It has been super-hot here but I think it's going to cool down, I used my drysuit last weekend when it was 80 degrees because I knew I'd be on the river late and was glad I had it - so if it's hot, like Lisa said, a drytop is fine but if you're not sure and have the room I'd be taking my drysuit too. When you're here should be a good time for a lower Skykomish River run, I think from Cabledrop down to Big Eddy would be good
http://www.professorpaddle.com/rivers/riverdetails.asp?riverid=587 if it's not too low, or you could do the Index to Cabledrop section and portage Boulder Drop. Maybe post on here as you get closer to that date and see who's doing what or who would be available to go.