SEATTLE - It's a collision of winds that occur due to the Olympic Mountains. When we get a northwest wind, the Olympics will split them. Some of the winds will go east down the Strait of Juan de Fuca and then turn south once they reach the I-5 corridor.
Meanwhile, the other branch of the winds will go around the southern end of the Olympics, and then turn north as they reach the I-5 corridor.
The Convergence Zone is where those two branches collide. The winds have nowhere to go but up, which creates clouds and eventually rain.
It usually forms in an east-west line across southern Snohomish County but can go as far north as Anacortes or as far south as Federal Way, depending on where the winds collide.
For More Information:
KOMO Weather FAQ -- www.komotv.com/weather/faq