Friday, November 16, 2012

Seattle: Part III Air Masses and Fronts

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 map shows the position and influences of air masses in the summertime.  Please note  the maritime tropical air mass and its position along the west coast. The maritime polar air mass has been pushed north to Canada and Alaska.
In the winter time, the maritime polar air mass extends down into the Seattle area. The maritime tropical air mass recedes to the south. Continental and maritime polar air move into Seattle, sometimes causing snowfall.
This image of the Rossby waves show how it dips into Washington State, bring cold air with it from the Arctic.

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.

This image shows the origin point of mid-latitude cyclones in the United States. The Gulf of Alaska low is most likely to affect Seattle. However, the Alberta Clipper can bring rainfall to Seattle as well.
As the cyclone begins to form, the leading low pressure edge brings clouds and rain to Seattle.  Since Seattle receives weather when the cyclone is just forming, it usually does not experience extreme or turbulent storms.
This image from the National Weather Service shows the circumstances under which Seattle might receive snow.  A warm front is moving into the Seattle area from the west, followed by a cold front. The warm front brings precipitation. As the warm front passes, the cold front will cool the air bringing more precipitation in the form of snow.

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 topographic map show the orographic areas of Seattle. Topography in green are lowlying areas, and pink to blue shades are uplifted areas. On the western side, Seattle is protected by the Olympic Mountains. The Cascade Range flanks Seattle to the east. The Olympic Mountains provide a barrier to the fronts brought by mid-latitude cyclones. The air must flow around the orographic barrier, creating a convergence zone (an area where air masses meet) over the Puget Sound and Seattle in what is called the Puget Sound Convergence Zone.
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. 
These are lenticular clouds over Mount Rainier. Lenticular clouds indicate turbulent skies to  pilots flying over mountainous areas. Lenticular clouds typically only form in mountainous regions due to orographic lifting. However, they can form in other areas if the wind shear is right.
Works Referenced

Allen, Casey. "Weather." n.d. Web. 14-16 November 2012.

Haveson, Scott. Forecasting for Western and Eastern Washington. n.d. Web. 14-16 November 2012.

"Into the Mystic Mount Rainier." 28 August 2012. All That is Interesting. Web. 14-16 November 2012.

Mast, Beth. "Climatology Seattle-Tacoma International Airport." 2010. Web. 14-16 November 2012.

"Mid-Latitude Cyclone." n.d. Web. 14-16 November 2012.

National Weather Service. "Puget Sound Convergence Zone." n.d. Web. 14-16 November 2012.

"Olympic Rain Shadow: Information and Resources." 2010-2012. Web. 14-16 November 2012.

Rossby Waves. n.d. Web. 14-16 November 2012.

"The Big Blog." n.d. SeattlePi. Web. 14-16 November 2012.

"What is the Puget Sound Convergence Zone?" 4 October 2006. Komonews. Web. 14-16 November 2012.

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