Check the articles on the home page of this site, left hand column. There are several on the selection and fitting of backpacks.

It's best to get the rest of your gear first and take it with you when shopping for a backpack. Be sure to include the approximate weight/volume of a week's food (or the longest period you expect to be out) plus a day's water.

The most important aspects of a pack are fit, fit and fit. The pack must fit your body (the important measure is torso length, not weight, height, or age), fit your gear, and be comfortable for you with your gear packed inside. Pack fit is almost as individual as shoe fit.

For what you want, you need a pack that can be compressed to fit a smaller load than what you'd take for, say, winter outings. However, it's more likely that you'll end up being more satisfied with two packs, one for your summer load and the other for winter.

There are many lighter weight options, still supportive, than the brand you've mentioned.

At the top of this Backcountry Beginners section is a very useful "sticky" post from phat, one of our longtime members who lives in Edmonton and therefore encounters the same conditions you'll be facing.