I'm thinking about getting a backpack to replace my messenger bag. The bag has been shifting around on my back, if it is very full and I am riding hard with a lower position and a flatter back. It works fine in other circumstances, but when I'm in the drops, low, sprinting, the bag will shift toward my right and throw me off balance. This didn't used to happen but perhaps I am riding harder lately, I like to think so anyway.

My needs are: waterproof enough for a Portland winter, durable (I've had my messenger bag for 14 years), a bright color, either lots of reflective or able to have reflective tape applied (and stick well), won't shift around when low and sprinting, as low wind resistance as possible for a backpack, and it has to have as much volume as a "large" messenger bag.

What do you recommend?

I looked at an Ortlieb messenger backpack, it looked pretty good. I also saw a Chrome backpack, it is totally different but also looked promising. My messenger bag from Timbuktu has held up so well for all this time, that I'm interested in looking at their backpack too, but so far I've not seen one in the flesh.