On the East Bay Scots event listing on FB (https://www.facebook.com/events/152895952077474/) they say it's a "rain or shine" event.

If it's really the "rivers of rain" some outlets are predicting, I'll be staying home. However, if the predictions are off or the storm winds itself out Friday night and things clear on Saturday morning, I'd still go. I've been in damp years and it's still fun.