1. Employers will have an enormously wider choice of employees. The pool of people who are willing to work for $15 hour will expand and employers will get some very talented people who otherwise wouldn't work for current minimum wage i.e. people will join the labor pool.
2. There will be an influx of people moving to Seattle to get one of those $15/hr jobs so the unemployment rate will go up. Not because employers will lay off but simply because more people looking.
3, There will be higher unemployment among people who now get $9 hour. They will not be able to compete when there is a larger pool.
4. The economy will grow since labor productivity will increase due to higher-skilled workers.