Wednesday, April 17, 2013

The Wetter the Better...

Washington State Geographic Map
When people think of Seattle and the Pacific Northwest, they often thing of rain, gray skies, and cold weather.  When in fact, they are correct.  Well half correct.  The state of Washington is known for its lush, beautiful green landscapes filled with evergreen trees and wet undergrowth.  But these common beliefs are only about one third the state.  From the Olympic coast to western slopes of the Cascades, the Seattle and Puget Sound areas are what most people think of in terms of the Pacific Northwest.  The western third of the state is very moist, with high amounts of precipitation each year.  Some of the highest in the continental United States.  In fact, the Olympic National Park rainforest with the highest amount of precipitation in the northern hemisphere is located on the Olympic peninsula.

Left & Right, Black & White, Wet & Dry...

Amount of Precipitation, Washington State

There are several reasons as to why the western third of Washington State is so wet.  One of which is the rain shadow effect.  As the moist cool air flows in from the Pacific Ocean, the Olympic peninsula and Puget Sound capture all that moisture.  This occurs not simply because the weather rolls inland, but mostly in part to the two mountain ranges that surround the Seattle-Tacoma area.  The Olympic mountains first to the west, and the Cascade Mountains to the east.  The abrupt increase in elevation from sea-level to the tops of the Olympic Mountains causes the already cool pacific air to quickly reach their due point.  As the cold moist air is forced over the mountains and cools, the water in the air is released on the Olympic Mountains and in the Olympic National Park.  This is why this part of Washington is known for its high precipitation amounts.  As the weather continues east towards the Puget Sound and the Seattle area, the air quickly descends, it mixes with the cool moist air being blow in through the Puget sound from the north by Victoria, British Columbia.  This helps to keep the moisture in the air that travels through the Puget Sound.  This is where the air again rises quickly up the western slope of the Cascades, reaching its due point quickly, allowing the rain to fall on the western slopes and Puget Sound areas of Washington.  The Olympic Peninsula receives around a high of 200 inches of precipitation a year, where on the eastern side of the state in Spokane is typically less than 20 inches of precipitation a year.

Seattle to Spokane
Now this is where the moisture stops, and the nomenclatures about the Pacific Northwest with them.  If you were to drive on I-90 over the Cascades from Seattle east towards Spokane, Washington, you would notice how the climate completely changes.  The eastern two thirds of Washington State are like most of California, Nevada, Utah, and Colorado; very dry and arid.  This is due to the Cascade rain shadow; the Columbia River Basin stays flat and dry.  The biggest rains and weather differences come when the prevailing winds blow from the south. 

Air Mass Flow Diagram

The moist air in the Pacific Northwest is a colder air.  It comes for the Maritime Polar air that flows in from the north Pacific.  This keeps the temperatures relatively cool and consistent, but is also the reason for all the rain.  Since it’s a maritime air mass, this infers that it will be loaded with moisture unlike continental air masses.
As the maritime polar air mass (cold front) flows towards the Pacific Northwest coast, it converges with the maritime tropical air mass (warm front), the cold front overtakes the warm front and forms an occluded front right over the pacific north west.  This occluded front creates the possibility of a high amount of precipitation to be produced.  This is the case in the Pacific Northwest and why Seattle and the surrounding areas are known for their excessive rain.  as the front moves east, it reaches down into Oregon and northern California. 

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