Why does it rain more in the winter than during the summer?
East of the Rockies, thunderstorms in the late spring and summer produce much of the yearly rainfall. Here in the coastal northwest, our rainy season usually runs from October through March (except this year!). That's because as the contrast in temperatures between the polar region and the midlatitudes (such as our area) sharpen, the jet stream strengthens and drops south. It's average position is over southern British Columbia. Because the jet stream acts to strengthen and direct weather disturbances, that period tends to produce the most rainfall in our area.
That north-south contrast in temperatures weakens in the summer, resulting in a weaker jet that shifts northward; it's average track is into northern B.C. or southeast Alaska. That's why we receive less rain during the summer months.