In the Northwest, October is the transition month to the wet, winter regime that extends through mid-February. And the next few weeks should reflect the typical changes.
But before I show you the latest forecasts, let's look at the typical precipitation of climatology at Seattle.
The figure below shows the climatological probability of getting at least 0.01 inch of precipitation in a day at SeaTac. After the dry minimum around August 1 (about 8% probability), there is a plateau (around 30%) in September, and then the big move upward in October to around 55% (October is delineated by the vertical lines).
This week will be a good one to keep your umbrella handy.
The map of the total accumulation through 5 AM next Wednesday shows rain over the entire region, with particularly heavy totals (as much as ten inches) over the western side of the terrain in BC. The Olympics and North Cascades get a piece of it.