A locals point of view.
Here's what you do. Fly up the East side of the Cascades to say Wenatchee. Zero chance of rain that way. Check the Wx from there or while enroute. If its raining in Arlington, stay put in Wenatchee until it clears or RON. Repeat as needed. Your 90% there.
Or stop in Hood River and we'll put you up and guard your planes.
DT