Stranger yet, it will be occurring during the climatologically driest period of the year (see plot below of average daily rainfall, which shows the dip to a minimum during this time)
The last month or so has been startling dry. Here is the plot of the observed and climatological rainfall at Seattle Tacoma Airport since June 5th. A few hundredths of an inch in June--we are down by roughly 2 inches. A trace (less than .01 inch) in July. Sahara in Seattle.
Fortunately, the latest radar image (below) is reason for joy for many parched Northwesteners, folks that have had moisture envy of our brethren down in southern CA, of all places. And it will be a potent tool for those fighting local wildfires. Associated with an approaching Pacific front, moderate rain is now falling over the NE Olympics and southern Vancouver Is. It is raining in Vancouver (see cam below).
Today, this front will slowly move through the region, reaching Seattle this afternoon and tonight. Here is the predicted 24h precipitation totals ending 5 AM Saturday. SW British Columbia, the Olympics and the northern Cascades get the most--roughy a half inch. The Puget Sound area will only get some light rain later today....not that much (see more detailed map below)
But this is just the appetizer. On Saturday, a weak upper-level disturbance will move though, bringing light showers (see 24h precip ending 5 AM Sunday). More over the southern Cascades.
The main event will be on Sunday as a robust upper level trough pushes into us (see upper air map for Sunday AM). Goodbye big ridge of high pressure.
And here is the 24- precipitation ending 5 AM Monday (both wide and close-in view). Extensive light rain, with particularly heavy amounts in the north Cascades and Olympics.
For most, this will not be a big rain event...but nearly all in the mountains and west of the Cascade crest will get some light rain.. Temperatures will drop back to the mid 60s to low 70s over the weekend.