Just left Ketchikan on May 7th. for 1 week I wore jeans/t-shirt/denim shirt as a jacket - from 5 a.m. being outdoors until after 9 in the evening.
I've spent 3 summers there - and have worn everything from ski wear in August to tank tops and shorts.
Just like the Boy Scouts-be prepared! A sweat shirt or Polar Fleece jacket covered by waterproof rain wear is sufficient. Sturdy walking shoes, preferably waterproof - are necessary. This year, people in Ketchikan are preparing for more warmth than is usual.