Seattle Weather. Live 36 hour weather forecast for Seattle WA, plus average and seasonal rainfall totals and reasons for Seattle’s weather patterns.
...Especially if you visit in late June through early September.
Seattle gets only 36.15 inches of rain per year, as averaged over the last 30 years. This is fully 3 inches less than the national average. The reason for this is partially attributable to a rain shadow effect. The edge of the Olympic Mountain rain shadow extends very close to, and occasionally includes the greater Seattle region.
Despite receiving only 36.15 inches of rain per year on average, Seattle typically receives measurable rainfall 152 days of the year, more than most US cities. Meaning, we don't get a whole lot of rain, but what we do get comes in "drips and drizzles" spread out over days. This is probably why many denizens of Seattle, and those in the rain-shadowed Olympic Peninsula, don't bother packing umbrellas. We've gotten used to the drips and the drizzles. If you hurry, you can scoot off to your next destination in between raindrops. That strategy will certainly fail us in fall or winter, however....
Seattle's yearly rainfall per season:
As clouds and moisture funnel into the greater Seattle region, they often drift past Seattle and continue to the east. Eventually they bang up against the Cascade Mountain Range and begin dumping their moisture on the western face of the Cascades. The farther east one travels between Seattle and the Cascades, the more precipitation falls, depending on the season. The more moisture, the wetter Seattle also gets, since it is only 28 miles away from the Cascades.
Once you cross the Cascades, you'll find Eastern Washington is very dry. The eastern half of the state sits in the Cascade Mountain rain shadow.
So yes, do check the weather forecast before you arrive. But it might not end up being as bad as you fear.