We went on a Thomson's cruise to the Norwegian fiords last year in June. We were very lucky with the weather which was like Spring in the UK. It can be quite warm on some days and cool on others.
As you would expect these days who can tell. Some people I have spoken to have said the it has been in the 70's others rather chilly. I am sure that the scenery will make up for any chilly conditions. Pick a ship that caters for indooor and outdoor cruising thats the best bet. Kind regard.