My disclaimer...some SAR experience, but in the eastern US in broadleaf forest, rather than mountainous areas.

Are you anticipating having to provide for only yourself for an overnight, or possibly providing for a victim for an overnight before a daytime evac? The area I worked in was very small (not really many potential searches that would involve an overnight), but in my pack I carried an 8x10 silnylon tarp and a couple of preformed aluminized bivy sacks made by Adventure Medical Kits (I think they now also offer one made of Thermolite). One for me, one for the patient. I never anticipated actually getting much sleep if I found myself in that situation, more concerned with keeping myslf and possibly a victim normothermic. Tarp would be set up as a roof, using either branches or tekking poles. I think the weight came in at less than a pound. (I should also mention that even winter temps only get to around 0 degrees F at most, barring unusual weather patterns) YMMV

I know that the possibility of an overnight is largely determined by the area you cover, but a RON scenario was one we consciously avoided, and would routinely postpone searches if the urgency wasn't very high. Again, YMMV.

Welcome to the family! I'm hoping that my recent app to a regional team will be accepted soon, SAR is too much fun to just stop doing (kind of like the UL thing...)