For the dates of your trip the average high is around 78-80 degrees. The record highs are generally in the low 90s (though one day has a 100 degree record high, I think I remember that if these are SeaTac stats).
Of course, historically the average rainfall for your days is between 0.25 and 0.5 inches. Presumably those average highs and average rains generally don't happen at the same time (Seattle is generally not one of those "warm rain" towns).