A friend recently pointed me to this fantastic outing planning tool: it's a 3-day precipitation forecast animation map for OR and WA from the weather scientists at the University of Washington:
http://www.atmos.washington.edu/~ove...d2_x_pcp1+///1
All else equal I prefer to ride when it's not raining, and this tool has been a remarkable help. The UW weather site has lots of other content. For cyclists this one far outshines "regular" forecasts.
Cheers,
Scott