Arc'Teryx earns a twofer: along with the Alpha SV bib comes the Alpha LT jacket, a stellar example of what good mountaineering kit should be. The LT barely nudges 13 ounces on the scale; yes, you read that right, a serious-duty waterproof, windproof shell at well under a pound. With the jacket and bib, all you need is a codpiece and you're good to hook.
It was difficult for me to give up my much-beloved Sigma SV soft shell, but the LT saves half a pound and a packs down even smaller, plus it's fully waterproof. My only real complaint about the Sigma was the pockets; problem solved on the LT with its reach-across chest pockets for easy access under a pack. An internal zippered pocket stashes wallet or MP3 player, and on the left and right hip are little foam bumpers (Hemlock) that prevent the LT from riding up through your harness. Pit zips, zipper garages, helmet compatible hood and a full length zipper flap round it out.
The fit is typical Arc'Teryx, which is to say just about perfect. The few adjustments are all you need to dial it in. The hem drops down in back just a bit, which is nice. The Gore-Tex 3L Pro Shell is getting a lot closer to soft shell, not exactly supple but minimal annoying crinkle and swishing sounds. My only concern was durability, but the LT got lived in on Denali with no visible scarring, including sucking up a sixty pound pack with no complaints. Breathability is excellent. The Alpha LT is top drawer all the way, but as usual with AT gear you'll have to save your pennies.