- jet stream
- El Niño / La Niña
- chapter 7 of NOAA's book "Practical Mountain Weather" (PDF pages 12-25, especially the section 17-19)
"The vast majority of precipitation that falls in the Cascades and Coast Ranges occurs in
the October through April period. Needless to say, winters in the region are typically cloudy and wet.
From the central Oregon area northward, May and June can be considered a transitional weather
period. What we mean is that the weather during these two months typically alternates from being
relatively cloud free and warm, to cool and wet. The cycle roughly last a week or so, and is linked to
the north-south migration of the polar jet."