Every bend in the western coast route represents a new microclimate. There is no way to predict accurately the isoalted weather systems through which you will pass. It will be wet, dry, foggy, rainy, windy, overcast, clear—sometimes all in one 10-12 hour period.
We've had an unusually wet 2010 winter and 2011 spring in the inland Northwest, Idaho and eastern Oregon and Washington. The ski season was long and profitable, reservoirs are full, rivers ran high, hydroelectric production and surface irrigation were good but we have had no measurable precipitation in most of Idaho since July. And yet, on the Ride Idaho 2011 tour, we experienced severe isolated mountain thunderstorms like I haven't seen in decades. Hail, sideways rain, big winds, lightning. Spooky.
You can make no prgnositcations about weather.