You can use this link,
weather near Outpost Camp to approximate weather at Outpost Camp. This is from altitude 10,266, which is just slightly higher than Outpost camp.
While I have a somewhat different take from Steve on the sleep elevation issue (I will grab an opportunity to push the sleeping elevation incrementally higher if it is there and all systems are working), I will note that the first time I climbed any part of Whitney, I did overnight at Outpost, and hiked up to Trail Camp the next morning. The Outpost-Trail Camp section is far easier without a backpack. I had the same do-I-don't-I back-and-forth that you are now doing when I did a summit climb last Labor Day. When we got to Outpost Camp, the answer just became clear, based on the time, weather, energy level, etc. No need to worry about it ahead of time, really.
