Grew up in Washington so can attest that if it decides to REALLY rain, then you prepare to hunker down until walking again becomes reasonable. Springtime also means a lot of lingering snowpack, unless hiking the coast and lower foothills.

A silnylon poncho can protect your pack while hiking and double as a small tarp for hanging in camp and cooking under. Gear list seems reasonable, and waiting to select the pack is exactly right. You need to know the bulk and weight of your kit, first. ETA: gaiters, you'll want some.

Plan on carrying a spare Hyperflow cartridge. Trust me.

Cheers,