Why is the weather temperate in Seattle?

Question

What causes the winters to be so mild in Seattle (compared to other northern cities) and for there to be regular rainfall? Is it because of its proximity to the coast or to the mountains, or what?

Answer 1

This is because winds above the surface in the midlatitudes (in other words, where most of the United States is located) are generally from the west, which is why most weather systems move from west to east.  Just to the west of Seattle lies the vast Pacific Ocean.  The water cools down a lot slower than land in the winter (and warms a lot slower in the spring/summer), and is thus much warmer than land masses at this time of year.  With the prevalent wind off the ocean, western Oregon and western Washington stay warmer than further inland areas.

However, this isn't the entire story.  The Cascade Mountains just to the east also play a part.  Air pressure is higher over cold areas (like the Northern Plains and Canada) than over the warmer Pacific Ocean, and air likes to flow from higher pressure to lower pressure.  Thus, the mountains actually impede this air for the most part into making it to areas west of the Cascades.  Only rarely does colder air flow southwest through the Fraser River valley in British Columbia and make it into western Washington, and that's when snow is most likely in the Seattle area.  The coldest Seattle has ever been was 0 degrees Fahrenheit, in 1950, and that is what occurred.  Generally, when this relatively cold air occurs, it doesn't last long, and flow soon reverts back from off the ocean once low pressure systems change the wind directions.

So in short, the mildness is because of its proximity to the coast (but more importantly, the fact that the ocean is just to the west and not the north, south or east), and secondly, because of its proximity to the mountains.

The rain is because the jet stream (upper level winds above the surface) is pointed largely to the Pacific Northwest that time of year, so a lot of low pressure systems move toward the area, or just to the north, bringing rain to the area from the Pacific Ocean.  Because of the mildness, this usually falls as rain as opposed to snow.  In the summer, most low pressure systems move too far north to provide rain to the area.