The Stormfront by Sitka does not stretch, it features an articulated cut to increase the range of motion. If you are looking for stretch, you would need to look at the Stormfront Lite or the newer version of the Downpour, these jackets are a Gore-Tex PacLite construction that does feature 2-way stretch.
The S.F. Lite and Downpour are nice for packing (compress small and very light), but for longevity, durability and a jacket for the long haul I would look at the Stormfront or Coldfront. As far as durability of the waterproofness, Gore-Tex guarantee's the waterproofness of their fabrics with a 100% satisfaction guarantee.
