About 18"-36" base depth.
Summit West is usually the first base area we're able to open due to its relatively smooth slopes (we try and get more base areas open as additional snow starts coming in). The amount of snow needed to open Summit West varies somewhat, depending on how much moisture is in the snow, but typically we're looking at a minimum of 18" - 24" of standing snow. If the initial snow is powdery and light in moisture content, we tend to need more snow as the grooming machines will knock a lot of the air out of it. If the initial snow is dense we can often get by with a little less. This is why we like our first snows of the season to be a bit on the heavier side as they build good base.
The other factor that determines openability of any slope is what's on the surface. If it's a mostly grassy slope with very little rocks, etc. and we've done intensive brush-cutting, then we only need enough snow base so that we're able to groom it. Our aggressive summer slope maintenance, brush cutting, and drainage improvement program helps improve the surface and reduces the amount of snow required to open. Of course, the rougher slopes or those with large rocks, brush, small trees, etc. require much more more snow before we feel we can create a skiing surface. Bottom line, there's really no finite number of inches of snow that determines what we can open, it always comes down to a final determination from our ski patrol and operations team using all of the above factors in the mix.