The wind, which can reach speeds of 150 mph, is formed from the boundary between the cold, polar air to the north and the warm, tropical air to the south. The greater the temperature difference, the stronger the jet stream can be.
Since storms tend to form along these boundaries, the jet stream also acts like a steering mechanism for storms coming off the Pacific.
If the jet stream is pointed right at us, as is usually the case in the fall and winter, that's when we get our series of rain storms.
When the jet is to the north of us, we tend to be warmer, and when it's further south or coming at us from the north or northwest, we usually get some colder air. The perfect recipe for snow is to have the jet stream pointing at, say, Portland, with storms coming in just to our south and having those storms pull down cold air from Canada. That's what happened during the big area-wide snowstorm in December of 1996.
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