Why are the states of California, Oregon, and Washington mild and rainy?

There's few reasons why these three states have mild and rainy climate, most important of which are:

- Pacific trade winds; the Pacific trade winds are bringing fronts of the Pacific that are moving eastwards towards the western part of the USA. These winds are warm and wet and they bring in lot of precipitation with them.

- Mountain ranges; the mountain ranges that are stretching through these three states and that lift up the terrain significantly east from the coast are crucial because they stop the Pacific trade winds from going further inland and disperse, so they stop their movement and give them the opportunity to accumulate more and more clouds and thus higher precipitation.

- Ocean currents; the dominant ocean currents along the shores of these states are cold ocean currents both from the south and the north, so the colder water manages to affect the coastal regions and drop their temperature by few degrees.

Clee8403, The pacific trade winds bring fronts off the pacific moving east. This provides wetter weather in the lowlands.