the coast of Alaska as the maritime tropical mass brings extra humidity and summer temperatures to Seattle. Both air masses are relatively unstable, leading to cloudy and drab weather.
|This image of the Rossby waves show how it dips into Washington State, bring cold air with it from the Arctic. http://www.geography.hunter.cuny.edu/~tbw/wc.notes/7.circ.atm/rossby.waves.jpg|
On a continental scale, mid-latitude cyclones may begin in the Pacific Northwest as cool air from the continental arctic, maritime polar and maritime tropical masses meet. Typically, Seattle is encountered by mid-latitude cyclones when they are in the formation stage, meaning that the city receives warm, cold fronts, and precipitation from the low side of the cyclone. Lows from the Gulf of Alaska provide Seattle with cold fronts and precipitation as they form and move east. Alberta clippers may affect the Seattle area as they move south east, bringing cold fronts and a small amount of precipitation to the region. Alberta clippers are fast moving and not very large. They occlude quickly over the northeast of the United States without dropping much moisture.
As wind blows from the west to the east, it comes into contact with the orographic barrier of the Olympic Mountains. If the air cannot flow over the mountains it must go around them instead. Air flows around the barrier it enters the Puget Sound Convergence Zone. As the air collides it is forced to rise. It condenses and forms clouds which lead to precipitation when the cloud has reached its vapor load. This is one reason why Seattle receives a large amount of moisture.
|This is a radar image of the Puget Sound Convergence Zone. The convergence zone typically brings rain to the Seattle area, as shown. The rain in this instance is light to moderate.|
From Seattle, one might see lenticular clouds form over the mountains, especially Mount Rainier. For Lenticular clouds to appear there must be winds which flow perpendicular to a topographic barrier. The moist air parcels are orographically lifted and form clouds as they meet the dew point. Lenticular clouds often seem as though they are not moving due to the constant supply of cool, moist air from the windward side of the mountain. High pressure gradients often set up conditions for lenticular clouds to form, as they bring strong winds. Lenticular clouds are usually only observed in mountainous regions.
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"Mid-Latitude Cyclone." n.d. Atmos.illinois.edu. Web. 14-16 November 2012.
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"What is the Puget Sound Convergence Zone?" 4 October 2006. Komonews. Web. 14-16 November 2012.