Western Washintgton Sports Car Council http://www.wwscc.org
is kind of the clearing house for all Seattle-area autocross events.
There is an SCCA event Sunday, July 5th at Bremerton Motorsports Park.
Membership is required for all events, but weekend memberships are available. Declutter your car, pump the tires up to 40PSI to prevent trashing the sidewalls and be there by 7:30AM for morning runs or 11:30AM for afternoon, depending on your class.
Between March and the end of October you will be hard pressed not to find an autocross going on somewhere in the area every weekend.