Set off an avalanche in those trees skiers left on Scotties last night on the twilight tour. I made two mistakes and after thinking about I can look back and learn. I remember when skinning up last Friday and I was looking at the big snow crystals on the top of the snow (surface hoar) and made the comment that this could be a weak layer come next storm. Had powder fever and totally forgot all about that observation when I headed up. I also made the classic mistake of seeing many tracks and assuming that sincemany people had skied it that its not going to slide. So me and my partner rode the first pitch, one at a time, with no problems. Same line we all took off the cornice and down to the flat section last Friday. Last time I rode that line I went skiers right of that flat section and into the main chute, however me and my partner decided to stay in the trees since that was where the best snow was. I started the second pitch and about my third turn snow was fracturing all around me. The fracture shot out about 15 ft to my left and about 50 feet to my right. I carved a hard left and was eventually able to get out to the side of it. I shouted out "AVALANCHE" for my partner and put the avalung in my mouth as I made the desperate carve to the left. Another thing that kinda freaked me out was the slide started in the glade within the trees and spread out to some thick tree sections of the bowl. Slab was about 75-100 feet wide and about 14 inches thick and ran for a hundred yards. I always kinda assume that trees were a safer choice, not last night. My theory is that the sun was out all day and got kinda warm. Even though those trees did not see any direct sun it got warm enough to consolidate that new snow layer into a thick slab. The cold nights really built up some surface hoar in those sheltered tree zones making a weak layer that was not found in the main chute. The main avalanche chute had enough daytime sun to keep those weak crystals building up actually making the gut a safer choice. Lesson learned.