On the West side of the Cascades, if it's raining when I get there I'll ride. Or if there's some chance of rain I'll go. If I know it's going to be a mess I'll probably think of something else to do.
On the East side, I and most of my friends will drive up to 200 miles to ride that sweet, sweet, wet desert
If it's pouring when I get to the Starvation Ridge track though (on the east side), I'll turn around and go home. Not worth it, never again
The weather forecast has to be read differently from one side to the other too. 20% chance of showers in the NW means it's pretty much guaranteed to storm. 80% chance of rain on the East means it will still probably be a dust bowl.