I think you need the Arcteryx Naos 55 pack. Introduced last year, the Naos 55 is part of Arcteryxs ACC" (for Advanced Composite Construction) line of packs. Theyre not sewn togethertheyre heat-welded, so theres no stitching. That means the packs can be made with waterproof materials without fussy (and, in packs, somewhat ineffective) seam-sealing. The result is a light, capable pack thats completely waterproof. The Naos 55 is a sort of mid-sized pack, with up to 3,400 cubic inches of capacity in the medium size. That should handle your anticipated load nicely.
The down side to the Naos is the priceit sells for $430, putting it in the upper echelons of pack prices. The materials are not cheap, and the assembly method is pretty high-tech.
There are other options. Ospreys excellent Aether 60 ($219; www.ospreypacks.com) is a little bigger than the Naos 55, and it has a wonderful suspension and the ability to handle more strap-on cargo than the Naos 55. The Aether 60 is not waterproof, but that can be remedied pretty easily with an Osprey (or any brand) pack cover for about $30.
REIs Cruise UL 60 pack ($130; www.rei.com) is right in this same size range and is lighter than either the Naos 55 or Aether 60. Add a rain cover, and youre good.
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