This is not a waterproof pack. And I do not think Osprey makes a rain cover for this series of packs. If you are looking for a more waterproof bag may I suggest checking out the Arcteryx Arrakis pack, it is pricey but amazing.

While this pack is made with very water resistant materials, Joshua is correct--it is not waterproof. However, Osprey does make rain covers and the Large raincover will fit this pack just fine:
http://www.backcountry.com/osprey-packs-ul-raincover-osp0131