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).
For more meaningful rain (a quarter inch in a day), there is a substantial increase in October, but the heaviest rain period is still a month away, centered in November.
This week will be a good one to keep your umbrella handy.
There will be a slow, steady accumulation of precipitation, as shown by the accumulated rain at Seattle (below) through October 22. This plot shows the precipitation accumulating from many model runs, something called an ensemble.
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.
The official NOAA 8-14 day outlook is for cool and wet over our region (see below).
No complaints will be allowed. We have been drier than normal for the past few months and soils are dry. This rain is good news and will be modest.
