You might hear meteorologists talk about the "convergence zone," but have you ever stopped to think about what it is?
A convergence zone is kind of like Seattle traffic, where I-5 and I-405 near Lynnwood converge, often causing traffic backups.
The same thing happens with the wind.
When wind comes off the ocean, it splits up around the Olympic Mountains, meeting up over Puget Sound. That's where the wind collides and the convergence zone forms.
The winds colliding from north and south force moist air up. The air cools and the moisture then turns into rain.
That's the Puget Sound convergence zone, which might explain why some parts of the area stay dry while others get drenched.