Another of the many things that affect the Northwest's weather is the Arctic Oscillation (AO). The AO seems to interact with the Pacific Decadal Oscillation and the North Atlantic Oscillation. It may have more of an effect on the winter weather of northern and eastern North America and western Europe than on the Northwest.
During the AO's positive phases the Northwest experiences drier than normal conditions. The negative phase brings colder temperatures and snowier conditions in Alaska, eastern North America and western Europe. During the last two winters, the Arctic Oscillation was strongly negative causing outbreaks of cold and snowy conditions in the U.S.
The AO has a seesaw atmospheric pressure pattern that fluctuates on the order of weeks and months, though it has a tendency to favor one phase or the other for years at a time. Strong Arctic Oscillation episodes typically last a few weeks and are difficult to predict more than one to two weeks in advance.