Was it your choice to go in June? The weather was actually pretty nice in late July (years earlier). Some days in the 70's and sun. You are right about it changing quickly though. What made you decide to do this peak?
I had a conference in Reykjavik, so I had no choice of dates and less time than I would have liked -- so I stayed in the southwest corner and stumbled upon this one by accident. Roads in the interior hadn't opened up yet.